TheExplosiveEarth_Volcanoes
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TheExplosiveEarth_Volcanoes

Course Number: EOSC 114, Fall 2012

College/University: University of British...

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The Explosive Earth - Volcanoes OVERVIEW In this Module, we will first study basic volcanic processes and the factors that determine a volcano's explosivity; this is helpful in assessing how dangerous an individual volcano may be. We will also investigate different types of volcanic hazards associated with specific volcano morphologies and learn how volcanologists predict volcanic eruptions. Finally, we will...

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Explosive The Earth - Volcanoes OVERVIEW In this Module, we will first study basic volcanic processes and the factors that determine a volcano's explosivity; this is helpful in assessing how dangerous an individual volcano may be. We will also investigate different types of volcanic hazards associated with specific volcano morphologies and learn how volcanologists predict volcanic eruptions. Finally, we will incorporate our knowledge from the previous volcanism units into a plate tectonic framework to better understand how volcanism is influenced by various plate interactions. INSTRUCTIONS and QUIZ Study and make notes based on the online course material and the textbook reading assignments, and read the commentaries that follow. The review questions will allow you to assess your understanding of the key concepts. This Module will be covered in the Volcanoes Quiz. Consult the FAQs for information on taking the Quizzes. LEARNING GOALS Use the following learning goals as a self-assessment tool to help you gauge your understanding of the course material presented in this Module. By the end of this Module, you will be able to: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. Explain what magma density and magma viscosity are. List the different categories of volcanic rocks and explain the differences between the magmas they came from. Explain why some magmas erupt explosively (as pyroclastic material) and some magmas erupt effusively (as lava). Explain the differences between pahoehoe and a'a lavas. Describe the different types of volcanic eruptions and how they are related to magma properties. Describe the morphology, dominant rock type and typical eruption style of the different types of volcanoes. Explain what lava flows, fire fountains, lava bombs, and volcanic ash are and how they form. Describe the particular hazards associated with lava flows, fire fountaining, lava bombs, and ash fall. Explain what pyroclastic flows, lahars, volcanic domes, sector collapses, lateral blasts, and toxic gases are and how they form. Describe the particular hazards associated with pyroclastic flows, lahars, dome collapses, sector collapses, lateral blasts, and toxic gases. Explain how the size of a volcanic eruption is estimated. List the different volcano monitoring techniques and the instruments that are used. Explain what a volcanic hazard map is and why they are useful. Use your knowledge of volcanic processes to map major hazards around different volcanoes. Evaluate the hazards to Vancouver associated with an eruption from Mount Baker. Describe the distribution of the world's active volcanoes. List the three types of plate boundaries and the different types of volcanoes that occur at these plate boundaries. Describe the type of volcanoes that occur at oceanic and continental hot spots. Describe the tectonic setting of British Columbia and determine the dominant type of volcano that occurs here. Page 1 of 54 ORGANIZATION The Explosive Earth contains four units (A - D). UNIT TOPIC A Why Do Volcanoes Have Different Characteristics? B What Threats Do Volcanoes Pose? C How Do We Prevent Destruction From Eruptions? D Where are Violent Volcanoes Found? READINGS: The Explosive Earth Important Notes: a) Don't memorize any Tables but understand the main points that the Tables illustrate. b) Figures contain very important information, too! Read the captions and understand the ideas being illustrated Read all of Chapter 6. Volcanic Eruptions and Landforms Read all of Chapter 7. Volcano Case Histories: Killer Events UNIT A TOPIC Why Do Volcanoes Have Different Characteristics? Outline In this unit you will examine the physical and chemical characteristics of magma and lava, learn about a wide range of eruptive products, and study different types of volcanic landforms. With this information, you will go on to study volcanic hazards in Unit B. 1. Formation of magma/lava 2. Composition of magma/lava 3. Volcanic products 4. Explosivity of magmas Factors influencing explosivity Mafic and felsic magma 5. Anatomy of an explosive eruption 6. Volcanoes Mafic volcanic landforms Intermediate/Felsic volcanic landforms Felsic volcanic landform: Calderas Volcanic landforms: Summary Page 2 of 54 7. Summary 1. Formation of magma/lava What is magma made of? What is the range of products erupted from volcanoes? Which magmas are the most explosive? What sequence of events takes place during an explosive eruption? The answers to all of these questions depend on the chemical and physical properties of the melted source rock. Click here to view an animation of how magma rises through the mantle and crust. Magma is formed by melting pre-existing rock in Earth's interior. Melted rock is less dense than solid rock, so it will rise towards the surface of the Earth. Some magmas are trapped in the crust, whereas other magmas travel through fractures in the crust and eventually extrude onto Earth's surface. For magma to rise through the crust it must be: (1) Less dense than the crust ; (2) Runny enough to flow (i.e., it must have low viscosity); and (3) Hot enough to stay liquid. All these properties depend on the magma's composition and temperature. Magma that has erupted and cools to form solid rock is called lava . Table VO.1 and Figure VO.1 below list the common elements of the Earth's crust, which all were formed by magma rising to the Earth's surface. Table VO.1 Common Elements of the Earth's Crust ELEMENT COMMON FORM, SYMBOL WEIGHT % Oxygen oxide, O-2 45.20 Silicon silica, Si +4 27.20 Aluminum aluminum, Al +3 8.00 Iron iron, Fe +3 and Fe +2 5.80 Calcium calcium, Ca +2 5.06 Magnesium magnesium, Mg+2 2.77 Sodium sodium, Na +2 2.32 Potassium potassium, K +1 1.68 Titanium titanium, Ti +3 and Ti +4 0.86 Hydrogen hydrogen, H+1 0.14 Phosphorus phosphate, P +5 0.10 Manganese manganese, Mn+2 0.10 Page 3 of 54 Figure VO.1 Elemental abundance of the Earth's crust by weight percent. CHECK YOUR UNDERSTANDING: For magma to rise through the crust it must be: (check all that apply) A. Cool enough to solidify B. Runny enough to flow C. Less dense than the crust D. Full of gas bubbles to float E. More dense than the crust F. Hot enough to stay liquid 2. Composition of magma/lava Chemical, mineralogical, and physical composition of magma. As listed in Table VO.1, most magmas are composed of varying proportions of 8 elements: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), magnesium (Mg), sodium (Na), and potassium (K), with O and Si being present in the greatest amounts. Lesser amounts of titanium (Ti), manganese (Mn), phosphorus (P), and hydrogen (H) are present. The amounts of the elements in rocks are reported as weight percentages of the oxides (e.g., SiO 2 rather than Si and O) because oxygen bonds with all other common elements. Table VO.2 compares the elemental composition of continental and oceanic crusts and shows that average continental crust is higher in SiO 2 than oceanic crust, a fact that you will find influences the locations of explosive volcanoes on Earth. Table VO.2 Crustal Elements in Weight-Percent Oxides CONTINENTAL CRUST SiO 2 60.2 Page 4 of 54 OCEANIC CRUST SiO 2 48.7 Al 2 O3 15.2 Al 2 O3 16.5 Fe 2 O3 2.5 Fe 2 O3 2.3 FeO CaO MgO Na 2 O 3.8 5.5 3.1 6.2 12.3 6.8 3.0 FeO CaO MgO Na 2 O K2O 2.9 K2O 0.4 2.6 When magma cools and crystallizes, these chemical components bond together to form minerals. A mineral is a naturally occurring element or compound that has an ordered internal structure, a characteristic chemical composition, crystal form, and physical properties. Because Si and O are the most common elements in Earth's crust and upper mantle, most of the minerals that crystallize out of magma are silicate minerals. Because the O-2 anion is very large, you can think of the crust of our planet as being a sea of O ions with all of the smaller cations in the interstices. Silicate minerals have interesting structures. The main building block of all silicate minerals is the (SiO4)-4 tetrahedron, a geometric form in which one Si with a +4 charge is bonded to four O's, each with a -2 charge. The Si-O bond is strong. Common silicate minerals in volcanic rocks include feldspar, quartz, micas, hornblende, pyroxene, and olivine. A graphical representation of the silicate radical is shown below. Figure VO.2 A silicon atom (purple ball) is linked to four oxygen atoms (red balls). The sizes of the crystals that form in a solidified magma are dependent on how long it takes the magma to cool. Magmas that cool slowly underground, in crustal magma chambers, are called intrusive rocks . The minerals crystallize in an interlocking texture of grains that are big enough to be seen with the naked eye. It might take hundreds to thousands of years to grow crystals that large. In contrast, magma that extrudes at Earth's surface cools quite quickly in terms of geologic time, commonly solidifying over days to weeks. These erupted magmas, called lavas, typically have very small sizes of crystals, most of which are too small to be seen with your eye. The resulting texture is called extrusive or fine-grained. 3. Volcanic products Page 5 of 54 a. Lava flows. When magma erupts at Earth's surface, it can form an array of eruptive products, depending on its chemical composition. Lava flows result when magma that is relatively low in gas content (less than a few percent) erupts effusively at Earth's surface. The surface of a lava flow may be ropy-textured, which is called pahoehoe (pronounced pah-hoy-hoy). This is a Hawaiian term that comes from the prevalence of these flows on Hawaii. A lava flow with a rubble-y flow top consisting of broken fragment of lava is called an aa (pronounced ah-ah) flow. This too is a Hawaiian term, meaning "a painful surface to walk on"! Figure VO.3 Lava flows from shield volcanoes in Hawaii: (left) pahoehoe and (right) aa. Figure VO.4 (left) Lava flow in a channel showing fluid but fairly slow flow. (right) Photo of a lava flow stopped between buildings in Heimaey, Iceland in 1974. These flows rarely kill people because they move very slowly. Click here to view an animation of pahoehoe flow. Click here to view an animation of aa flow. b. Volcanic glass. In some cases magma erupts and cools so quickly that crystallization does not occur. The magma solidifies into glass, a supercooled liquid. Glass is not crystalline, although volcanic glass may contain some microscopic crystals of common minerals. This glass is called obsidian if it is massive, or pumice if it contained many bubbles (called vesicles ). The bubbles form from gas that escaped from the magma during eruption. Page 6 of 54 c. Pyroclastic material. Pyroclastic (Greek for "fire piece", or hot fragment) material refers to fragments of all sizes that are erupted explosively out of a volcano. These range from the finest material, ash (< 2 mm diameter) through lapilli (2 - 64 mm) to larger blocks and bombs (> 64 mm). Many pieces of pumice are actually pumice lapilli. Page 7 of 54 Figure VO.5 (left column, top to bottom) Ash, lapilli, and blocks/bombs. (above) Very large bombs. In this course, we study pyroclastic materials in detail, as it is these explosive fragments that are the cause of most hazardous volcanic eruptions. Pyroclasts can erupt as ballistic fragments (bombs or blocks), flows (pyroclastic flows), or fall deposits (air fall deposits). d. Gaseous products. Gaseous compounds bubble out of magma during eruptions and form vesicles in the solid rock. H 2O (steam) is by far the most common volcanic gas, but CO2 (carbon dioxide), SO 2 (sulfur dioxide), H 2S (hydrogen sulfide), and HCl (hydrogen chloride) are other important volcanic gasses. H 2S gives volcanic gas its rotten egg smell. CO2, is a colorless, odorless gas, and is denser than air and thus may accumulate in low areas. When inhaled at concentrations much higher than usual atmospheric levels (0.0385% or 385 ppmv) , it can produce a sour taste in the mouth and a stinging sensation in the nose and throat. Figure VO.6 Acidic fumes emitting from Masaya Volcano in Nicaragua (left) and effect on coffee farms about 15 km downwind of the volcano (right). 4a. Factors influencing explosivity of magmas The gas content of magma contributes to the explosivity of an eruption. Gas content in a magma can vary from 1 - 10%. H 2O and CO2 are the primary gases present. As magmas rise towards Earth's surface, dissolved gases, which are compressed at depth, expand and try to escape the magma. Gas bubbles are formed, grow larger, and eventually explode. Magmas with higher gas contents have more explosive, and thus more hazardous, eruptions. Viscosity is defined as resistance to flow. This physical property is easy to visualize when you think of every day liquids with which you come in contact. When you tip over a glass of milk, it flows quite easily. But if you knock over a jar of honey, although it is still runny, it flows more slowly than the milk. This is because honey has higher viscosity than the milk. Peanut butter has even higher viscosity and will flow more slowly than either milk or honey. Figure VO.7 (left to right) Viscosity of materials: milk=low viscosity Page 8 of 54 (runny); honey=middle (less runny); peanut butter=high viscosity (sticky). The more easily magma flows, the more likely it is that gas bubbles in it will dissipate and the magma will erupt effusively rather than explosively. It is the combination of high gas content and high viscosity that leads to the most explosive volcanic eruptions. What factors influence viscosity? First, the chemical composition of the magma exerts a profound control on its viscosity. Magmas have varying amount of SiO 2: the more SiO 2, the higher the viscosity. Remember that the Si-O bond is quite strong, so the more Si and O that are in a magma, the more will it resist flow. Second, temperature has an effect on viscosity. As magma cools, more Si-O bonds form, and the magma is said to polymerize. This makes the magma stickier and thicker as it cools, or in other words, more viscous. Table VO.3 Table of Viscosity (Orders of Magnitude, m) MATERIAL TEMPERATURE, C m (log of viscosity) rhyolite magma 1400 4 rhyolite magma 1000 7 rhyolitic obsidian (solid) 700 13 basalt magma 1400 0 basalt magma 1200 1 average solid rock 25 21 water 25 -3 eruption column 1000 -4 to -5 Data from Carmichael et al., (1974) Igneous Petrology, McGraw-Hill, and Hyndman (1985) Petrology of Igneous and Metamorphic Rocks, McGraw-Hill. Table VO.3 above is organized to show the individual and combined effects of the 2 factors discussed above on explosivity. Note that the higher the m value (log of the viscosity), the more viscous the lava and more explosive the volcanic eruption. For magmas of different chemical composition at the same temperature, the magma with the higher SiO 2 content will be more viscous. For two magmas with the same SiO 2 content, the one at lower temperature will be more viscous. To summarize, viscosity is proportional to the weight percent of SiO 2 in a magma; and viscosity is proportional to 1 / T of a magma, where T = temperature Cool Volcano Video Click here to view an explosivity experiment - TRY IT at home! Page 9 of 54 4b. Explosivity, viscosity and chemical composition of magmas The relationship between chemical composition of magmas, viscosity, temperature, color, and rock names is summarized in Table VO.4 below. An examination of this table shows that mafic magmas (those with the lowest SiO 2 content) exist at high temperatures, have the lowest viscosity, have low gas contents, and tend to erupt effusively (non-explosively). They form dark colored mafic rocks called basalts . Felsic magmas form at lower temperatures, have high viscosity, and tend to erupt explosively. They form lighter-colored felsic rocks called rhyolites . Felsic magmas also tend to have higher gas contents than mafic magmas, and it is this combination of high viscosity and high gas content that leads to explosive eruptions. Table VO.4 Comparison of Three Types of Magma MAFIC (low silica, 45 - 52%) INTERMEDIATE (silica at 52 - 68%) FELSIC (high silica, > 68%) High temperature (1200 - 1400 C) Low temperature (600 - 1000 C) Low viscosity High viscosity Low gas content High gas content Dark color Light color BASALT ANDESITE DACITE NON-EXPLOSIVE (effusive) RHYOLITE EXPLOSIVE Data from Carmichael et al., (1974) Igneous Petrology, McGraw-Hill, and Hyndman (1985) Petrology of Igneous and Metamorphic Rocks, McGraw-Hill. CHECK YOUR UNDERSTANDING: High viscosity encourages high explosivity because ______. A. gas content decreases viscosity B. gas content increases viscosity C. bubbles have trouble growing in highly viscous magma D. highly viscous magma lets gas escape E. highly viscous magma lets bubbles grow and pressure goes up Page 10 of 54 5. Anatomy of an explosive eruption 1. High silica magma forms in the crust by melting of preexisting rock. Gas is dissolved in this magma due to the high pressure underground. 2. The magma rises towards Earth's surface because it is less dense than the surrounding solid rock. 3. As the magma rises, the dissolved gas expands and forms bubbles in the magma. This process is called vesiculation. 4. Because the magma is also cooling at it rises, its viscosity is increasing (Si-O bonds are forming). The gas bubbles are trapped in the viscous magma and gas pressure builds within the bubbles as they get closer to the surface. 5. When the percentage of the bubbles in the magma is about 75%, they are close enough that they touch. The walls between adjacent bubbles are so thin that the gas pressure overcomes the viscosity of the magma. 6. BOOM! An explosive eruption takes place, and fragments of magma are erupted as pyroclasts. Figure VO.8 Anatomy of an eruption. As magma rises to levels of lower pressure, gas comes out of solution, forming bubbles that overwhelm magma and create a gas jet leading to a buoyant plume. Figure 6.14 of Abbott and Samson, 2012. If the magma was mafic (low in silica content), the included gas bubbles would be able to escape from the rising magma and pop easily upon eruption, resulting in less explosive eruptions. 6. Volcanoes A volcano is a mass of material that forms at Earth's surface in response to one or more eruptions of magma. Most volcanoes form hills or mountains. Within a volcano is a crater, a steep-walled, bowl-shaped depression surrounding the volcanic vent from which new material erupts. In this class, we will learn about four main types of volcanoes: cinder cones, shield volcanoes, stratovolcanoes, and calderas. Page 11 of 54 Figure VO.9 (left) Mount Baker, Washington is an ice-clad volcano in the North Cascades. The volume of snow and ice on Mt. Baker is greater than that of all the other Cascades volcanoes (except Mt. Rainier) combined. (right) Aerial view across Mount Pinatubo crater and lake. Images by Ken McGee (left) and Willie Scott (right) of the U.S. Geological Survey. 6a. Mafic volcanic landforms: shield volcanoes and cinder cones Mafic lavas are very fluid (low viscosity) and can travel long distances from a vent. They may flow downhill at speeds of up to 30 km/hr, and travel tens to over one hundred kilometers from their vent. One common mafic volcanic landform is the shield volcano. It is tens of kilometers high and may be over 100 km across. Early observers thought that the shape of this type of volcano, which is much larger in diameter than it is high, reminded them of the shape of a Viking's shield, hence the name. Repeated eruption of basaltic lava flows forms most of a shield volcano, although small eruptive centers called cinder cones (see below) are present on the flanks. The large volcanoes in Hawaii such as Mauna Loa, Mauna Kea, and Kilauea, are all examples of shield volcanoes. Mauna Loa is the world's largest active volcano; it rises 28,000 feet above the sea floor. Figure VO.10 A view of Mauna Kea from Mauna Loa, on the Island of Hawaii. Page 12 of 54 Figure VO.11 A schematic diagram of a shield volcano. Note the layers of basaltic lava flow (tan and gray layers, which could be either pahoehoe or aa). A cinder cone is a conical hill formed from the accumulation of pyroclastic material (cinders are 4 - 32 mm in size) around a volcanic vent. The cones are commonly 10's to 100's of meters high and 100's of meters across. One or more lava flows may issue from the vent after the cone forms. In general, cinder cones are composed of mafic magma, although andesitic cinder cones are not unknown. Figure VO.12 (left) Eve Cone, a young, well-preserved cinder cone at Mount Edziza, BC. (right) A schematic diagram of a cinder cone. Photo by C.J. Hickson, Geological Survey of Canada. 6b. Intermediate to felsic volcanic landforms: stratovolcanoes When intermediate to felsic magma erupts explosively, it can produce ash columns that penetrate to higher than 40 km into the atmosphere. Ash is the finest grained pyroclastic material, and if it gets into the stratosphere, it can travel around the world in upper level winds. When the winds die, ash falls out of the atmosphere due to gravity and it forms layers that blanket the Page 13 of 54 landscape. This type of eruption occurred at Mt. St. Helens, Washington State, on May 18, 1980; it was only one component of the most recent volcanic eruption to affect western North America outside of Alaska. Air-fall ash eruptions typically take place at composite volcanoes and calderas. Figure VO.13 Shishaldin Volcano, a symmetrical stratovolcano rising 2,857 m (9,372 ft) above sea level, Aleutian Islands, Alaska. Photograph of the volcano as seen from aboard the Clipper Odyssey. (Photo by Devon Ducharme). A composite volcano, also known as a stratovolcano, is composed of alternating layers of lava flows and pyroclastic layers, along with volcanic domes (see below) and tabular intrusions known as dykes and sills . These volcanoes are tall, conical, and have steep slopes. They can be several kilometers high and over 10 km across. Composite volcanoes get their name because they erupt magmas of a full range of silica contents. The mafic and some intermediate eruptions tend to form flows, whereas magmas with higher silica contents commonly erupt explosively as pyroclastic material, either in the form of bombs, flows, or falls. Figure VO.14 A schematic diagram of a stratovolcano. Note the interbedded lava flows, pyroclastic flows, and lahars. Another type of intermediate to felsic landform is a lava dome . A dome is a pile of viscous lava that forms over a vent. It commonly has a mushroom-like shape because the lava is viscous and does not travel far from the vent. Domes may stand alone, or be erupted in the center or on the flank of an existing volcano. Page 14 of 54 Figure VO.15 Novarupta lava dome formed as hardened magma plugged the central magma pipe of the 1912 eruption of Katmai Volcano in southern Alaska. The dome is 800 ft across and 200 ft high. Aerial view of the dome, U.S. Geological Survey photograph by Gene Iwatsubo, July 29, 1987. The dome in the center of the crater at Mt. St. Helens initially grew between 1980 and 1986, and then underwent a period of quiescence. Another dome began erupting next to the 1980 - 1986 dome in late 2004. These domes are dacitic (intermediate, but more SiO 2-rich than andesite) in composition. Read the latest information on dome growth at the U.S. Geological Survey's web site on current activity at Mt. St. Helens. 6c. Felsic volcanic landform: calderas One type of volcano that is dominantly felsic is known as a caldera . Calderas are very large depressions in Earth's crust, caused by voluminous eruption of explosive, felsic pyroclastic material. They are typically 10's to over 100 km across and kilometers deep. Calderas form because so much magma is erupted that the existing crust founders into the void left by the evacuated magma chamber. Do not confuse a crater with a caldera! A crater is a depression on top of a volcanic vent, and it typically is only 10's to 100's of meters across, although craters on some of the Hawaiian shield volcanoes are kilometers across. A crater normally does not form because a block of crust is downdropped. Rather, it forms by default as piles of erupted material surround a volcanic vent. The sequence of events that forms a caldera is as follows: 1. Formation and accumulation of a very large body of felsic magma in a near-surface magma chamber. This magma is buoyant and it domes the overlying crust, creating both radial and ring fractures. 2. Gas pressure in the felsic magma chamber increases to the point that explosive fragmentation occurs. Pyroclastic material erupts through fractures in the overlying crust in the form of pyroclastic flows, and produces a deposit known as an ash flow tuff sheet. This tuff sheet blankets the countryside for many kilometers out from the caldera, and some of the tuff sheet is deposited in the center of the caldera. 3. So much magma erupts that the roof of the caldera collapses back into the void left by the evacuated magma, producing a very large closed depression in the surface. Calderas are the most explosive volcanoes, expending tremendous amount of energy on the landscape. Luckily for us, they erupt infrequently in terms of our lifetimes. There are several large fairly young calderas (in terms of geologic time!) in western North America. Crater Lake, Oregon was formed 6700 years ago by a caldera-forming eruption that destroyed a composite cone known as Mt. Mazama. Other large nearby calderas include Yellowstone, Wyoming, and Long Valley, California, both of which formed calderas about 1 million years ago. Geophysical imaging reveals that smaller magma chambers exist under each of these calderas today, so another eruption, although probably not a caldera-forming eruption, is certainly possible. Page 15 of 54 Figure VO.16 (top) How Crater Lake formed. (a) Mount Mazama volcano stood high just prior to its eruption. (b) In 5677 BCE, a gaseous eruption emptied a huge volume of viscous magma. (c) The gigantic eruption left a void inside the weakened mountain, and the unsupported collapsed into the emptied magma chamber. (d) The waters of Crater Lake now fill the caldera, and a small new volcanic cone (Wizard Island) has built above lake level. (bottom left) Schematic map of the Yellowstone hotspot area. The North American plate is moving southwest, thus the hot-spot magma plume erupts progressively farther northeast with time. Three giant calderas have erupted in the last 2 million years - at 2, 1.3, and 0.6 million years ago. Cross-hatched area was covered by hot, killing pyroclastic flows during the eruption of 600,000 years ago. (bottom right) Map showing the Long Valley caldera formed by massive eruption. Bishop Tuff is uneroded remains of pyroclastic debris from the last major eruption. 6d. Volcanic landform: Summary Page 16 of 54 Figure VO.17 illustrates the relative sizes of several volcanic landforms. Figure VO.17 A shield volcano such as Mauna Loa has a great width compared to its height (~100 km wide and 4-10 km high). A stratovolcano such as Mount Rainier has the same magnitude height and width (~10 km wide and 5 km high). A cinder cone such as Sunset Crater is about 4 as wide as it is high (<2 km wide and <500 m high). A cinder cone is about 1 order of magnitude smaller than a stratovolcano, which is about 1 order of magnitude smaller than a shield volcano. Click here to view an animation of the Types of Volcanoes. QUESTION FOR FURTHER THOUGHT : If you had a choice, which type of volcano would you AVOID living next-door to? Why? A. stratovolcano B. caldera C. shield volcano D. cinder cone E. dome Page 17 of 54 7. Summary Table VO.5 below summarizes the relationship between viscosity, volatiles, and volume and how these factors control the type of volcanic landform. Magma physical properties such as viscosity are dependent on chemical composition. Mafic magmas with low silica have low viscosity; they tend to erupt quietly as lava flows and form landforms such as shield volcanoes and cinder cones. Felsic magmas have high viscosity. Because of their high viscosity and high gas content, most felsic magmas erupt explosively as pyroclastic material. This can be in the form of ballistic fragments, pyroclastic flows, or ash falls. Felsic magmas are found in landforms such as composite volcanoes, domes, and calderas. Table VO.5 Volcanism Control by the Three Vs: Viscosity, Volatiles, Volume VISCOSITY + VOLATILES + VOLUME = VOLCANIC LANDFORM Low Shield Volcanoes Low Very Large Flood Basalts Low/Medium Medium/High Small Scoria Cones Medium/High Medium/High Large Stratovolcanoes High Low Small Lava Domes High B Large Low UNIT Low High Very Large Calderas TOPIC What Threats Do Volcanoes Pose? Outline Why are fatalities associated with eruptions increasing with time? Examine the graph below. Figure VO.18 Cumulative fatalities from volcanoes during the 500 years, 1500 to 2000. The seven eruptions that dominate the record, all claiming 10,000 or more victims, are named. These account for two-thirds of the total and heavily influence studies based on number of fatalities alone. Plot from Simkin, et al., 2001 ("Volcano Fatalities: Lessons from the Historical Record", Science 291:255). Note that the rate at which humans have been killed by volcanic eruptions has greatly increased since the year 1800. The main reason is that as world population increases, more people are living closer to active volcanoes . Page 18 of 54 In this unit you will examine the volcanic hazards associated with volcanoes and eruptions according to the outline below. Case studies of important eruptions are used to study these hazards in more detail. 1. Eruptive Volcanic Hazards Lava flows Pyroclastic material Pyroclastic flows Lahar Toxic gasses Volcanic landslide 2. Volcanic Explosivity Index 3. Summary of Volcanic Hazards 1. Eruptive volcanic hazards Volcanoes can have multiple types of eruptions and thus the hazards due to these will vary with time. Not all volcanic hazards are directly associated with explosive eruptions; even mafic lava flows are dangerous if one is not careful. Neither are all volcanic hazards directly associated with eruptions; lahars may occur without warning during times of volcanic quiescence. 1a. Lava flows Low viscosity mafic lava flows can flow and spread laterally out of a stream canyon onto a valley floor. They erupt at 1200 1400 C. However, they are NOT explosive. Most people are able to run out of the way or avoid a particular valley through which a flow is progressing. Thus, the main hazard associated with mafic flows is infrastructure damage. Road and buildings are easily overrun and incinerated. Page 19 of 54 Figure VO.19 On shield volcanoes, lava flows from fissures like the one shown on top left; cooling as it runs down slope (bottom left). Lava flows from shield volcanoes in Hawaii erupt for long periods of time and cover very large areas, as shown on the map of the Island of Hawaii above. This island is composed of 5 shield volcanoes, listed from youngest to oldest, Kilauea, Mauna Loa, Mauna Kea, Hualalai and Kohala. Sometimes, basaltic lava can contain lots of gas which create small explosive eruptions called fire fountains. As partially liquid drops fall back to the ground, they may coalesce to re-form a lava flow. Figure VO.20 Basaltic lava flows move fluidly though fairly slowly (left). An erupting fire fountain at Puu Oo, one of the vents on Kilauea volcano, Hawaii (right). Page 20 of 54 Figure VO.21 These photos emphasize the low level of threat due to lava flows. Much of the damage is on infrastructure. Cool Volcano Videos Click here to view a YouTube video of pahoehoe at Hawaii Volcanoes National Park. Click here to view a'a at Hawaii Volcanoes National Park. QUESTION FOR FURTHER THOUGHT : How can we reduce the risk of damage by lavas? A. build barriers and/or dams before an eruption B. prevent development in valleys C. divert or attempt to slow active lava flows D. evacuate residents or move towns and villages away from volcanoes E. prevent development by making the area around the volcano a National Park 1b. Pyroclastic material Most pyroclastic material (tephra) form from high-silica magma, although mafic and intermediate-composition pyroclasts are not unheard of. Mafic pyroclasts are called scoria . i. Ash particles are the smallest pyroclasts (< 2 mm diameter). These form from fragmentation of a frothy magma, when the pressure within the gas bubbles exceeds the strength of the viscous magma. Page 21 of 54 Figure VO.22 Ash pyroclasts form due to bubble formation and magma fragmentation. Schematic diagram on the left shows the sequence of formation of ash. Figure above is an Scanning Electron Microscope (SEM) image of ash from the Yellowstone caldera (eruption 2.1 Ma). See also Figure 7.26 of Abbott and Samson, 2012. A description of the process shown in the diagram above follows: 1. Gas bubbles form in a high-silica magma but do not pop because of the high viscosity. Therefore, the pressure in each bubble increases as the magma makes it way towards the surface. 2. The bubbles expand until they are almost touching. At this point the magma is frothy, as only thin walls of silicate melt are present in between adjacent bubbles. 3. BOOM! The gas pressure in the bubbles exceeds the strength of the thin walls of silicate melt, causing explosive fragmentation, and triangular glass shards called ash rise into the atmosphere in an eruption column. Some larger fragments (see definitions below) are left from incomplete fragmentation of the gas-rich magma. During explosive volcanic eruptions, ash falls downwind of the volcano. In the case of very large eruptions, ash columns (Figure VO.23 and VO.24 below) may rise to altitudes of > 40 km, where the fine ash particles are picked up by global winds and circulated around the world. This allows for ash to be deposited over a vast area (also see Figure 7.27 of Abbott and Samson, 2012 ). Ash injected into the stratosphere can contribute to a lower amount of solar radiation reaching Earth's surface, and therefore to global cooling. Figure VO.23 (above) The eruption column from Mount Pinatubo taken from the east side of Clark Air Page 22 of 54 Base, June 12, 1991. (right, A) The spreading of Pinatubo eruption cloud as derived from Japanese GMS satellite images at the given times. (right, B) The transition from ash-laden eruption cloud to SO 2 dominated stratospheric cloud mapped by TOMS satellite. Photo taken by Dave Harlow of the U.S. Geological Survey. (right) Data and plots from NASA Goddard Space Flight Center. Figure VO.24 Space Shuttle photograph of the Earth over South America taken on August 8, 1991, showing double layer of Pinatubo aerosol cloud (dark streaks) way above high cumulonimbus tops. Ash can choke vehicle and airplane engines (see Figure 7.28 of Abbott and Samson, 2012 ), aggravate respiratory ailments, bury homes and other structures and/or cause their collapse, and cover valuable agricultural land, thereby decreasing the food supply and leading to famine. When Mount St. Helens had its catastrophic eruption on May 18, 1980, approximately 1300 vertical feet was removed from the top of the mountain and about 2.92 km3 of material was removed. See Figure 7.11 of Abbott and Samson, 2012 to view the entire chronology of the events of that day. Page 23 of 54 Figure VO.25 (above) Aerial view showing buildings and vegetation damaged by ash fall as of 3 days after the June 12, 1991 Mount Pinatubo eruption. Photo by Willie Scott of the U.S. Geological Survey. (right) Figure 7.24 of Abbott and Samson, 2012 showing a map of significant distribution of ash in Western Canada. ii. Lapilli are pyroclasts in the 2 - 64 mm range. These are associated with large eruption columns and are mostly fragments of vesicular felsic rock called pumice . Lapilli-sized pumice fragments fall out of an eruption column and blanket surrounding areas, forming a pumice fall deposit. iii. Bombs and blocks (> 64 mm in diameter) are the largest pyroclasts. These are explosively ejected during an eruption as ballistic projectiles, most of which land near the volcanic vent. Pyroclastic flows, which consist of hot gas and pyroclastic fragments, will be discussed in more detail in the next section. QUESTION FOR FURTHER THOUGHT : Which of the following are HAZARDS to people from ash fall? (choose all that apply) A. Breathing in ash Page 24 of 54 B. Blanket of ash (like snow) on everything C. Electrical fires D. Total darkness (even during daylight) E. Extra weight on roofs F. Aircraft engines sucking in ash G. Buildings destroyed by a fast-flowing or fast-falling ash 1c. Pyroclastic flows Pyroclastic flows are turbulent mixtures of hot gas and pyroclastic fragments that are emplaced laterally from a volcanic vent. They consist of ash, lapilli, crystals, and other rock fragments called lithic fragments . These flows are density currents that move at speeds of 300 - 400 km/hr. The largest pyroclastic flows can travel 10's of kilometers from their source. The temperature of a pyroclastic flow is generally in the 500 - 700 C range. Pyroclastic flows tend to be confined to low areas in the topography such as stream valleys radiating from a volcanic cone. The town of St. Pierre, Martinique, was destroyed by pyroclastic flows from Mount Pele in 1902. The map on the left shows how ridges radiating from the volcano channeled the pyroclastic flow towards the town. Pyroclastic flows originate in several ways. The most common way is by collapse of an eruption column. Another method is by sudden collapse of part of an erupting volcanic dome. This was the case at Montserrat in the early 1990s. Thirdly, is when a small pyroclastic eruption builds up and overspills the crater rim resulting in a high-speed flow of pyroclastic materials. Lastly, failure of one side of a volcanic edifice may lead to a lateral blast of pyroclastic material which then flows down slope. Page 25 of 54 Figure VO.26 (left) Pyroclastic flows descend the southeastern flank during the 1984 eruption of Mayon Page 26 of 54 Volcano, Philippines. Maximum height of the eruption column was 15 km above sea level, and volcanic ash fell within about 50 km toward the west. A U.S. Geological Survey photograph. (right) Pyroclastic flows after the eruption of Mount Unzen, Japan in June 1993. Part of the dome collapsed, resulting in a pyroclastic flow which had a very dense core of materials. Photo only shows the billows of rising ash. Photo by S. Nakada. Pyroclastic flows are the most lethal of the eruptive products. These flow mostly along channels, ridges, and valleys, but can also jump the banks as it travels at high speed over tens of kilometres. Effects from pyroclastic flows are mostly limited to the immediate vicinity of the source volcano, but all life and structures in its path are destroyed. CHECK YOUR UNDERSTANDING: Which statement about pyroclastic flows is FALSE? A. Pyroclastic flows can travel up to 200 km/h. B. Pyroclastic flows can be as hot as 1500 C. C. Pyroclastic flows can form from a collapsing lava dome. D. Pyroclastic flows can flow uphill. E. Pyroclastic flows contain pumice, ash, chunks of lava dome, and high temperature gas. 1d. Lahar There are additional hazards associated with volcanoes that do not necessarily occur when a volcano is actively erupting. Lahars (volcanic mud or debris flows) are extremely destructive mixtures of water, ash, rock fragments of all sizes, and debris that travel swiftly down slope from a volcano, at speeds up to 50 km/hr. They follow stream valleys. Some lahars are hot when they are emplaced because they are associated with pyroclastic eruptions, but other lahars may occur long after volcanic activity has ceased. If you search the web, you will find many photographs of destruction caused by lahars. Roads, bridges, houses, livestock, and people may all be caught up in their destructive path. They do a lot of damage because they travel long distances away from a volcano so their presence may not be associated with it, and they can occur in between times of volcanic activity. Page 27 of 54 Figure VO.27 (left) Lahars from Vulcanian-type eruption of Mount Pinatubo destroyed bridges and flooded communities in the Philippines. A U.S. Geological Survey photograph. (above) Mount St. Helens and remains of the lahar (the dark deposit on the snow) flowing from the crater into the river valley below. Photo by T. Casadevall of the U.S. Geological Survey. Some volcanic eruptions can cause their own micro-weather system which result can in lahars. Water, a common gaseous product of eruption, can be blown high into the atmosphere as steam, eventually to fall as heavy rain. Rain falling on thick accumulations of pyroclastic material on steep slopes set off lahars. This sequence of events buried the city of Herculaneum days after the eruption of Mount Vesuvius. As mentioned earlier, formation of a lahar requires the following: pyroclastic material and water. Lots of water! The water may come from melted snow and ice high on the flanks of a volcano. One of the Cascades volcanoes, Mount Rainier in Washington State, has a history of producing large lahars. Moreover, because of its great height, extensive glacial cap, frequent earthquakes, and active hot-water spring systems, it is inherently unstable, thus a very dangerous volcano for the heavily populated areas surrounding it. The mountain itself could fail in a massive avalanche and/or rapidly melted ice can cause floods and lahars. Page 28 of 54 Figure VO.28 Lahar Hazard Potential. A map of Mount Rainier and vicinity showing areas at risk of lahars, lava flows, and pyroclastic flows. The Seattle and Tacoma suburbs of Puyallup, Sumner, Orting, and Auburn are at risk. 1e. Toxic gasses Toxic gases may be released by a volcano during or between active eruptions. Many types of gas are emitted from volcanoes, but water (H2O) and carbon dioxide (CO2) are the most common components. Other gases include sulfur dioxide (SO2), hydrogen sulfide (H2S), and hydrogen chloride (HCl). About 1 - 10% of a magma consists of gas. There are many hazards associated with volcanic gases. SO 2 may combine with H 2O to form dilute sulfuric acid (H2SO 4). This in turn forms "acid rain" which can destroy crops, leach heavy metals from steel structures, nails, and paint, and contaminate cisterns full of drinking water. CO2 is dangerous because it is denser than air and can accumulate in low areas. Lake Nyos in Cameroon, western Africa, is a crater lake in the middle of a dormant volcano. In 1986, the sudden release of 109 m3 of CO2 that was previously trapped in sediments on the floor of the lake asphyxiated 1,700 people and all livestock in the area, with no warning. (This event is also described in Abbott and Samson, 2012 pages 174-176.) Page 29 of 54 Figure VO.29 (above) Schematic cross section of Lake Nyos and how a "river" of carbon dioxide erupted out of the lake and flowed down slope and into the river valley. Solid lines show the flow of gas, dashed lines represent the flow of water. From Y. Zhang, 1996, Nature , vol 379, pp 57-59. (right) A photo of Lake Nyos in 1986. 1f. Volcanic landslide A volcanic landslide (also known as a sector collapse ) is a down slope failure of a portion of a volcanic edifice. Eruptive products that compose the volcano are altered to clay minerals by hydrothermal fluids. Clays have low shear strength. The combination of water percolating into the volcano (either from rain, melted snow, or hot springs) combined with clay can lead to failure under the influence of gravity with no warning or precursor activity. Volcanic landslides may turn into mudflows or debris flows down slope. You should recognize from the description of the materials involved in a sector collapse, that the landslide that ensues can be classified as a debris avalanche. Volcanic landslides can be triggered by volcanic activity, earthquakes, rapid melting of snow and ice, or heavy rainfall events. On May 18, 1980, at 8:32 am Pacific Daylight Time, a magnitude 5.1 earthquake triggered a landslide off of the north flank of Mount St. Helens. The landslide in turn depressurized a near-surface magma body and precipitated a catastrophic eruption that included a lateral blast, a vertical ash column, and pyroclastic flows. Figure VO.30 The May 18, 1980 eruption of Mount St. Helens left it with an elevation of only 2,550 meters and a 1.5 kilometer-wide horseshoe-shaped crater. The view shown on the left is from the northwest. Pits in the foreground are from phreatic explosions (violent expulsion of steam and pulverized rocks) within the debris avalanche. Photo by T. Casadevall of the U.S. Geological Survey. The photo on the right shows the debris avalanche deposits on the entire valley down slope from Mount St. Helens. Photo by J.J. Clague. Page 30 of 54 Cool Volcano Animation and Videos Click here to view an animation of the eruption of Mount St. Helens from Abbott and Samson, 2012 . Click here to view a YouTube video of the Mount St. Helens eruption of 1980. Click here to view a YouTube video of a more recent eruption (2006) of Mount St. Helens. 2. Volcanic Explosivity Index What makes a volcano explosive? For each of the following factors, note what variation will indicate a more explosive eruption. Viscosity? Temperature? Gas content? Silica content? Volcanic landforms? Volcanologists C.G. Newhall and S. Self created a Volcanic Explosivity Index (VEI) to provide a relative measure of the explosiveness of volcanic eruptions and to classify eruptions as to their potential for disaster. The classification considers: volume of eruption products; eruption plume or cloud height; duration of eruption; and qualitative observations (with terms that range from non-explosive, gentle, cataclysmic, to mega-colossal) Because the index is based on a relative comparison of known eruptions, the scale is open-ended, with the largest volcanoes in history given an index of 8. A "0" represents a non-explosive eruption (for example, Hawaiian shield volcanoes) while an "8" represents a mega-colossal explosive eruption. The VEI is a logarithmic scale, mainly based on the volume of ejecta, the height of the eruption column, duration of eruption, and the eruption style. The last category is a general classification for types of volcanoes, as described in Figure 6.15 of Abbott and Samson, 2012 (shown below). This is not to be confused with the volcano types we learned in Unit A. Page 31 of 54 Page 32 of 54 The eruption of Lake Toba in 73,000 BP was a VEI 8; a similar eruption has not been experienced since. Read more about Lake Toba here. A relatively recent very large eruption (VEI 6) was the 1883 eruption of Krakatau. View the video below and read more about this eruption from the USGS website here. On this scale, the eruption of Mount St. Helens in 1980 was a VEI 5 and the June 1991 eruption of Mount Pinatubo was a VEI 6. The world has not seen an eruption with a VEI 7 since the catastrophic eruption in 1815 of Tambora caldera, in Sumatra. Click here to view the eruption of Krakatau. QUESTION FOR FURTHER THOUGHT : Why do you think scientists use eruption plume height as a basis for VEI? A. It is a good estimate of the volume of the eruption. B. It is a good estimate of the power of the eruption. C. It is a good estimate of the heat involved in the eruption. D. It is a good estimate of the length of the eruption. E. It is a good estimate of the area affected by the eruption. 3. Summary of volcanic hazards Volcanoes may have multiple types of eruptions and thus the hazards at a volcano will vary with time. Not all volcanic hazards are directly associated with explosive eruptions; even mafic lava flows are dangerous if observers are not careful. Neither are all volcanic hazards directly associated with eruptions; lahars may occur without warning during times of volcanic quiescence. The figures below highlight some of the volcanic hazards from the Cascades volcanoes for residents of B.C. and the rest of the Pacific Northwest. Be aware of these hazards, know the extent and limitations of effects from these, and be prepared with an avoidance and mitigation plan. Click here to read the B.C. Provincial Emergency Program's volcano preparedness publications. Page 33 of 54 Page 34 of 54 Figure VO.31 Mount Baker Lahar. A large volcanic debris flow (lahar) moving down the Nooksack River from Mount Baker might spill into British Columbia at Sumas. CHECK YOUR UNDERSTANDING: Given a simplified Hazard Map for a stratovolcano (as shown below) and the key to the various hazard zones: A - ash fall B - pyroclastic flow C - lahar D - lava dome E - lava flows Can you explain WHY: 1. Zone A covers the largest area and follows the wind direction? 2. Zone B only covers the periphery of the volcano within less than 20 km from the summit? 3. Zone C follows river/stream valleys over extended distances downslope from the volcano? 4. Zone D, like Zone B, covers the periphery of the volcano's flanks but unlike Zone B, covers only areas within short distances from the summit? 5. Zone E, like Zone C, flows along river/stream valleys but unlike Zone C, covers only short distances from the summit? Page 35 of 54 For more information on current volcanic activity around the world, visit the Smithsonian's Weekly Volcanic Activity Report UNIT C TOPIC How Do We Prevent Destruction from Eruptions? Outline In this unit we will examine the common methods used to monitor volcanoes and how they help us to predict eruptions. 1. Facets of eruption prediction 2. What do we measure? 3. Monitoring techniques 4. Predicting eruptions: Communicating with the public 5. Why it is difficult to predict eruptions Our goals in monitoring volcanoes and trying to predict volcanic eruptions are to reduce losses of life, infrastructure, and disruption to society. In this unit we will learn what types of data are collected from volcanoes and how they are collected, as well as the usefulness of these data for predicting eruptions and mitigating (reducing) volcanic hazards. 1. Facets of eruption prediction a. Collect existing (baseline) geologic data. The first step in any volcanic hazards assessment is to collect all existing data about a particular volcano. What types of products have erupted from the volcano in the past? What is their distribution, and how much area do they cover? What are the percentages of magmas of different silica contents that Page 36 of 54 have erupted in the past? Analyses of the chemical components of the eruptive products, such as the weight percentage of SiO 2, are used to answer this question. And lastly, what is the frequency at which the volcano has erupted in the past? Radiometric dating techniques such as the potassium-argon (K - Ar) and argon-argon (Ar - Ar) methods are used to date young eruptive material. These existing data are compiled into a series of maps and a database. b. Develop a monitoring scheme. Ideally, the synthesis of baseline data and real-time monitoring of an active or potentially active volcano should begin long before an eruption starts. But sampling, mapping, and installing specialized monitoring equipment all take time, money, and political will. In reality, many monitoring schemes are not started until a volcano shows obvious signs of an imminent eruption, or until after an unexpected eruption occurs. This is especially true in the economically under-developed countries where funds are scarce. c. Create geologic and volcanic hazards maps. Figure VO.32 Volcano hazard zonation map showing which areas in the vicinity of Mount Hood, Oregon will be at risk when hazardous geologic events occur in the future. For Zone A, the 30-year probability of an eruption affecting this area is estimated to be 1 in 15 to 1 in 30. For Zone B, the 30-year probability of an eruption affecting this area is estimated to be about 1 in 300. Map courtesy of USGS. Spatial data for a volcano, such as the distribution of eruptive units, are plotted on a geologic map. The map is then used to identify hazards, based on past history, and to produce a volcanic hazards map showing zones of likely eruptive activity, and specific hazards. 2. What do we observe and/or measure? Page 37 of 54 a. Ground deformation - changes in the shape of the volcano b. Visible signs of volcanic activity - new eruptive material c. Increased seismic activity d. Increased gas emissions e. Increased heat flow 3. Volcano monitoring strategy Scientists monitor volcanic activity to detect and measure changes in its state caused by magma movement beneath the volcano. Rising magma typically will: trigger swarms of earthquakes and other types of seismic events; cause swelling or subsidence of a volcano's summit or flanks; and lead to the release of volcanic gasses from the ground and vents Thus, by monitoring these phenomena, scientists are sometimes able to anticipate an eruption days to weeks ahead of time and to detect remotely the occurrence of certain volcanic events like explosive eruptions and lahars. 3a. Volcano monitoring techniques: Increased seismic activity i. Seismic monitoring is probably the most useful technique for predicting volcanic activity. Increased seismic activity is associated with eruptions or precursor activity before eruptions. The location, frequency, and types of earthquakes are all important. As buoyant magma rises through the crust, it fractures the overlying rock to make room for itself. This causes concentrations of earthquakes in space and time. Pre-eruption 'quakes are typically swarms consisting of dozens to hundreds of events, M < 5, and increasing in frequency or intensity as an eruption is imminent. These quakes occur closer to the eruption location and become shallower as eruption nears. Volcanic eruption 'quakes are typically sustained with M = 2-6 and occur close to the eruption location. Figure VO.33 A comparison of seismograms produced by eruption events, earthquakes, and landslides. Image downloaded from USGS Volcano Hazards Program. Page 38 of 54 The onset of activity at Mount St. Helens in March 1980 was initially observed by seismologists who detected both an increased frequency of earthquakes located directly beneath the volcano, as well as a trend to shallower earthquake foci with time. Mount St. Helens was coming to life after over 100 years of quiescence! Earthquake S waves are not transmitted through liquids, so areas where there are gaps (an absence of earthquakes) in seismic profiles beneath volcanoes may be interpreted as bodies of magma. For example, recent seismic data from Yellowstone caldera, Wyoming, image the top of a magma chamber some 90 km across at a minimum depth of 5 km (Figure VO.34 below). Earthquakes at Yellowstone are plentiful at depths < 5 km, indicating brittle fracturing of the overlying rocks as the magma tries to expand towards the surface. Figure VO.34 Lateral variations in focal depths of earthquakes of the Yellowstone caldera are thought to reflect variations in the depth to the brittle-ductile transition. Image downloaded from R. B. Smith, University of Utah. ii. Acoustic flow monitors (AFM) are a specialized type of seismometer that are optimized to detect and record high frequency (10 - 250 Hz) vibrations caused by lahars, rather than lower frequency vibrations in the 0.5 - 10 Hz range that are typical of earthquakes. These units are generally solar-powered and are buried in the ground adjacent to stream Page 39 of 54 valleys radiating out from a volcano. When an AFM senses vibrations typical of a lahar, it sends a signal from an antenna to a receiving station where an alarm can be sounded to alert people downstream to get to higher ground. Figure VO.35 Photo (left) of an AFM station in Hoala, near Mount St. Helens. On the right is a comparison of the response to ground vibration of typical seismometer and acoustic-flow monitor. Photo and image courtesy of USGS. 3b. Volcano monitoring techniques: Ground deformation A volcano changes shape when magma and/or gas moves into a reservoir beneath or within the volcanic edifice. For example, between March and May, 1980, Mount St. Helens, Washington, developed a pronounced bulge on its north flank. A cryptodome of dacitic magma was emplaced close to the surface, under pressure from volcanic gas trapped inside the existing stratovolcano. This bulge was destroyed during the May 18, 1980 eruption. To learn more about Mount St. Helens, read the extensive documentation by the U.S. Geological Survey on its Mount St. Helens web site. There are several ways to measure ground deformation; these vary in cost and ease of use. i. Simple, inexpensive, yet effective direct measurements of horizontal displacement can be made by physically taping the distance between two metal stakes, or by measuring an increase in the width of cracks growing on a volcano, and keeping track of the change in distance over time. The trade-off to these simple and inexpensive techniques is that their use can be hazardous to field personnel, as they involve being physically present within or on an active volcano. At Mount St. Helens, these techniques were used as often as weather permitted over the period from mid-1980 to 1986, and they provided excellent information that was used to predict upcoming eruptions. Geologists at Mount St. Helens measured the increase in distance between stakes and other markers within the crater, and plotted the increase versus time. In this way they were able to correlate a rapid increase in the rate of movement between markers with an increased possibility of an eruption. In many instances, these simple measurements accurately predicted dome-growth eruptions within 24 hours. ii. Tiltmeters are sensitive instruments that record changes in the tilt of a surface. These instruments are placed around the flanks of a volcano and the change in tilt is measured over time. They have been used successfully to predict eruptions at Hawaiian shield volcanoes such as Kilauea. Prior to an eruption, tiltmeters on the broad flanks of Kilauea record inflation of the volcano. Individual tiltmeters record tilting outward from the central crater as the volcano inflates with magma in its interior. After an eruption, the tiltmeters record decreasing tilt as the volcano deflates. Page 40 of 54 Figure VO.35b A schematic diagram of the type of volcanic tiltmeters used currently used at the Hawaiian Volcano Observatory. Drawing courtesy of USGS. iii. The Global Positioning System (GPS) can be used to pinpoint horizontal and vertical movement of targets on a volcano through repeated measurements over time. This technique may involve placing automated GPS receivers at known locations around a volcano, or geoscientists may make the measurements directly at each site using portable GPS receivers. As is typically the case with monitoring volcanoes, having fixed receivers is safer - less personnel are put in harm's way. However, expensive equipment can be destroyed by on-going eruptions. Geoscientists must often judge whether to place equipment in dangerous areas, or to visit these areas in times of relative volcanic quiescence to manually take measurements. iv. A safer but more expensive way of making ground deformation measurements is to use InSAR (satellite radar interferometry) . This technique involves having satellites using radar waves to measure the elevation of large areas of the crust in the vicinity of active volcanoes over an interval of time. These remotely sensed images can be superimposed over each other using digital techniques and areas of uplift can be identified by the presence of deformation fringes, which show up as colored irregularly shaped rings on an InSAR image. In 2001, geoscientists found that InSAR imagery from the central Oregon Cascade Range indicated that there was a roughly circular area about 20 km in diameter, coincident with the Three Sisters volcanoes, which had experienced uplift of up to 10 cm over the preceding 5 years. Computer modeling suggests that this ground deformation is consistent with a magma body at about 7 km depth. Should these predictions be correct, don't be surprised if there is a volcanic eruption in central Oregon in our lifetimes. 3c. Volcano monitoring techniques: Increased gas emissions Increased gas emissions may reflect a change in magma supply rate, a change in the type of magma that is erupting, or a change in the underground fracture system that is routing gas towards the surface. Volcanoes typically record 2 - 3 orders of magnitude increase in SO 2 emissions during or prior to an eruption. The Table below shows the significant change in the rate of gas emission as a volcano progresses from a fuming to an eruptive state. Table VO.6 Typical Sulfur Dioxide Emission, Mg per day (=tonnes/day) Volcano Mount St. Augustine (Alaska) Mount Etna (Sicily) Page 41 of 54 Kilauea (Hawaii) Fuego (Guatemala) fuming 20 - 400 1000 - 5000 150 - 250 160* erupting 24,000 up to 25,000 > 10,000 200 to 400** Data from McGuire, B., Kilburn, C., and Murray, J. eds., 1995 (Monitoring Active Volcanoes: Strategies, Procedures, and Techniques, University College London Press, 421 pp.); *Andres, R.J. et al., 1993 (Bulletin of Volcanology , 55:379-388); **Rodrigues, L.A. et al., 2004 (Journal of Volcanology and Geothermal Research , 138:325-344). Direct collection of gases emitted at fumaroles is possible, but in an active volcanic setting this may be getting too close for comfort for most people! Fortunately, the correlation spectrometer (COSPEC) allows us to estimate SO 2 emissions in an eruption plume from afar and thus with little danger. A COSPEC measures the amount of solar ultraviolet light absorbed by sulfur dioxide and compares it to a standard. The amount of absorption is proportional to the concentration of sulfur dioxide. Measurements are made over a period of time and the data are used to calculate the sulfur dioxide daily emission rate. Figure VO.36 USGS scientists use a COSPEC (correlation spectrometer) to estimate sulfur dioxide gas emission from the Mount St. Helens dome by measuring the absorption of ultraviolet light. The setup was located on Sugar Bowl, on the northwest flank of Mount St. Helens. Photo courtesy of USGS. 3d. Volcano monitoring techniques: Visible signs of volcanic activity - new eruptive material Has a new lava dome appeared in the crater? In which direction is it expanding? Is a new lava flow issuing out of an existing volcano in an area where there has been no previous activity? Have parts of an existing volcano been destroyed due to recent explosive eruptions or landslides? Are there any new craters, fumaroles, or hot springs? The answers to all of these questions can be obtained from visual observations . On an active volcano, field data MUST be collected as often as possible, with considerations for safety, ease of access, weather conditions, and of course, funding. i. Still photography and video footage are used extensively to document changes to active volcanoes. ii. LIDAR (light detection and ranging) images such as the one of the dome at Mount St. Helens in September - October, 2004 (Figure below) can be used to calculate the volume of new material added over time. Page 42 of 54 Figure VO.37 A LIDAR image of Mount St. Helens taken sometime between September 2003 and October 4, 2004, showing the deformed crater floor. New dome rocks did not penetrate the floor until October 9, 2004. Image courtesy of USGS. Animation of LIDAR images are helpful in visualizing the progress of change in the volcano. Access the animated version of the LIDAR images of Mount St. Helens here. 3e. Volcano monitoring techniques: Increased heat flow Increased heat flow from a volcano may indicate the presence of new magma at the surface, fracturing of the crust due to magma upwelling, or increased activity at hot springs and/or fumaroles. There are a number of techniques used to measure heat flow. i. Direct observations of glowing magma or glowing cracks in a volcano are certainly possible. An optical pyrometer , a hand-held device, may be used to determine the temperature of glowing lava. ii. Aerial remote sensing of infrared spectra can record anomalously hot areas. iii. Forward looking infrared (FLIR) images, taken by airplanes or satellites, are also being used extensively to monitor dome growth at Mount St. Helens. Page 43 of 54 Figure VO.38 Visual image compared to FLIR (Forward Looking Infrared) image of new growth on dome. USGS images taken on 13 October 2004. Photo courtesy of USGS. 4. Predicting eruptions: Communicating with the public How do geoscientists communicate the potential danger at a volcano to the public? Normally there is a table of warning levels for an area, such as the following system used for Alaskan volcanoes. Table VO.7 Volcanoes in Alaska: Level of Concern Colour Code GREEN Volcano is in typical background, non-eruptive state or, after a change from a higher level, volcanic activity has ceased and volcano has returned to non-eruptive background state. YELLOW Volcano is exhibiting signs of elevated unrest above known background level or, after a change from a higher level, volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase. ORANGE Volcano is exhibiting heightened or escalating unrest with increased potential of eruption, timeframe uncertain, OR eruption is underway with no or minor volcanic-ash emissions (ash-plume height specified, if possible). RED Eruption is imminent with significant emission of volcanic ash into the atmosphere likely OR eruption is underway or suspected with significant emission of volcanic ash into the atmosphere (ash-plume height specified, if possible). Adapted from USGS Volcano Hazards Program Page 44 of 54 The levels of concern are color coded, with green used for background level activity. Yellow indicates restlessness, orange indicates small ash eruption expected or confirmed, and red indicates a large ash eruption expected or confirmed. Variations in the meaning of the colors exist for different volcanoes. View current alerts for volcanoes worldwide at the SI / USGS Weekly Volcanic Activity Report. These reports are provided by the Global Volcanism Network (GVN) and is a cooperative project between the Smithsonian Institute's Global Volcanism Program and the US Geological Survey's Volcano Hazards Program. 5. Why it is difficult to predict eruptions As we learned in the previous sections, a comprehensive volcano monitoring scheme MUST include the following in order to achieve its goals: a. installation and upkeep of real-time seismic network b. installation and upkeep of real-time GPS network c. gas emission sampling (COSPEC) d. regular ground measurements (e.g., crack widths) e. chemical analyses of eruptive deposits f. daily observations **It is important to note that equipment do not last long in the harsh environment of an active volcano, so upkeep of monitoring equipment is both ongoing and expensive. If we know all of this information about a volcano, why is it difficult to predict eruptions? No one precursor event can predict the exact time of an eruption, especially with enough time to safely evacuate the local populace. More precursors increase the chance of successfully predicting an eruption. But every volcano behaves differently, and any one volcano can change its behavior with time. For example, Mount St. Helens was quite predictable over the period from mid-1980 through 1986. Dome-building eruptions Page 45 of 54 could be predicted within 24 hours by noting a rapid increase in the rate of ground deformation (through direct measurement). But a different set of precursors (recognized in hindsight) heralded the catastrophic eruption on May 18, 1980, and between 1986 and 2004, Mount St. Helens was quiescent. Geoscientists had little warning that the volcano would start another period of dome-building eruptions in late 2004. In summary, every volcano behaves differently from others, and any volcano may change its behavior over time. Therefore, a monitoring scheme that is successful at one volcano may not work at another volcano, and any monitoring scheme should be continually reassessed and updated as eruptive conditions change. QUESTION FOR FURTHER THOUGHT : Where would it be safest to build an observation post to monitor a stratovolcano (refer to Hazard Map below)? A , B , C , D , or E UNIT D TOPIC Where are Violent Volcanoes Found? Outline In this unit we will learn about the different plate tectonic settings where volcanoes are found, and study the relationship between plate tectonic setting and sub-aerial volcanoes, with particular emphasis on the setting of the most explosive volcanoes. 1. Review of plate boundaries and the chemical composition of Earth's upper layers 2. Distribution of active volcanoes on Earth 3. Hotspot settings Page 46 of 54 4. Convergent plate margins 5. Summary - Our nearby volcanoes Active volcanoes are not randomly distributed on the surface of our planet. Rather, they are associated with specific types of plate tectonic settings. In this unit we will learn where active volcanoes are located, including the details of the tectonic settings and the conditions that lead to formation and eruption of explosive magmas. 1a. Review of plate boundaries and the chemical composition of Earth's upper layers: Plate boundaries (Note: this material is covered in part in the Fragile Systems and Earthquakes Modules.) There are three types of plate boundaries. At divergent margins , two plates spread apart due to upwelling of the upper mantle and the injection of magma. Most divergent margins are located in the ocean basins and are called mid-ocean ridges (MOR), as they form undersea mountain ranges. The on-land equivalent of a MOR is known as a continental rift zone. An example would be the East African Rift. Where two plates collide, one plate will subduct into the mantle beneath the other plate to form a convergent margin. This holds true for ocean-ocean collisions and ocean-continent collisions, and volcanic arcs form in the overlying plate in these settings. Where two continental plates collide, however, neither will subduct, as both are too buoyant to descend into the dense mantle. Rather, they suture together in a massive collisional zone to form a large mountain chain. This is what happened when India collided with Asia to form the Himalayan Mountains. Magmatism is rare in continent-continent collision zones. A third type of plate boundary is a transform fault , where two plates slide laterally past one another. Magmatism is also rare along this type of plate boundary. 1b. Review of plate boundaries: Chemical composition of the upper mantle and the crust The chemical composition of each of Earth's layers has a profound effect on the types of magmas that are produced there. The upper mantle is ultramafic in composition; that is, the rocks (called peridotites) found there have < 45 weight percent SiO 2 (silicon dioxide), and they are high in MgO (magnesium oxide) and FeO (iron oxide). Oceanic crust is mafic, containing igneous rocks (basalts, gabbros) with 45 - 52% SiO 2, along with less MgO and FeO and more CaO (calcium oxide) and Al 2O3 (aluminum oxide) than the upper mantle. In contrast, continental crust contains a wide range of igneous rocks with SiO 2 contents ranging from mafic through intermediate to felsic, from 45% to > 68%, with the most felsic rocks containing about 75% SiO 2. Overall, continental crust has an average SiO 2 content of 60%, which is much more SiO 2-rich than oceanic crust, and which accounts for the higher buoyancy (lower density) of the continents as opposed to the ocean basins. Why is this background information on the chemical composition of Earth's layers important for understanding the distribution of explosive volcanoes? Well, the melting temperature of a rock or magma is inversely proportional to its SiO 2 content. Therefore, mafic magma is hot enough to melt average continental crust but not hot enough to melt mafic oceanic crust. This leads to more explosive volcanism being present in continental locations. 2. Distribution of active volcanoes on Earth Page 47 of 54 Figure VO.39 Map showing the Earth's tectonic plates and boundaries, and location of active volcanoes and the "Ring of Fire". Image downloaded from USGS. Sub-aerial active volcanoes. Examine the map of active volcanoes around the world, and search the web for similar maps. About 80% of active sub-aerial volcanoes (those located above ground level) are associated with convergent plate margins. Many active volcanoes are found along the margins of the Pacific Ocean basin, where their presence is due to the convergent margin setting. This has lead to the term "Pacific Ring of Fire" to describe this concentration of volcanoes. Divergent plate boundaries are host to about 15% of Earth's active sub-aerial volcanoes, but many more submarine volcanoes are found at the mid-ocean ridge (MOR). In fact most volcanic activity on Earth is probably taking place at the MOR, but the majority of these volcanic vents are under water and therefore not so dangerous to us. Additional active sub-aerial volcanoes are found within oceanic and continental plates, and not located along a plate boundary. These are called hotspot volcanoes. Click here and here to view animations of the tectonic settings of volcanic activity. Animations from Samson and Abbott, 2012 . 3a. Hotspot settings: Oceanic Page 48 of 54 Hotspot volcanoes are located where hot, buoyant mantle material (a mantle plume ) rises into the upper mantle and initiates melting. Mantle plumes are thought to originate at the core-mantle boundary, and they appear to be fixed in the mantle; that is, their positions do not migrate with time. A chain of hotspot volcanoes exhibiting a definite age progression along its length forms due to movement of a plate over a mantle plume. When a mantle plume induces melting in the mantle near the base of an oceanic plate, mafic magma is produced. This mafic magma rises to the surface through mafic crust with little interaction because the melting temperatures of the magma and crust are similar. Volcanoes formed in oceanic hotspot settings are typically shield volcanoes (along with some small cinder cones) that erupt mafic magma. The resulting volcanic rocks are dark-colored, low in viscosity, and low in SiO 2. They are called basalts . Page 49 of 54 Figure VO.40 Map of the Pacific basin showing the location of the Hawaiian RidgeEmperor Seamount Chain. Image downloaded from USGS. The Hawaiian-Emperor chain of volcanoes formed as a result of the Pacific Plate moving in a north-northwest direction over a mantle plume from 60 to 43 million years, with a change in the direction of plate motion to more northwesterly at about 43 mya. Active volcanism in this chain today is centered on Kilauea volcano on the Big Island of Hawaii and at Loihi Seamount, which is located 30 km south of the Big Island. Click here to view an animation of hotspot formation. 3b. Hotspot settings: Continental Mantle plumes found beneath continental plates are no different than mantle plumes found beneath oceanic plates. However, continental hotspot volcanoes are very different from the shield volcanoes found in oceanic hotspot settings. This is because of the higher silica content of the continental crust and the resulting interactions of mantle-derived magmas with that of continental crust. A rising mantle plume will induce melting in the upper mantle or the lithosphere near the base of a continental plate. The resulting melts are mafic. As mafic magma rise through the continental crust, the crust melts. This is because continental crust has, on average, a higher silica content and hence a lower melting temperature than mafic crust. Thus in this manner, mafic magmas can produce large volumes of felsic melt. The production of large volumes of high silica, high viscosity magma in continental hotspot settings leads to the formation of giant calderas. Calderas are the most explosive type of volcano, but luckily for us, they have the longest recurrence interval between eruptions. The light-colored, high silica magmas produced by eruption of SiO 2-rich magmas are called rhyolites . Because of their high viscosity and high gas content, these felsic magmas typically erupt as large volumes of ash flow tuff. Yellowstone caldera, located in northwest Wyoming, U.S.A. had its most recent caldera-forming eruption about 0.6 mya. Air-fall ash deposits from this eruption covered much of the western U.S. If a similar eruption were to occur today, a catastrophe far worse than any that you have ever seen would affect most of North America. Page 50 of 54 Yellowstone sits at the northeast end of a chain of volcanoes (the Yellowstone hotspot chain) that began to form in Oregon about 16 mya as the North American Plate moved to the southwest over a mantle plume. From where Yellowstone caldera is today, in which direction would you expect future volcanoes to form? 4a. Convergent margin settings: Ocean-ocean convergence At convergent plate boundaries, oceanic lithosphere is subducted. At depth, the lithosphere dehydrates and water is introduced into the overlying asthenosphere (upper mantle). This water induces melting of the mantle (by lowering the melting temperature), which leads to the formation of mafic magma. Being less dense than the surrounding mantle, mafic magma rises into the crust and towards the surface. If the plate overlying the rising magma is oceanic, the mafic magma will have limited interaction with it. Intermediate magmas may form from differentiation processes that change the composition of the mafic magma and slightly raise the SiO 2 content. Thus, magmas produced in oceanic arcs are mostly mafic, with some intermediate magmas. By-and-large, volcanoes in oceanic arcs are composite volcanoes, and the rocks that form from them are basalts and andesites. The western Aleutian arc is an example of an arc formed from ocean-ocean plate convergence (see Figure VO.41 below). Figure VO.41 Map showing volcanoes of the Alaska Peninsula and Aleutian Islands. Image downloaded from USGS. 4b. Convergent margin settings: Ocean-continent convergence At ocean-continent convergence zones, mafic magma forms in the mantle through the same mechanism described in the previous section; that is, water introduced by the subducted slab lowers the melting temperature of the mantle . However, as the resulting mafic magmas rise through continental crust, they interact with it to produce intermediate and felsic magmas. These interactions include melting of the continental crust (to produce felsic magma) and/or changing (or differentiating ) the composition of mafic magma to a more SiO 2-rich composition (to form intermediate and/or felsic magmas). Volcanoes that result from eruption of magmas with a wide range of silica contents are called composite volcanoes , and they Page 51 of 54 consist of rocks that range from basalt , to andesite, and rhyolite. The higher silica magmas produce the most explosive and therefore most hazardous eruptions at these volcanoes. The Garibaldi-Cascade arc (the Cascade Range volcanoes) formed from subduction of the Juan de Fuca plate beneath the North American Plate. This array of volcanoes stretches from Mount Meager in southern B.C. to Mount Lassen in northern California. Mount St. Helens has had the most recent activity in this arc, but could other volcanoes, perhaps nearer to where you live, become active? Page 52 of 54 The Cascades volcanoes have been quite active over the past 4,000 years, based on dated eruptions from the major volcanoes as shown in the Figure above. Note how many of these volcanoes have been active in the past 200 years, and you will understand why it is important to study the recent history of many of the Cascade volcanoes. If you had been asked in the 1970s to predict which of these volcanoes would likely be the next one to erupt, which one would you have picked? 5. Summary - our nearby volcanoes Volcanoes are found at divergent and convergent plate boundaries as well as where mantle plumes ascend beneath plates. Physical constraints in each of these tectonic settings delimit the silica contents of the resulting magmas, which in turn constrain the type(s) of volcanoes that are present. Although any volcano is potentially hazardous, the most hazardous volcanoes are composite volcanoes and calderas because of their tendency to erupt gas-rich viscous magmas. The volumes of these explosive eruptions may be high, especially for eruptions associated with a caldera. On a positive note, calderas erupt infrequently. Within the Garibaldi-Cascades arc, eruptions are frequent enough that, based on historical activity, we will likely witness another eruption during our lifetime. Based on the longer recurrence interval of catastrophic events from caldera eruptions, it is harder to predict whether we will experience one from any of the three youngest calderas in western North America (Yellowstone, Long Valley, and Jemez calderas) during our lifetime. We now know, however that it is not impossible. Page 53 of 54 Page 54 of 54

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