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Unformatted text preview: The Wrath of Vulcan: Volcanic Eruptions Volcanic Eruptions Volcanoes are … Erupting vents through which molten rock surfaces Erupting Mountains built from magmatic eruptions Caused by tectonic activity Volcanoes pose a number of hazards to humans Mexico City Seattle Naples, Italy Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcanic Eruptions Volcanic In 79 C.E. Mount Vesuvius erupted violently Pyroclastic flows destroyed Pompeii, killing 20,000 Pyroclastic A record of Etruscan life was preserved under ash record Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcanic Eruptions Unpredictable, dangerous Unpredictable, Eruptions can… Build large mountains Blow mountains to bits Provide highly productive Provide soils to feed a civilization soils Create new real estate Can extinguish a civilization Can in a matter of minutes in Eruptions affect climate Eruptions in the short term Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcanic Materials The products of volcanic eruption take three forms: The Lava flows – Molten rock that moves over the ground Lava Pyroclastic debris – Fragments blown out of a volcano Volcanic gases – Vapor and aerosols that exit a volcano Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Lava Flows Lava can be thin and runny or thick and sticky Flow style depends on viscosity, which depends upon... Composition, especially silica (SiO2), Fe, and Mg content Temperature Gas content Crystal content Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Lava Compositions Lavas with high silica / low Fe and Mg are called... Lavas with low silica / high Fe and Mg are called… Silicic, felsic, or rhyolitic Mafic or basaltic Lavas with moderate silica, Fe, and Mg are called… Intermediate or andesitic Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Basaltic Lava Flows Mafic lava – Very hot, low silica and low viscosity Mafic Basalt flows are often thin and fluid They can flow rapidly (up to 100 km/hr) They can flow for long distances (up to several 100 km) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Basaltic Lava Flows Pahoehoe (pa-hoy-hoy) - a Hawaiian word describing Pahoehoe basalt with a glassy, ropy texture basalt Pahoehoe forms when extremely hot basalt forms a skin Pahoehoe With flow, the skin is rolled into ropy ridges and furrows With Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Basaltic Lava Flows A’a’ (ah-ah) is a Hawaiian word describing basalt that A’a’ solidifies with a jagged, sharp, angular texture solidifies A’a’ forms when hot flowing basalt cools and thickens With flow, lava crumbles into shards and fragments Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Basaltic Lava Flows Solidified flows may contract with vertical fractures that Solidified are hexagonal in cross section This feature is called columnar jointing This Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Andesitic Lava Flows Higher SiO2 makes andesitic lavas viscous Unlike basalt, they do not flow rapidly Instead, they mound around the vent and flow slowly The outer crust fractures, creating rubble Andesitic lava flows remain close to the vent Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Rhyolitic Lava Flows Rhyolite, with the highest SiO2, is the most viscous lava Rhyolitic lava rarely flows Rather, lava plugs the vent as a lava dome Sometimes, lava domes are blown to smithereens Sometimes, Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcaniclastic Deposits Accumulations of fragmented igneous material Pyroclastic debris – Lava that freezes flying through air Pre-existing rock fragments Pre-existing Landslide debris Mudflows Kitchen sink Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Basaltic Pyroclastic Debris Glass shards and fragmented lava in a range of sizes Basaltic eruptions generate a lot of spatter Lapilli – Pea to plum-sized material Lapilli Pea Blocks and bombs – Apple to refrigerator-sized Blocks Pele’s Hair – Strands of glass created by flying lava droplets Bombs – Streamlined fragments of ejected lava Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Explosive Pyroclastic Debris Intermediate and felsic magmas erupt explosively More viscous than basaltic magma (more SiO2) Contain more gas Produce large quantities of volcanic ash Produce Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Pyroclastic Flows Pyroclastic flows (or, nuées ardentes - French): Pyroclastic French): Avalanches of hot ash (200Co–450oC) Race downslope at speeds up to 300 kmh They incinerate all in their path; fast killers Famous examples: Mt. Vesuvius, Mt. Pelee, Mt. Augustine Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Pyroclastic Deposits Tephra – Deposits of pyroclastic debris of any size Tephra Tuff – Lithified ash with or without lapilli Tuff Lithified Air-fall tuff – Accumulations of ash that fell like snow Ignimbrite (welded tuff) – Tuff that is deposited while hot and fused together while cooling Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcaniclastic Deposits Blocks – Preexisting rock fragmented by eruption Blocks Preexisting Blown out of a volcanic vent, blocks pile up nearby Create unstable slopes that easily fail Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Lahars Tephra is readily moved by water as a debris flow Tephra is Known as lahars, these flows are often deadly Known lahars Lahars move rapidly (up to 50 km per hour) They have the consistency of wet cement A distinct hazard to people living in volcanic valleys Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcanic Gas Volcanic Up to 9% of magma may be gas Carbon dioxide (CO2) – Second in abundance Water (H2O) – Most abundant gas Sulfur dioxide (SO2) – Rotten egg smell Magma composition controls gas content Magma Felsic magmas are gas-rich; mafic magmas are less so Felsic Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcanic Gas Volcanic Gases are expelled as magma rises (P drops) SO2 reacts with water to form aerosol sulfuric acid Style of gas escape controls eruption violence Style Low viscosity (basalt) – Easy escape; mellow eruption High viscosity (rhyolite) – Difficult escape; violent eruption Gas bubbles in rock are Gas called vesicles vesicles Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcanic Architecture Volcanoes have characteristic features Volcanoes Magma chamber Fissures and vents Craters Calderas Distinctive profiles Shield volcanoes Cinder cones Stratovolcanoes Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Magma Release Some magma rises via a conduit to the surface Some Magma may also erupt along a linear tear, a fissure Magma Fissure eruptions may display a “curtain of fire” Fissures evolve into discrete vents Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Craters Crater – A bowl-shaped depression atop a volcano Craters are up to 500 m across; 200 m deep Form as erupted lava piles up around the vent Summit eruptions – Located within the summit crater Flank eruption – Located along the side of a volcano Flank Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Caldera A gigantic volcanic depression gigantic Up to several tens kilometers across, Up larger than a crater, with steep sidewalls and flat floors with Form from massive eruptions The volcano collapses Crater Lake, Oregon Yellowstone National Park Figure 4.1a Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Crater Lake Caldera, Oregon Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcano Types Shield volcanoes Broad, slightly domed-shaped (like an inverted shield) Made by lateral flow of low-viscosity basaltic lava Made Have a low slope and cover large geographic areas Mauna Loa on Hawaii is a good example Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcano Types Cinder cone – Conical piles of tephra The smallest type of volcano Built of ejected lapilli-sized fragments piled up at a vent Slopes are at the angle of repose Often symmetrical with a deep summit crater Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcano Types Stratovolcanoes (also called Composite volcanoes) Stratovolcanoes Composite Large, cone-shaped volcano Composed of alternating layers of lava and tephra Often symmetric; can be odd shapes from landslides, etc Examples include Mt. Fuji, Mt. Rainier, Mt. Vesuvius Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Will it flow, or will it blow? Will Two dominant eruptive styles Effusive eruptions – Produce lava flows Explosive eruptions – Blow up Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Eruptive Style Effusive eruptions – Produce lava flows Effusive eruptions Lava flows stream away from vents Lava lakes can form around the vent Can produce huge lava fountains Commonly basaltic, these create shield volcanoes Commonly Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Eruptive Style Explosive eruptions – Produce pyroclastic flows Explosive eruptions Caused by gas pressure in the more viscous magma Pressure is released explosively Pressure Create stratovolcanoes/ composite volcanoes May create calderas Blanket the landscape with tephra Andesitic and rhyolitic compositions Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Controls on Eruptive Style Viscosity – Controls the ease of lava flow Gas Pressure – Greater P favors explosive style Basalt – Low viscosity lava flows away from vent Felsic – High viscosity lava builds up at the vent Basalt – Low viscosity allows gas release Felsic – High viscosity prevents gas release Environment – Where eruption occurs is important Environment Subaerial lava flowing on land cools slower than… Submarine lava which is quickly quenched Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Maar: Magma meets Groundwater Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Eruptions to Remember Mt. St. Helens – Erupted May 18, 1980, 8:32 A.M. Earthquake triggered landslide released pressure Initial vertical blast followed by a much stronger lateral Initial blast that tore off the entire north side 396 m was blasted away Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Eruptions to Remember Mt. St. Helens – Erupted May 18, 1980, 8:32 P.M. The blast devastated 600 km2 and killed 61 people Lahars plugged the Toutle River; closed the Columbia Lahars Ash fell in North Dakota; highways and rail lines stopped Ash Destroyed timber valued at several 100 million dollars Destroyed Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Mid-Ocean Ridge Eruptions Most lava erupts along the mid-ocean ridge (MOR) MOR-generated oceanic crust covers 70% of Earth Basalt erupted from fissures quenches as pillows Pillow mounds are pulled apart with plate motion Pillow Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Convergent Boundaries Most volcanoes form at convergent boundaries Most Volatiles from subducting plate initiate melting Volatiles Arc volcanoes develop on the overriding plate May cut through either oceanic or continental crust The “Ring of Fire” dominates Pacific margins Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Continental Rifts Yield an array of volcano types reflecting... Partial melting of the mantle (mafic magmas) Partial melting of the crust (felsic magmas) Examples: East African Rift Basin and Range Province Mid-continent Rift Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Oceanic Hot Spot Volcanoes Plume under an oceanic plate Plume Basalt erupts at the seafloor and forms a growing mound Basalt A volcano builds above sea level to form an island Then, basalt will not quench and can flow long distances Lava builds upward and outward and the island grows Lava Submarine slumps remove large masses of the volcano Submarine Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Continental Hot-Spot Volcanoes Continental Hot Spot – Cuts a continental plate Continental Yellowstone – Eruption ~ 640 Ka created a 100 km caldera 1,000 times more powerful than Mt. St. Helens Deposited vast ignimbrite deposits Magma beneath the caldera continues to fuel geysers Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Flood-Basalt Eruptions Voluminous lava eruptions above a plume Thinned lithosphere erupts magma from long fissures Thinned Lava spreads over large areas; great thicknesses stack up Create plateaus called Large Igneous Provinces (LIPs) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Heimaey Eruption 1973 Iceland is a hot spot beneath a mid-ocean ridge Lava has built the hot spot/ridge above sea level The island is being torn apart by plate motion Volcanoes trace the mid-ocean ridge rift valley Volcanoes Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcanic Hazards Volcanic eruptions cause great harm to humans Volcanic Many populated areas ring active volcanoes Many Eruptions have profoundly influenced human history Eruptions In the past 2,000 years: an estimated 250,000 deaths More humans live in volcano hazard areas than ever before More Understanding volcanic Understanding behavior is the best defense Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcanic Hazards Lava flows – Lava threats are mostly from basalt Lava Lava may completely destroy immovable objects It is rare for lava flows to kill people Usually there is enough notice Lava rarely moves faster than people Sometimes, people watching lava flows are killed Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcanic Hazards Pyroclastic flows – aka Nuées ardentes Pyroclastic Nuées Clouds of roiling ash and gas that race downslope Ride over a carpet of superheated air Eases passage of the flow Permits high velocities Immediately deadly to anything living Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcanic Hazards Ash and lapilli fall around the volcano Can completely bury landscapes, killing plants and crops This material is heavy; it causes roof collapses This material is gritty; it abrades car and airplane engines Water easily moves this material as deadly lahars Water Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcanic Hazards Blast – Rarely, explosions are ejected sideways Mt. St. Helens – Lateral blast tore off north side. Destroyed over 600 km2 of forest and killed 61 people Blast hazard is uncommon; most eruptions are vertical Blast Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcanic Hazards Landslides – Eruption-related slope failures Eruptions can trigger landslides Large masses of material are deposited rapidly near vent Earthquakes initiate failure of unstable slopes Mt. St. Helens The eruption immediately followed a 3 km3 slope failure Slide material moved more than 20 km from the peak Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcanic Hazards Lahars – Mudflows result when ash is moved by water Like concrete, this material is more dense than water Like Lahars carry away everything (people, houses, bridges) Nevada del Ruiz, Colombia, buried Armero +25,000 people Nevada Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcanic Hazards Earthquakes – Moving magma causes earthquakes Earthquakes Although usually small in magnitude, they are frequent Can cause slope failures and damage to structures Tsunamis – Water explosions create giant waves Tsunamis from Krakatoa (1883) killed 36,000 Tsunamis Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcanic Hazards Gas – Volcanic gases are poisonous (H2S, CO2) Lake Nyos, Cameroon, 1986 Magmatic CO2 built up in the lake located in a crater built The lake overturned (burped) and the CO2… Moved down the valleys as a heavier-than-air underflow Killed 1,742 people; 6,000 cattle Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Active vs. Extinct Recurrence interval – Average time between eruptions Recurrence Active – Erupting, recently erupted or likely to erupt Active Erupting, Dormant – Hasn’t erupted in hundreds to thousands of years Dormant Hasn’t Extinct – Not capable of erupting Extinct Not Tectonics can shut off magma After extinction, erosion takes over Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Devils Tower, Wyoming Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Predicting Eruptions Warning signs precede most eruptions Earthquake activity – Magma flow increases seismicity Heat flow – Magma causes volcanoes to “heat up” Changes in shape – Magma causes volcanoes to inflate Emission increases – Changes in gas mix and volume These signs indicate that an eruption is imminent They cannot predict eruption timing or style Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Mitigating Hazards Danger assessment maps Delineate danger areas Pyroclastic flows Lahars Landslides Used for planning, zoning Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Mitigating Hazards Evacuation – Moving those at high risk saves lives Mt. St. Helens – Timely evacuation saved hundreds Sometimes eruptions don’t occur, large expenses Sometimes Diverting flows – Flowing lava can be diverted Explosives Heavy equipment Seawater Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcanoes and Climate Volcanic eruptions can be large enough to alter climate Volcanic for a few years for Ash and aerosols high in the atmosphere block sunlight >>> atmospheric cooling 1815 was the “year without a summer” due to Tambora Volcanic output of CO2 minute compared to that from anthropogenic sources anthropogenic Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions Volcanoes in Our Neighborhood Volcanic activity evident on the Moon and planets Volcanic Lunar maria (dark “seas”) are regions of flood basalts Lunar (dark Olympus Mons – Extinct Martian shield volcano The Jovian moon, Io, has active volcanoes Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic Eruptions ...
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This note was uploaded on 02/25/2012 for the course EARTH 2 taught by Professor Thomas during the Summer '11 term at UCSB.

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