Lecture6 - ways to melt rock isothermal decompression...

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Unformatted text preview: - ways to melt rock --- isothermal decompression --- increase temperature --- increase pressure Volcanoes Volcanoes Name the largest volcano on Earth Name the tallest mountain, measured from top to bottom, on Earth Visualize your expectations of what this mountain should look like Mauna Loa 9 km high! 1 A size comparison of the three types of volcanoes - shape will say something implicitly about its composition 9 km high! Igneous activity & plate tectonics - basaltic rock - mafic - probably from melting mantle - granite - felsic - some form of melting mantle - Global distribution of igneous activity is not random • Most volcanoes are located within or near ocean basins • Basaltic rocks are common in both oceanic and continental settings; granitic rocks are rarely found in the oceans • Ocean crust (mafic) = (modified) mantle - mostly at edges of Pacific plate - active plate margin Distribution of some major volcanoes 2 - divergent (spreading centres) margins -- decompression melting - pulls apart - divergence Igneous activity & plate tectonics Igneous activity along plate margins 1) Spreading centers – Greatest volume of volcanic rock is produced along the oceanic ridge system (divergence) – Mechanism of (decompression) melting – – – – Lithosphere pulls apart Less pressure on underlying rocks Results in partial melting of mantle Large quantities of basaltic magma are produced two tectonic plates - creation of rock in this case 100km Ocean crust (mafic) = (modified) mantle 3 ridge system - creating material - convergent (subduction zones) margins Igneous activity & plate tectonics Igneous activity along plate margins 2) Subduction zones – Occur in conjunction with deep oceanic trenches (convergence) – Descending plate partially melts + release H 2O – Magma slowly moves upward – Rising magma can form either: – An island arc if in the ocean – A volcanic arc if on a continental margin – Both are andesitic (intermediate) composition - releasing water that are trapped on the sediments -- change in melting temp - melting rock + water 4 - oceanic arc - Indonesia Igneous activity & plate tectonics - Ring of Fire - ring of volcanoes that are currently active around the Pacific Ocean Basin - Pacific plate --- generally all due to subduction 2) Subduction zones, cont. • Associated with the Pacific Ocean Basin – Region around the margin is known as the 'Ring of Fire' – Most of the worldʼs explosive volcanoes Igneous activity & plate tectonics Intraplate volcanism • Activity within a tectonic plate • Associated with plumes of heat in the mantle • Form localized volcanic regions in the overriding plate called a hot spot - middle of tectonic plate - Intraplate volcanism -- mantle plume - rises through mantle and acts like a blowtorch at the base of the lithosphere - if blowtorch is under oceanic - basaltic - under continental crust - granitic –Produces basaltic magma sources in oceanic crust (e.g., Hawaii and Iceland) –Produces granitic magma sources in continental crust (e.g., Yellowstone Park) 5 - mantle is solid - moving at the speed of glass - but very hot Volcanism on a tectonic plate moving over a hot spot - these hot spots make up most of Hawaii and the Midway Islands - because the Pacific plate is always moving Tectonic plate moving over a hot spot Tectonic plate moving over a hot spot 6 - Yellowstone - super volcano - has erupted a number of times over the past million times - ash fall beds Tectonic plate moving over a hot spot Tectonic plate moving over a hot spot Tectonic plate moving over a hot spot 7 The nature of volcanic eruptions Factors determining the "violence" or explosiveness of a volcanic eruption • Composition of the magma (#Si = hi visc) • Temperature of the magma (hi T = lo visc) • Dissolved gases in the magma - 1-9 wt% of a magma - Mainly H2O vapor & CO2 (SO2 , N, Cl) - composition - high amount of silica - very viscous won't flow well - temp - high temp - low viscosity - flows well - gases in magma - can make about 10 percent to magma - can add to the volatility …factors actually control the viscosity of a given magma which in turn controls the nature of an eruption Rhyolitic - granitic Andesitic - more viscous - Aa - vesicles preserved gas bubble - tend to be the most explosive Materials extruded from a volcano Rhyolitic • Si-rich, v. viscous; hi-H2O & gas (pumice) Andesitic (intermediate) • more viscous (aa, vesicles; most explosive) Basaltic • low Si = much more fluid-like • types of basaltic flows – Pahoehoe lava (resembles a twisted / ropey texture) – Aa lava (rough, jagged blocky texture) – Pillow basalts (underwater) Pahoehoe lava flow Iceland - Pahoehoe texture 8 Typical aa flow Pillow basalt Materials extruded from a volcano Pyroclastic materials – "fire fragments" Types of pyroclastic debris • Ash & dust - fine, glassy fragments • Pumice - porous rock from 'frothy' lava • Lapilli - walnut-sized material • Cinders - pea-sized material • Particles larger than lapilli Aa - blocky and jagged divergent margins - pillow basalt - all just textural terms -- shapes are all dictated by how they are extruded -- most of the stuff is ejected as red lava -crystalize through air - cool in transit – Blocks - hardened or cooled lava (meters) – Bombs - ejected as hot lava; teardrop shape 9 Phreatic Eruption: steam from hot groundwater snow caps - one of the first things that are going to melt - steam - first signs of volcanic eruption Plinian Eruption (Pliny the Elder) - column collapse - snow caps... etc collapsing into caldera - giving rise to volatility Materials extruded from a volcano Pyroclastic flows & surges • Mixture of hot (~500-700oC) gas & rock that flows like an avalanche. Material size: ash to blocks • Velocities from 10’s - 100m/sec; travel kms - 10 kms • Generally confined to valleys • Originate from column collapse or dome collapse • "Ash hurricane" responsible for the holocaust at St. Pierre, Martinique, 1902; 30,000 people killed in minutes 10 Pyroclastic flows & surges nueé ardente • Fiery pyroclastic flow made of hot gases infused with ash and other debris • Move down slopes at speeds up to 200 km/h - several hundred degrees, ashes and gases Pyroclastic flows & surges lahar • Water-saturated pyroclastic flow • Melting glacier provides excess water Nevado del Ruiz 11 Nevado del Ruiz - blocks, bombs, and ash - represents a paleovolcanic eruption paleolahar fossil lahar 12 - SO2 - in atmosphere = acid rain Volcanic eruptions Secondary effects - SO2 + H2 O atm = H2 SO4 (acid rain) - dust/ash = reflects sun radiation lowers T of Earthʼs surface by 2-3°C for as much as 10 yrs Volcanoes volcanic cone - hill or mountain volcanic crater / caldera - depression surrounding vent not necessarily steep - shield volcano - very large and round - flood basalts - flood over the landscape upon eruption 1) Effusive eruptions: • from/near main vent = shield volcano • from fissures (linear features) on land = flood basalts • from subaqueous (= pillow basalts) & subglacial (= lahars) Fissure 13 Lava Tube Lava Tube Shield volcano - lava travelling through these tubes at some point - like a warrior shield - composition - low in silica and low in volatile composition - mafic rock - basaltic Mauna Loa is a classic shield volcano Slope angles are gentle, size is large Composition? 14 Shield Volcanoes - on Mars - same shape of Mauna Loa --- shield volcano - prob same composition - composite cone - mixture of bunch of things - both lava flows and pyroclastic material Volcanoes 2) Pyroclastic eruptions: • airborne material w/ little / no magma .. v. viscous = tephra • gas-rich, viscous magma = ash flow eruption a) Composite cone (Stratovolcano) • Composed of interbedded lava flows and layers of pyroclastic debris • Large, classic-shaped volcano (1000ʼs of m high & several km wide at base) • Most are located adjacent to the Pacific Ocean (e.g. Fujiyama, Mt. St. Helens, Ranier) Volcanoes a) Composite cones, cont. • most violent eruption (e.g. Mt. Vesuvius) • often produce a nueé ardente • may produce a lahar 15 - steep sided compared to shield volcanoes A composite volcano Mt St Helens: lateral blast Mt. St. Helens following the 1980 eruption 16 2005 Volcanoes b) Pyroclastic or cinder cone • loose pyroclastic material; small, steep sided; Sunset Crater, AZ 17 - connected a volcano to the surface - eroded above Volcanic landforms Lava Domes • Bulbous mass of congealed lava • Most are associated with explosive eruptions of gas-rich magma Volcanic pipes and necks • Pipes are short conduits that connect a magma chamber to the surface - need to predict these things -- all this movement causes seismicity - little earthquakes Volcanic precursors • Increase of ground water temp (>20°C) • Topographic changes (tilt/bulge) • Increase in seismicity • Magnetic, electrical, gravity field changes • Changes in gas composition • Changes in animal behavior ---- physical and chemical parameters that we can monitor - animal behaviour - they are attuned to electrical, magnetic, gravity field changes 18 - tiltmeter - tilts and records tilt changes - after eruption - will collapse - Yellowstone - earthquakes - related to the cracking and breaking of rocks associated with magma - 19 - Plutonic activity - not all melted material makes it to the surface - intrusive - Plutonic material - most magma is actually implaced at depth Most magma is emplaced at depth in the Earth • An underground igneous body, once cooled and solidified, is called a pluton Classification of plutons • Shape – Tabular (sheetlike) – Massive (irregular) • Orientation with respect to the host (surrounding) rock – Discordant – cuts across rock units – Concordant – parallel to rock units - Cordiara - stuff that intruded at depth - laccolith - some type of head on it - mushroom-shaped Plutonic activity Types of intrusive igneous features • Dike – a tabular, discordant pluton • Sill – a tabular, concordant pluton (e.g., Palisades Sill in New York) • Laccolith – Similar to a sill – Lens or mushroom-shaped mass – Arches overlying strata upward 20 - putting bunch of plutons together - makes up most of Cordiara Plutonic activity Intrusive igneous features • Batholith – Largest intrusive body – Surface exposure of 100+ km2 (smaller bodies are termed stocks) – Frequently form the cores of mountains Pipe Some intrusive igneous structures Laccolith Sill Dike Stock/ Batholith 21 ...
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