Volcanoes Tectonics Seamounts and Guyots Lava Viscosity silica

Volcanoes & Tectonics Seamounts and Guyots

Lava Viscosity (silica, Temp. ) & explosiveness • Basaltic lava - less silica and higher temperature, low viscosity, gasses escape at surface low pressures, not explosive e. g. Shield volcanoes, Rifts, MOR, cinder cones • Intermediate silica, explosive, composite = stratovolcanoes, Andes, Cascades, Aleutians Japan, Philippines, often pyroclastic clouds = nuée ardente • Rhyolitic lava - more silica and lower temperatures, high viscosity, gasses cannot escape, explosive.

Plate Tectonics and Volcanism

Volcanism on a tectonic plate moving over a hot spot

Pacific Basin 90%Volcanic Output Seamounts and MOR => ocean floor

Great Basalt Plateaus Volcanic geomorphic provinces Plumes under Continents yield more silica rich, explosive eruptions

A size comparison of the three main types of volcanoes Seamounts are these largest Plume below ocean Above subduction zone Divergent Zone Cinder Cones

Volcanic Activities and their products Upper surface pitted Scoria Jagged A'a' away from vent Lav a Flo w • Exhalative (gas: H 2 O, CO 2, SO 2) • Effusive (lava) • Explosive (tephra) Pahoehoe Near vent Hawaii, low silica, low viscosity, notice no tephra in view

http: //volcanoes. usgs. gov/Products/Pglossary/ancientseq. html Basaltic Pillow Lavas The reason there is so much water carried down into a subduction zone

Volcanic Activities and their products • Explosive (tephra) nuée ardente = pyroclastic cloud Intermediate to high silica - Andesitic to Rhyolitic

Tephra • Coarse = close • Thick = close

Lahars - volcanic mudflows Melting collapse of ice embedded cone Copyright © Stephane Veyrat-Charvillon 2002

Characteristic Eruptive Types Special case: Phreatic eruptions occur when magma hits water. And the water flashes to steam explosively

Rift Valley Flows Volcanic Landforms 1: Basaltic lava flows • Layered – Lava/sediment • Jointed – Columnar Copyright © Richard Kesel 2002

• Columbia Plateau • Deccan Traps • Very large in scale, worldwide effects Basalt Plateaus - Flood Basalts

Basaltic Cinder and Spatter Cones • Projectiles – Ballistic – Wind-borne • Divergent Margin +

Volcanic Landforms, Basaltic continued Shield Volcanoes Mauna Kea from Mauna Loa • Form above Plumes, huge • also Basaltic, gentle eruptions • As plate moves old volcanoes become inactive, new volcano forms over the plume

Volcanic Landforms 2: Andesitic Volcanoes Andesitic - Composite cones "Stratovolcano" Mt. Fuji in summer Form above Subduction zones

The Cascades, Washington State

Composite cones - section Lava and tephra alternate

Andesitic Eruptions Extensive Welded Tuff, Pumice Lavas, and Lahars Cleveland Volcano, Aleutians

Plug Domes

Rhyolite Eruptions • Rare, extremely explosive • Size of ash cover = violence • Toba and our near-extinction

Volcanism on a tectonic plate moving over a hot spot

Calderas

Caldera – Crater Lake Copyright © Ann Dittmer 2002

Caldera topography – Crater Lake

Maars A maar is a broad, low-relief volcanic crater that is caused by a phreatomagmatic eruption, an explosion caused by groundwater coming into contact with hot lava or magma Ukinrek Maars Tuff Ring

Mono Co, CA Rhyolite, very high silica, very viscous, can’t get rid of gas bubbles, explodes Panum Crater is a good example of both a tuff ring and a Rhyolite dome Rising magma hit groundwater, explosion left crater, then fountain of hot cinders made ring, then remaining magma rose as dome

• • • Iceland Is on the mid-Atlantic Ridge, 13 volcanoes have erupted since AD 874. most active & volatile is Grímsvötn. Over the past 500 years, Iceland's volcanoes have erupted a third of the total global lava output. One quarter of Iceland ‘s population died due to eruption of the craters of Laki in 1783 -84. The craters are a part of a larger volcanic system with the subglacial Grímsvötn as a central volcano. Because most of Grímsvötn’s eruptions have been subglacial, the interaction of magma and meltwater from the ice causes phreatomagmatic explosions

Tephra forms Tuff Ignimbrite or Welded Tuff Source: Gerald & Buff Corsi/Visuals Unlimited, Inc.

Erosional Remnants • Plugs • Dikes Ship Rock, NM

Intrusive terrains - Laccoliths First studied in 1875 -1876 by Grove Karl Gilbert. The term laccolith) is used to describe the characteristic shapes of some of the igneous intrusions that core the mountains. • Laccoliths - Henry Mts. • Colorado Plateau

Intrusive terrains - Sierra Nevada Batholith