Volcanism Flashcards
What is a volcano?
Structure in the Earth’s crust with an opening at the end of central vent or pipe through which magma rises.
Define magma and lava.
Magma: molten rock, crystals, gases beneath surface.
Lava: magma once it reaches surface.
List three brief reasons why magma forms.
Decompression of hot rocks.
Addition of volatiles (mainly water).
Transfer energy to surrounding rocks.
Volatiles break _____ bonds allowing the material to _____.
Silica; flow.
Water comes from rock that is what? What margin does this happen in?
Subducted.
Convergent.
Overlying rocks are less _____ so magma _____.
Dense; pools.
Heat transfer occurs where?
Subduction zones.
Decompression is a release of pressure akin to what?
Like from a soda can.
Decompression occurs where?
Divergent margin, mantle upwells.
Mantle plumes are also known as what?
“Hot spots.”
What are the three “types” of volcano?
Active: currently erupting or showing unrest; if it has erupted in historical time.
Extinct: scientists consider unlikely to erupt again.
Dormant: not currently active, but could become restless/erupt again.
What are the two types of eruption?
Central vent.
Fissure.
Volcanoes produce what three things?
Lava: basalt, andesite, rhyolite.
Tephra (fragmental material): ash, lapilli, bombs, blocks.
Gas.
Volcanic behaviour is controlled by _____.
Viscosity.
What three elements control viscosity regarding volcanoes?
Silica content: higher content = stronger bonds; higher iron content, silica not linked.
Volatile content: reduced pressure allows gases to form vapor.
Temperature: higher = lower viscosity.
What are effusive eruptions?
Low viscosity magma, enormous volumes of lava, flow directly from asthenosphere and upper mantle.
What are explosive eruptions?
Violent explosions of magma, gas, pyroclastics driven by buildup pressure in magma conduit.
The composition of magma can consist of basalt, andesite, and rhyolite. Explain each with regards to silica, gas content, and temperature.
Basalt: low silica, low gas content, high temperature.
Andesite: intermediate in all things.
Rhyolite: high silica, high gas content, low temperature.
Volatile content generally increases with what? How does this relate to basalt, andesite, and rhyolite?
Silica content.
Basalt: <1% volatiles by weight.
Rhyolite and andesite: 2-5% volatiles by weight.
High volatile content are more likely to be explosive. Why is this?
Dissolved volatiles are released from the magma during decompression.
90% of lava is _____.
Basalt.
How are lava tubes formed and what do they do?
Crust of rock insulates the lava, allows lava to flow 10s of kilometres.
Describe shield volcanoes in terms of size, slope, and magma content.
Largest on Earth.
Gently sloping sides.
Basaltic magma.
What is another term for a composite volcano? What is their eruption frequency and their magma content?
Stratovolcano.
Erupt less frequently than shield.
Andesitic magma composition.
Describe the layering of composite volcanoes and provide two examples of these volcanoes.
Interlayered lavas and pyroclastic deposits.
Mt. Fuji, Mt. Vesuvius.
Describe cinder cone volcanoes in terms of size, what they are made up of, magma content, and eruption frequency.
Relatively small (100-200m high).
Made up of nut-to apple-sized pieces of “scoria.”
Low viscosity basaltic magma encounters groundwater.
Erupt for short periods of time (few months to years).
Describe lava domes in terms of how they are formed, eruption style, and slope.
Formed by viscous magma erupting effusively, then piling up around vent.
Insufficient pressure to erupt explosively.
Steep sided.
Describe the process of explosive eruptions.
Steam bubbles separate from rhyolitic magma. Froth expands to form pumice. When whole volcano explodes, pumice forms ash and gas is released.
What are phreatic eruptions?
Violent, steam driven; magma vaporizes shallow groundwater or surface water; very little solid material.
What is laze and vog?
Laze: lava + haze. Acidic air pollution from steam explosions where lava pours into the ocean.
Vog: sulfur dioxide and particles from volcano react with moisture in the presence of sunlight.
What is the danger of pyroclastic eruptions?
Volcanic ash may be blown downwind, spread for 1000s km.
In order, what are the four primary gases produced by volcanoes?
Water vapor.
Carbon dioxide.
Sulfur dioxide.
Hydrogen sulfide.
What are four hazards posed by volcanic gases?
Ozone depletion (chlorine).
Aerosols (climate cooling).
Acid rain (sulfur, chlorine).
Poison (chlorine and fluorine).
What is a pyroclastic flow? What temperatures do they reach and how far can they travel?
Hot gas and ash (tephra), heavier parts of cloud collapse onto volcano flank.
Temperatures 400-800C.
Air trapped below the ash, increases speed; travel 20+km at 200km.
What is a pyroclastic surge?
Contain higher proportion of gas versus rock fragments than a pyroclastic flow, can flow over higher topographies.
What are lahars? How do they form?
Mud or debris flow of pyroclastic material, debris, and water typically flowing along a river valley.
Volcanism melts ice cap, or flows into river.
In Nevado del Ruiz, Columbia, an eruption melted the ice cap. What was the largest lahar this produced, and why was evacuation of the town hampered?
30m deep, moved 12 m/sec.
Miscommunication (slow notices, newspapers printed maps w/o scales, officials underestimated risk).
Volcanoes can cause tsunamis. How is this?
Lateral blast sends volcano flank crashing into sea, tsunami forms and propagates to distant coastlines.
