Volcanism

Question 1

What is a volcano?

Answer 1

Structure in the Earth’s crust with an opening at the end of central vent or pipe through which magma rises.

Question 2

Define magma and lava.

Answer 2

Magma: molten rock, crystals, gases beneath surface.

Lava: magma once it reaches surface.

Question 3

List three brief reasons why magma forms.

Answer 3

Decompression of hot rocks.

Addition of volatiles (mainly water).

Transfer energy to surrounding rocks.

Question 4

Volatiles break _____ bonds allowing the material to _____.

Answer 4

Silica; flow.

Question 5

Water comes from rock that is what? What margin does this happen in?

Answer 5

Subducted.

Convergent.

Question 6

Overlying rocks are less _____ so magma _____.

Answer 6

Dense; pools.

Question 7

Heat transfer occurs where?

Answer 7

Subduction zones.

Question 8

Decompression is a release of pressure akin to what?

Answer 8

Like from a soda can.

Question 9

Decompression occurs where?

Answer 9

Divergent margin, mantle upwells.

Question 10

Mantle plumes are also known as what?

Answer 10

“Hot spots.”

Question 11

What are the three “types” of volcano?

Answer 11

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.

Question 12

What are the two types of eruption?

Answer 12

Central vent.

Fissure.

Question 13

Volcanoes produce what three things?

Answer 13

Lava: basalt, andesite, rhyolite.

Tephra (fragmental material): ash, lapilli, bombs, blocks.

Gas.

Question 14

Volcanic behaviour is controlled by _____.

Answer 14

Viscosity.

Question 15

What three elements control viscosity regarding volcanoes?

Answer 15

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.

Question 16

What are effusive eruptions?

Answer 16

Low viscosity magma, enormous volumes of lava, flow directly from asthenosphere and upper mantle.

Question 17

What are explosive eruptions?

Answer 17

Violent explosions of magma, gas, pyroclastics driven by buildup pressure in magma conduit.

Question 18

The composition of magma can consist of basalt, andesite, and rhyolite. Explain each with regards to silica, gas content, and temperature.

Answer 18

Basalt: low silica, low gas content, high temperature.

Andesite: intermediate in all things.

Rhyolite: high silica, high gas content, low temperature.

Question 19

Volatile content generally increases with what? How does this relate to basalt, andesite, and rhyolite?

Answer 19

Silica content.

Basalt: <1% volatiles by weight.

Rhyolite and andesite: 2-5% volatiles by weight.

Question 20

High volatile content are more likely to be explosive. Why is this?

Answer 20

Dissolved volatiles are released from the magma during decompression.

Question 21

90% of lava is _____.

Answer 21

Basalt.

Question 22

How are lava tubes formed and what do they do?

Answer 22

Crust of rock insulates the lava, allows lava to flow 10s of kilometres.

Question 23

Describe shield volcanoes in terms of size, slope, and magma content.

Answer 23

Largest on Earth.

Gently sloping sides.

Basaltic magma.

Question 24

What is another term for a composite volcano? What is their eruption frequency and their magma content?

Answer 24

Stratovolcano.

Erupt less frequently than shield.

Andesitic magma composition.

Question 25

Describe the layering of composite volcanoes and provide two examples of these volcanoes.

Answer 25

Interlayered lavas and pyroclastic deposits.

Mt. Fuji, Mt. Vesuvius.

Question 26

Describe cinder cone volcanoes in terms of size, what they are made up of, magma content, and eruption frequency.

Answer 26

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).

Question 27

Describe lava domes in terms of how they are formed, eruption style, and slope.

Answer 27

Formed by viscous magma erupting effusively, then piling up around vent.

Insufficient pressure to erupt explosively.

Steep sided.

Question 28

Describe the process of explosive eruptions.

Answer 28

Steam bubbles separate from rhyolitic magma. Froth expands to form pumice. When whole volcano explodes, pumice forms ash and gas is released.

Question 29

What are phreatic eruptions?

Answer 29

Violent, steam driven; magma vaporizes shallow groundwater or surface water; very little solid material.

Question 30

What is laze and vog?

Answer 30

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.

Question 31

What is the danger of pyroclastic eruptions?

Answer 31

Volcanic ash may be blown downwind, spread for 1000s km.

Question 32

In order, what are the four primary gases produced by volcanoes?

Answer 32

Water vapor.

Carbon dioxide.

Sulfur dioxide.

Hydrogen sulfide.

Question 33

What are four hazards posed by volcanic gases?

Answer 33

Ozone depletion (chlorine).

Aerosols (climate cooling).

Acid rain (sulfur, chlorine).

Poison (chlorine and fluorine).

Question 34

What is a pyroclastic flow? What temperatures do they reach and how far can they travel?

Answer 34

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.

Question 35

What is a pyroclastic surge?

Answer 35

Contain higher proportion of gas versus rock fragments than a pyroclastic flow, can flow over higher topographies.

Question 36

What are lahars? How do they form?

Answer 36

Mud or debris flow of pyroclastic material, debris, and water typically flowing along a river valley.

Volcanism melts ice cap, or flows into river.

Question 37

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?

Answer 37

30m deep, moved 12 m/sec.

Miscommunication (slow notices, newspapers printed maps w/o scales, officials underestimated risk).

Question 38

Volcanoes can cause tsunamis. How is this?

Answer 38

Lateral blast sends volcano flank crashing into sea, tsunami forms and propagates to distant coastlines.