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ANSWERS TO STUDY QUESTIONS CHAPTER 5 - Volcanism

1. SILICA - makes magmas more viscous - slowing down the flow of lava, but increasing the danger of a violent explosion if the magma contains a lot of gas.

TEMPERATURE - cooler magma temperatures affect an eruption in much the same way as high silica contents do.

GAS CONTENT - although elevated gas concentrations will make a magma more fluid, the main role that gas plays is in controlling the power of an eruption. Without gas, the eruption of any type of magma will be rather passive. With gas, the degree of violence depends on the viscosity of the magma (see silica and temperature above).

2. A highly viscous magma will inhibit the release of gas and thereby promote the build-up of gas pressure. Eventually the pressure may become high enough to tear apart the viscous magma that confines the gas, and all the pent-up gas is released at once creating a colossal explosion. More fluid magmas release gas more gradually and evenly, thereby preventing gas pressures from becoming dangerously high.

3. For each of the following combinations of magma composition, temperature, and amount of gas, describe the nature of its eruption, and name the type of volcanic feature(s), rock type(s) and texture(s) produced.

SILICA CONTENT

TEMP

GAS CONTENT

ERUPTION

VOLCANIC FEATURE(S)

ROCK TYPE(S)

TEXTURE(S)

Intermediate to high

High

High

Lots of ash, possible nuee ardent

Composite volcano

Tuff, tuff-breccia, welded tuff

Pyroclastic

Intermediate to high

High

Low

Viscous lava flows

Composite volcano

Rhyolite, andesite

Aphanitic

Intermediate to high

Low

High

Catastrophic explosion

Composite volcano, possible caldera

Breccia, tuff-breccia, tuff, welded tuff

Pyroclastic

Intermediate to high

Low

Low

A big dud. Lava barely flows from vent.

Volcanic dome on composite volcano

Rhyolite, andesite (porphyry?)

Aphanitic and often porphyritic

Low

High

High

Lava fountain feeding pa hoe hoe flows

Shield volcano, or fissure and flood basalt

Basalt

Aphanitic

Low

High

Low

Pa hoe hoe lava flows

Shield volcano, or fissure and flood basalt

Basalt

Aphanitic

Low

Low

High

Coarse pyroclastics: bombs, cinders, lapilli

Cinder cone

Volcanic breccia, scoria

Pyroclastic and/or vesicular

Low

Low

Low

Blocky lava flows (aa)

Shield volcano

Basalt

Aphanitic

4. Crater Lake formed following a giant eruption. The top of the volcano collapsed into it’s own magma chamber. Collapse occurred because the eruption left a void in the magma chamber.


5.

PLATE TECTONIC SETTING

CAUSE(S) OF MELTING

TYPE(S) OF MAGMA PRODUCED

VOLCANIC ACTIVITY

Ocean-ocean convergence

wet melting of asthenosphere above subducted plate

gas-rich and generally intermediate (andesitic)

both pyroclastic and lava flows erupt so composite volcanoes form

Ocean-continent convergence

wet melting of asthenosphere above subducted plate

gas-rich and generally intermediate, but more differentiation and partial melting happen in continental crust so mafic and felsic magma is also produced

both pyroclastic and lava flows erupt so composite volcanoes form, but greater magma variability can result in domes, cinder cones and rarely even shield volcanoes

Ocean-ocean divergence

decompression melting in asthenosphere

mafic (basaltic) and generally gas-poor

basaltic lava flows form oceanic crust

Continent-continent divergence

decompression melting in asthenosphere

mafic (basaltic) and generally gas-poor, but felsic can also form by partial melting at base of continental crust

basaltic lava flows form mostly  shield volcanoes but felsic magma combined with continental groundwater can lead to much more explosive volcanism

Intraplate volcanism (oceanic hot spot)

decompression melting in asthenosphere

mafic (basaltic) and generally gas-poor

basaltic lava flows form shield volcanoes

Intraplate volcanism (continental hot spot)

decompression melting in asthenosphere

mafic (basaltic) and generally gas-poor, but felsic can also form by partial melting at base of continental crust

basaltic lava flows form mostly  shield volcanoes but felsic magma combined with continental groundwater can lead to much more explosive volcanism

 

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This page was updated on 08/18/2014 09:03 PM

Last Updated: 01/13/2015
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