Pages within Study Question Answers

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.

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

This page has been viewed times since 8-18-14.

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

Last Updated: 01/13/2015

Housing

ANSWERS TO STUDY QUESTIONS CHAPTER 5 - Volcanism