magma composition at volcanoes

magma composition

There are as many types of volcanic eruptions as there are eruptions. Actually more since an eruption can change character as it progresses. Each volcanic eruption is unique, differing in size, style, and composition of erupted material. One key to what makes the eruption unique is the chemical composition of the magma that feeds a volcano, which determines (1) the eruption style, (2) the type of volcanic cone that forms, and (3) the composition of rocks that are found at the volcano. Different minerals within a rock melt at different temperatures. The amount of partial melting and the composition of the original rock determine the composition of the magma. The words that describe composition of igneous rocks also describe magma composition. Mafic magmas are low in silica and contain more dark, magnesium- and iron-rich mafic minerals, such as olivine and pyroxene. Felsic magmas are higher in silica and contain lighter colored minerals such as quartz and orthoclase feldspar. The higher the amount of silica in the magma, the higher is its viscosity. Viscosity is a liquids resistance to flow. Viscosity determines what the magma will do. Mafic magma is not viscous and will flow easily to the surface. Felsic magma is viscous and does not flow easily. Most felsic magma will stay deeper in the crust and will cool to form igneous intrusive rocks such as granite and granodiorite. If felsic magma rises into a magma chamber, it may be too viscous to move, so it gets stuck. Dissolved gases become trapped by thick magma. The magma churns in the chamber and the pressure builds. Magma collects in magma chambers in the crust at 160 kilometers (100 miles) beneath the surface. Click image to the left or use the URL below. URL:

instructional diagrams

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questions

knowing the magma composition will determine

a) eruption style

b) type of volcanic cone that will form

c) composition of rock found at the volcano

-->  d) all of the above

felsic lava is more likely to stay deeper in the crust and cool slowly.

-->  a) true

b) false

this is a region below a volcano where magma and gases collect.

-->  a) magma chamber

b) vent

c) pipe

d) crater

if magma is high in silica and more viscous, what type of eruption will form?

a) effusive

-->  b) explosive

c) alternative

d) pahoehoe

what causes an eruption to take place?

a) the composition of the magma.

b) the location of the magma beneath the volcano.

c) the viscosity of the magma.

-->  d) the amount of dissolved gases that become trapped by the magma.

lava cools so quickly that there are never any visible mineral grains in it.

a) true

-->  b) false

if you find a light colored ash, containing minerals such as quartz, you know that this represents

a) a mafic magma that erupted effusively.

b) a felsic magma that erupted effusively.

-->  c) a felsic magma that erupted explosively.

d) a mafic magma that erupted explosively.

magma composition is determined by

-->  a) the composition of the parent rock and the amount of partial melting it underwent.

b) the amount of fractional crystallization and the composition of the parent rock.

c) the viscosity of the parent rock and whether it undergoes partial melting or fractional crystallization.

d) the plate tectonic setting of the volcano.

diagram questions

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