chapter 5 geology

Solidified lava flows and volcanic ash deposits are examples of

extrusive igneous rocks

The difference between sills and dikes is that

sills are parallel to layering in surrounding rocks while dikes cut across the layering.

The grain size of an igneous rock is determined by the

rate of cooling, which controls the amount of time ions have to migrate to crystal seeds.

Formation of large grains in an igneous rock is aided by

magma with a low viscosity, very slow cooling, and the presence of water in the magma

Glassy igneous rocks

have no minerals.

A fine-grained texture most likely formed when a magma cooled

close to the Earth’s surface

A porphyritic texture indicates that an igneous rock had

two different cooling rates.

__________________ occurs at ocean ridges, whereas __________________ occurs at subduction zones.

Decompression melting; flux melting

The presence of water

lowers the melting temperature during flux melting, helps the magma rise toward the surface, and causes explosive volcanic eruptions

Where are intermediate igneous rocks (andesite and diorite) most commonly found?

subduction zones

What causes the steep slopes of stratovolcanoes (composite cones)?

large volumes of hot tephra that weld to one another

Which of the following is least likely to be associated with eruption of a shield volcano?

a pyroclastic flow (nuée ardente)