Geology chapter 4

from the partial melting of the ultramafic rock peridotite

How are basaltic magmas thought to be formed?

by their textures and compositions

How are igneous rocks classified?

magma mixing crystal settling assimilation magmatic differentiation

How can a magma body change its composition?

Dikes are discordant igneous intrusions; sills are concordant igneous intrusions; both are tabular.

Select the correct description of dikes and sills.

Melt

Which component of magma includes mobile ions of silicon?

Gabbro is mafic; obsidian is felsic; granite is felsic; andesite is intermediate.

Which of the following choices correctly classifies the four listed igneous rocks by composition?

After the earliest formed minerals have settled, the resulting magma will be more felsic than the original magma.

How do the crystallization and settling of the earliest formed minerals affect the composition of the remaining magma?

When hot, solid mantle rock ascends, it moves to areas of lower pressure. This reduction in pressure allows the hot material to melt into a less compressed liquid phase.

How does decompression melting occur?

Magma exists strictly below Earth's surface, whereas lava is found on Earth's surface.

How does magma differ from lava?

The liquid igneous material cools, the ions within slow in their movement, and they pack more closely together. When sufficiently cooled, ions become confined to a crystalline arrangement.

How does the crystallization of igneous material occur?

at a mid-oceanic ridge and at an oceanic-continental convergent boundary

In which two plate tectonic settings would one expect magma to be generated?

The geothermal gradient is the increase in temperature with depth under the Earth's surface. The melting curve of peridotite is consistently higher than the geothermal gradient.

What is the geothermal gradient, and how does it relate to the melting curve of peridotite?

aphanitic, phaneritic, porphyritic, vesicular, glassy, and pyroclastic

What are the six major igneous rock textures?

the liquid portion (mobile ions of the eight most common minerals found in the Earth's crust) called melt, the solid portion which consists of silicate mineral crystals, and the gaseous portion called volatiles

What are the three components of magma?

The rock began cooling under the Earth's surface, but then was moved to above the Earth's surface where it finished cooling.

What does a porphyritic texture indicate about the cooling history of an igneous rock?

the rate at which the rock cools, the amount of silica that is present in the magma from which it forms, and the amount of dissolved gases present in said magma

What factors influences the texture of an igneous rock?

the order in which specific minerals cool and form within melted magma, from hottest to coolest

What is Bowen's reaction series?

Extrusive igneous rocks cool and solidify on the Earth's surface, whereas intrusive igneous rocks cool and solidify beneath the Earth's surface.

What is the difference between extrusive igneous rocks and intrusive igneous rocks?

A basaltic pond of magma becomes trapped below continental crust and partially melts this silica-rich crust into a granitic magma.

What is the process that is thought to generate most granitic magmas?

The addition of water and other volatiles to rock allows the rock to melt at lower temperatures than if they had not been added.

What role do water and other volatiles play in the formation of magma?

Silicates

Which group of minerals makes up most igneous rocks?

granitic, andesitic, basaltic, ultramafic

Which of the choices below lists the four basic compositional groups of igneous rocks, ranging from the highest silica content to the lowest silica content?

Diorite is intermediate with a phaneritic texture; rhyolite is felsic with an aphanitic texture; basalt porphyry is mafic with a porphyritic texture.

Which of the following choices classifies the three listed rocks by composition and texture?

phaneritic texture

Which of the following is a characteristic of an igneous rock found in a batholith?

rhyolite

Which of the following is an aphanitic igneous rock?

pyroxene

Which of these minerals would be the first to crystallize in a body of magma?

Because different minerals melt at different temperatures, minerals that melt at lower temperatures will melt from the rock first, creating a magma formed from the minerals that melted first.

Why does partial melting result in a magma having a composition different from the rock from which it was derived?

Plagioclase feldspar and quartz; pyroxene and olivine

____ are minerals typically found in igneous rocks that have a high silica content, whereas ____ are minerals that are typically found in igneous rocks with a low silica content.

basalt

A dike is mostly likely to be composed of which of the following igneous rocks?

phaneritic

An igneous rock with a ______ texture solidified deep within Earth.

the introduction of water

At a subduction zone, melting is triggered by _____.

Large crystals in pegmatites are the result of a high percentage of the melt being composed of water, carbon dioxide, and other materials. These materials enhance ion migration, allowing the crystals to grow larger than usual.

How do crystals in pegmatite grow so large?

Assimilation occurs when magma that is injected into bedrock melts the rock directly surrounding it, incorporating it into the magma, whereas magma mixing occurs when two ascending bodies of magma join and mix together into a composition that is intermediate to the two original magmas.

Choose the statement that describes two processes that alter the composition of magma.

felsic; crystal size

Granite and rhyolite are both ____, but they differ in ____.

Batholiths are the largest type of igneous bodies and occur in a linear fashion with a distance of 100km or more; stocks are smaller than batholiths; laccoliths bend the sedimentary layers above them, whereas the sedimentary layers below remain relatively undeformed.

How do batholiths, stocks, and laccoliths differ?

Dark-colored igneous rocks have a higher iron and magnesium content than light-colored rocks.

How do light-colored igneous rocks differ from dark-colored rocks?

Quartz and potassium feldspar

The most abundant minerals found in granitic (felsic) igneous rocks include ______.

peridotite

The partial melting of ____ will yield mafic (basaltic) magma.

the appearance of a rock based on the size(s), shape(s), and arrangement on the mineral grains it contains.

The texture of an igneous rock describes

the crustal rocks into which igneous bodies intrude

To what does the term "country rock" refer?