Igneous Rocks

How are igneous rocks formed?

Igneous rocks are formed by the cooling and crystallization of molten rock.

Intrusive and extrusive igneous rocks are classified based on what criteria?

How quickly the molten rock cools and crystallizes, and where this occurs with respect to Earth’s surface

What is required to generate an igneous rock with a fine-grained texture?

A fine-grained texture is the product of rapid cooling and crystallization of lava.

A phaneritic texture is characteristic of a(n) __________ igneous rock that cooled __________.

intrusive; below the surface

What does it mean if an igneous rock has an aphanitic texture?

The igneous rock exhibits mineral crystals too small to see with the naked eye.

What three components make up most magmas?

Liquid Portion, Solid Portion, Gaseous Portion

name two minerals that are unlikely to coexist in the same igneous rock.

Muscovite and pyroxene

Igneous rocks of felsic composition have __________ and __________ relative to mafic igneous rocks.

greater silica; lower melting temperature

Which of the following is true about mafic igneous rocks?

Mafic igneous rocks are rich in Fe, Mg, and Ca.

You have found a mafic rock with a phaneritic texture. What is a potential rock name?

Gabbro

what minerals make up rhyolite?

Quartz, K-feldspar, and plagioclase feldspars, with relatively fewer biotite and amphiboles

Which mineral class makes up most igneous rocks?

Silicates

Peridotite is the main constituent of which part of the Earth?

Upper mantle

Which two minerals define a felsic composition?

Quartz and Potassium Feldspar

What is one way a glassy texture forms?

Lava cools rapidly.

Which of the following describes a texture containing bubble-shaped cavities?

Vesicular

Which of the following textures describes an igneous rock with anomalously large crystals?

Pegmatitic

What does an igneous rock with a pyroclastic texture tell a geologist?

the rock was likely formed by a violent volcanic eruption.

Which of the following textures indicates two stages of cooling and crystallization?

Porphyritic

Which of the following igneous rocks contains no mineral crystals?

obsidian

What is the term used to describe a very large, widespread, and deep intrusive body of igneous rock?

batholith

In what type of igneous feature would you find rocks with a vesicular texture?

the surface of a lava flow

Working in the field, you see a thin, dark-colored igneous sill across the valley. Without seeing the sill up close, what texture would you expect it to possess?

fine-grained

Working in the field, you come upon a light-colored, fine-grained igneous rock that contains some very large dark-colored crystals. How do you account for this complex mineralogy?

The dark crystals formed first, undergoing slow cooling, and the remainder cooled quickly.

Ultramafic rocks contain __________ and are commonly found in __________.

olivine; mantle

What is an accessory mineral?

A mineral that makes up a relatively small portion of the total rock composition

Which of the following best describes the difference between granite and rhyolite?

Granite is phaneritic and rhyolite is aphanitic.

What is the rock name of an intermediate rock with two distinct grain sizes?

Andesite porphyry

What do pumice and scoria have in common?

They both exhibit a vesicular texture.

At a subduction zone, melting is triggered by _____.

the introduction of water

Which is the following statements best describes the relationship between pressure and melting point in the Earth’s Interior?

Higher melting points are determined by higher pressures

What is the term used to describe increased temperature with depth in the Earth?

Geothermal gradient

How can a magma body change its composition? CHOOSE ALL THAT APPLY.

Magmatic differentiation Crystal settling Magma mixing Assimilation

________ describes how an entire suite of silicate minerals can form from a single basaltic magma as it cools and crystallizes.

Bowen’s Reaction Series

Which mineral on the Bowen’s Reaction Series will be the last to melt if an igneous rock is heated?

Olivine

Which mineral on the Bowen’s Reaction Series will be the first to melt if an igneous rock is heated?

Quartz

Once a source rock partially melts, what does it produce?

Felsic magma and mafic residue

According to Bowen’s Reaction Series, __________ is one of the first minerals to melt, but last to crystallize.

quartz

How is basaltic magma transformed into felsic magma?

Basaltic magma partially melts the continental crust, which is more felsic in composition.

What causes an elevated geothermal gradient in a divergent plate setting?

Decompression melting of the mantle

In a subduction zone, water driven from subducted oceanic crust causes __________.

the melting temperature of mantle rocks to decrease

Which process is thought the generate most felsic magmas?

Heat from basaltic magma partially melting overlying crust

What is the term for a tabular igneous pluton that occurs in an orientation that is discordant with the bedding surfaces of adjacent sedimentary rocks?

dike

What is the term for a tabular igneous pluton that occurs in an orientation that is concordant with the bedding surfaces of adjacent sedimentary rocks?

sill

Which of the following terms is best described as a massive pluton?

batholith

Batholiths are known to form the cores of mountain ranges such as the Sierra Nevada. What is the implication of this observation?

Because batholiths form deep within Earth, the occurrence of one at the surface implies that it has since been uplifted and/or the rocks above have been removed.

While working in the field, you find a dike cutting across some sedimentary rocks. What grain size would you expect to find in this pluton?

fine-grained