Geology Ch. 4

Rank the four major compositional groups of igneous rocks from highest silica content to lowest silica content.

felsic, intermediate, mafic, ultramafic

Has openings produced by escaping gases

Vesicular

Of the descriptions given, which rock type and environment has the highest concentration of rare elements? A pegmatite crystallized from a water-rich, highly differentiated residual magma Pumice crystallized in a mass of extrusive rock Peridotite crystallized at depth in the upper mantle Basalt dike crystallized in a vertical fracture at a shallow depth

A pegmatite crystallized from a water-rich, highly differentiated residual magma

Consists of crystals that are too small to be seen without a microscope

Aphanitic

Which of the following statements about crystallization from magma is false? (refer to Bowen's Reaction Series) As a magma cools, the plagioclase feldspar minerals formed become less sodium rich and more calcium rich with a decrease in temperature. As magma cools and minerals crystallize, the remaining melt becomes richer in silica. Quartz would be the last mineral to crystallize from a basaltic magma. Olivine would be the first mineral to crystallize from a basaltic magma. The first minerals to crystallize from a cooling magma have the lowest silica content and highest melting points.

As a magma cools, the plagioclase feldspar minerals formed become less sodium rich and more calcium rich with a decrease in temperature.

A vertical wall-like feature a few meters wide and hundreds of meters long.

Dike

Felsic magmas are less viscous than mafic magmas.

False

Results when magma cools too fast for mineral crystals to form; texture of obsidian

Glassy

A dome-shaped mountainous structure flanked by upturned layers of sedimentary rock.

Laccolith

Consider Bowen's reaction series. Which mineral would you expect to see as a phenocryst in a porphyritic basalt?

Olivine

Coarse-grained with crystals of roughly equal size

Phaneritic

Contains a matrix of fine crystals surrounding phenocrysts

Porphyritic

Which mineral gives some granites a pinkish color?

Potassium feldspar

A texture characterized by rock fragments welded together

Pyroclastic

A relatively thin layer of basalt sandwiched between horizontal layers of sedimentary rocks exposed along the walls of a river valley

Sill

A huge expanse of granitic rock forming a mountainous terrain tens of kilometers wide.

Stock

Which of the following statements about melting of rock is false? The first minerals to melt are those with the lowest silica content. Partial melting of intermediate rocks tends to yield felsic magmas. Most basaltic magmas form due to partial melting of ultramafic mantle rocks either by decompression melting or by hydration at subduction zones.

The first minerals to melt are those with the lowest silica content.

A porphyritic texture suggests two stages of cooling. An early stage of slow cooling and a subsequent stage of faster cooling.

True

Peridotite is the main constituent of the upper mantle.

True

The two basic characteristics used to classify igneous rocks are texture and mineral composition.

True

______________ refers to the removal and isolation of early-formed mineral grains that can cause the composition of the remaining magma to change.

crystal settling

What is the "melt" of a magma composed of?

mobile, silicon, oxygen, aluminum, potassium, calcium, sodium, iron, and magnesium ions