GEOL 101 Chapter 4

How are igneous rocks classified?

By their textures and compositions.

Why does magma composition change fractional crystallization?

Different elements in the magma form crystals at different rates, leaving behind more of the unused elements.

Use your understanding of Bowen's Reaction Series to determine which of the rocks below will crystallize the fastest. The chemical composition of the rocks is: - DIORITE: amphibole, sodium- and calcium-rich plagioclase feldspar, pyroxene, and biotite - GABBRO: pyroxene, calcium-rich plagioclase feldspar, amphibole, and olivine - GRANITE: quartz, potassium feldspar, sodium-rich plagioclase feldspar, amphibole, muscovite, and biotite - PERIDOTITE: olivine, pyroxene, and calcium-rich plagioclase feldspar

Peridodite

Most igneous rocks are primarily composed of:

Silicate minerals.

Which mineral group makes up most igneous rocks?

Silicates

Ultramafic rocks contain ___________ and are commonly found in __________.

olivine; mantle

Can you determine what type of igneous rock would result from various cooling histories and locations? Review the following rock photographs and determine their cooling histories. Place each image in its appropriate bin.

Intrusive Igneous Rock- DIORITE & GRANITE Extrusive Igneous Rock- RHYOLITE Porphyritic Igneous Rock- ANDESITE PORPHYRY

What element is principally responsible for making olivine, pyroxene, amphibole, and biotite dark?

Iron

What is one way a glassy texture forms?

Lava cools rapidly.

What type of magma would you expect to see after crystallization of minerals containing significant amounts of iron?

A magma depleted in iron.

What is an accessory mineral?

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

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

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

What is the rock name of an intermediate rock with 2 distinct grain size?

Andesite Porphyry

A geologist sends you a sample that was collected in the field. You find that it is a gray-colored igneous rock containing amphibole and intermediate plagioclase feldspar. Using Bowen's Reaction Series as a guide, how would you describe its composition?

Andesitic

How is basaltic magma transformed into felsic magma?

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

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.

What is fractional crystallization?

Crystallization of part of a magma, leaving behind melted rock with a different composition from the original magma.

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

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

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

Decompression melting of the mantle.

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

Select the correct description of dikes and sills.

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

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

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

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

Felsic magma and Mafic residue

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

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

Granite is phaneritic and rhyolite is aphanitic.

Which of the choices below lists the 4 basic compositional groups of igneous rocks, ranging from the highest silica content to the lowest silica content?

Granitic, Andesitic, Basaltic, Ultramafic

Match the igneous composition to its silica content.

Mafic~ 50% OF SILICA CONTENT Felsic~ 70% OF SILICA CONTENT Intermediate~ 60% OF SILICA CONTENT

How does magma differ from lava?

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

The light silicate minerals include ____________.

Muscovite, Orthoclase feldspar, Plagioclase feldspar, and quartz

If an igneous rock is heated, which mineral will be the very last mineral will be the very last mineral left to melt according to the Bowen's Reaction Series?

Olivine

The progressive nucleation of minerals from the melt leaves the magma depleted in certain elements. This change in magma composition is called magmatic differentiation and accounts for the great variety of igneous rocks found in nature. The separation of liquid and solid phases through crystal settling further contributes to this diversity. Use your understanding of Bowen's Reaction Series and crystal settling to determine which of the following mineral associations should not be found in nature.

Olivine, Potassium Fledspar, Quartz

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

Pegmatitic

Which of the following textures indicates 2 stages of cooling and crystallization?

Porphyritic

Basaltic igneous rocks contain which of the following minerals:

Pyroxene and Calcium-rich plagioclase feldspar.

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

Quartz

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 statements best represents the relationship between magma and crystals formed from that magma?

The crystals are denser than the magma.

What are the 3 components of magma?

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 gaseous portion called volatiles.

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

The rock was likely formed by a violent volcanic eruption.

What do pumice and scoria have in common?

They both exhibit a vesicular texture.

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

Vesicular

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

the melting temperature of mantle rocks to decrease.