GEOL 101 – Chapter 4: Magma, Igneous Rocks and Intrusive Activity

GEOL 101 – Chapter 4: Magma, Igneous Rocks and Intrusive Activity

Igneous Rocks
Form as molten rock (magma) cools and solidifies
Parent material of igneous rocks

Forms from partial melting of rocks

Magma at surface is called Lava

Magma at surface
Dissolved gases in the melt that vaporize at surface pressure
The cooling of magma which results in the systematic arrangement of ions into orderly patterns
Plutonic (Intrusive Igneous Rocks)
Magma that crystallizes at depth

These rocks are observed at the surface following periods of uplifting and erosion of overlying rocks.

** Inside the earth**

Volcanic (Extrusive Igneous Rocks)
The solidification of lava or volcanic debris

**Outside the Earth**

Granitic (Felsic Composition)
Light-colored silicates

Composed almost entirely of quartz and potassium feldspar

Termed felsic (feldspar and silica) in composition

High silica (SiO2) content

Major constituent of continental crust

Basaltic (Mafic Composition)
Dark silicates and calcium-rich felspar

Termed mafic (magnesium and ferrum, for iron) in composition

Higher density than granitic rocks

Comprise the ocean floor and many volcanic islands

Andesitic (Intermediate Composition)
Contain 25% or more dark silicate minerals (amphibole, pyroxene, and biotite mica)
Ultramafic Composition
Rare composition of mostly olivine and pyroxene

Composed almost entirely of ferromagnesium minerals

Peridotite is an example

The overall appearance of a rock based on the size, shape, and arrangement of mineral grains.
Aphanitic (Fine-Grained) Texture
Rapid rate of cooling

Microscopic crystals

Phaneritic (Coarse-Grained) Texture
Slow cooling

Large, visible crystals

Porphyritic Texture
Some minerals can grow large before others form from the magma

The magma can move to a different environment which causes the remaining minerals to form quickly

Large crystals (phenocrysts) are embedded in a matrix of smaller crystals (groundmass)

Large crystals
Smaller crystals
Vesicular Texture
Rocks contain voids left by gas bubbles in the lava

Common feature of an extrusive igneous rock

Glassy Texture
Very rapid cooling

Ions are frozen in place before they can unite in an orderly crystalline structure

Once used as arrowheads, they used in scalpels (less scarring)

Pyroclastic (fragmental) Texture
Forms from the consolidation of individual rock fragments ejected during explosive eruptions
Pegmatitic Texture
Exceptionally coarse-grained

Form in late stages of crystallization of magmas

Rocks with this texture are called pegmatites

Coarse-grained (phaneritic)

One of the best known igneous rocks

Very abundant

Natural beauty, especially when polished

10% – 20% quartz, roughly 50% potassium feldspar

Small amounts of dark silicates

Some granites have a porphyritic texture

Contain elongated feldspar crystals

Extrusive equivalent of granite

Composed essentially of light-colored silicates

Typically pink or light gray in color

Less common and less voluminous than granite

Dark-colored, glassy rock

Forms when silica-rick lava cools quickly at Earth’s surface

Usually black to reddish-brown

Similar chemical composition of granite

Dark color is the result of small amounts of metallic ions in an otherwise clear, glassy substance

Glassy textured rock that forms when large amounts of gas escape from lava

Voids are quite noticeable

Resembles fine shards of intertwined glass

Typically found in deposits with obsidian

Will float when placed in water

Medium-gray, fine-grained rock

Volcanic origin

Commonly exhibits a porphyritic texture

Intrusive equivalent of andesite

Coarse-grained rock

Looks like gray granite, but lacks visible quartz crystals

Can have a salt-and-pepper appearance

Very dark green to black, fined-grained rock

Composed mostly of pyroxene and calcium-rich plagioclase feldspar

When porphyritic, contains small, light-colored feldspar phenocrysts

Most common extrusive igneous rock

Upper layers of oceanic crust are composed of basalt

Intrusive equivalent of basalt

Very dark green to black, coarse grained rock

Composed mostly of pyroxene and calcium-rich plagioclase feldspar

Uncommon on the continental crust, but makes up a significant portion of the oceanic crust

Geothermal Gradient
Temperatures in the upper crust increase about 25 degree C per kilometer
Decompression Melting
Melting occurring at higher temperatures with increasing depth

Reducing pressure lowers the melting temperature

Solid, hot mantle rocks will ascend to regions of lower pressure, inducing melting

Bowen’s Reaction Series
Minerals crystallize in a systematic fashion based on their melting points

As minerals crystallize, the composition of the liquid portion of the magma continually changes

Crystal Settling
Earlier-formed minerals and denser than the liquid portion of the magma and sink to the base of the magma chamber
Magmatic Differentiation
The formation of one or more secondary magmas from a single parent magma
As magma migrates through the crust, it may incorporate some of the surrounding rock
Magma Mixing
During the ascent of two chemically different magma bodies, the more buoyant mass may overtake the slower-rising body
Cooled, emplaced magma into preexisting rocks

Classified by their orientation to the surrounding rock

Cut across existing structures
Parallel to features like sedimentary strata
Irregularly shaped
a tabular, discordant pluton

Serves as tabular conduits to transport magma

Parallel groups are called dike swarms

A tabular, concordant pluton

Tend to accumulate magma and increase in thickness

Closely resembles buried lava flows

Largest intrusive body

Surface exposure of 100+ square kilometers (smaller bodies are termed stocks)

While expansive, most are less than 10km thick

Suspended blocks of country rocks found in plutons
Forcibly injected between sedimentary strata

Causes the overlying strata to arch upward

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 ________.

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
Based on the diagram shown in the video, 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?
Based on what you learned in this video, 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?
Peridotite is the main constituent of which part of the Earth?
Upper Mantle
Which two minerals define a felsic composition?
Quartz and Potassium Feldspar
Can you determine what types of igneous rocks share the same minerals composition, but are different in texture, and are therefore different rocks?
Can you determine what types of igneous rocks share the same minerals composition, but are different in texture, and are therefore different rocks?
What is one way a glassy texture forms?
Lava cools rapidly.
Which of the following describes a texture containing bubble-shaped cavities?
Which of the following textures describes an igneous rock with anomalously large crystals?
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?
Which of the following igneous rocks contain no mineral crystals?
What is the term used to describe a very large, widespread, and deep intrusive body of igneous rock?
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?
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 _______.


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?
The both exhibit a vesicular texture.
At a subduction zone, melting is triggered by ______.
The introduction of water
Which is the following statement 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?

Magma mixing

Crystal settling

Magmatic differentiation

_______ describes how an entire suite of silicate minerals can form a single basaltic magma as it cools and crystalizes.
Bowen’s Reaction Series
Which mineral on the Bowen’s Reaction Series will be the last to melt if an igneous rock is heated?
Which mineral on the Bowen’s Reaction Series will be the first to melt if an igneous rock is heated?
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.
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 settling?
Decomposition 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 to generate most felsic magmas?
Heat from basaltic magma partially melting overlying crust
Which is the term for a tabular igneous pluton that occurs in an orientation that is discordant with the bedding surfaces of adjacent sedimentary rocks?
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?
Which of the following terms is best describes as a massive pluton?
Batholiths are known to form the cores of mountains 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 if 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?