Download Characteristics of Igneous Rocks

Characteristics of Igneous
Rocks
Dana Desonie, Ph.D.
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Printed: January 31, 2016
AUTHOR
Dana Desonie, Ph.D.
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C HAPTER
Chapter 1. Characteristics of Igneous Rocks
1
Characteristics of Igneous
Rocks
• Compare and contrast intrusive and extrusive igneous rock.
Are these both igneous rocks?
These rocks don’t even look like they’re the same type! They are, at least in the same way that fish and mice are
both vertebrates. They both cooled from magma, but the similarities end there. Can you tell what’s different?
TABLE 1.1: Igneous Rock References
1:
2:
3:
4:
Basalt
Gabbro
Granite
Obsidian
5:
6:
7:
8:
Pegmatite
Pumice
Rhyolite
Scoria
Magma & Igneous Rocks
Molten rock inside Earth is called magma. There are large pools of magma inside Earth’s upper mantle and lower
crust. Sometimes, magma rises through the cracks in rocks into the upper part of Earth’s crust. here, the temperature
is cooler than the temperatures inside the mantle. As magma rises through the cracks in Earth’s crust, it cools and
crystalizes forming intrusive igneous rocks. Intrusive igneous rocks have large crystals because they cool slowly
under Earth’s surface.
Granite is the most common intrusive igneous rock. Which of your rocks are intrusive igneous rocks, showing large
crystals?
Lava & Igneous Rocks
When magma rises through cracks in rocks and reaches Earth’s surface it is called lava. Lava cools upon contact
with air or water. Cooling makes lava harden into extrusive igneous rocks. Extrusive igneous rocks have small
crystals or no crystals at all due to the quickly cooling lava.
1
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FIGURE 1.1
Pegmatite is an intrusive igneous rock that has quartz, mica and feldspar
crystals that cooled slowly (large) then quickly (small).
(A) Lava cools to form extrusive igneous rock. The rocks here are basalts. (B) The strange rock formations of
Chiricahua National Monument in Arizona are formed of the extrusive igneous rock rhyolite.
Some extrusive igneous rocks cool so rapidly that crystals do not develop at all. These form a glass, such as obsidian.
Others, such as pumice, contain holes where gas bubbles were trapped in the lava. The holes make pumice so light
that it actually floats in water. The most common extrusive igneous rock is basalt. It is the rock that makes up the
ocean floor. Shown below are three types of extrusive igneous rocks ( Figure 1.2). Which of your rocks are extrusive
igneous rocks and have small crystals or no crystals?
FIGURE 1.2
Different cooling rate and gas content resulted in these different textures.
Igneous Rock Classification
The first step in classifying an igneous rock is to know its composition or what type of minerals it contains. Igneous
rocks are made up of many different kinds of minerals. However, there are only six minerals that are commonly
found in igneous rocks: quartz, feldspar, mica, olivine, amphibole, pyroxene. Look at a piece of granite. Can you
identify the three main minerals that compose this rock?
The composition of an igneous rock can be classified as felsic or mafic. The composition/color depends on the
minerals the rock includes. A felsic rock will contain felsic minerals. Felsic rocks are mostly light in color. Mafic
rocks are mostly dark in color. Try classifying your igneous rocks by their composition.
The mineral compositions are listed below ( Table 1.2). The table includes examples of minerals.
TABLE 1.2: Properties of Igneous Rock Compositions
Composition
Felsic
2
Color
Light
Density
Low
Minerals
Quartz,
feldspar
orthoclase
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Chapter 1. Characteristics of Igneous Rocks
TABLE 1.2: (continued)
Composition
Mafic
Color
Dark
Density
High
Minerals
Olivine, pyroxene
The second step in classifying igneous rocks is to identify the texture. Texture indicates how the magma cooled.
Lava that cools rapidly has small or no visible crystals. Magma that cools slowly has large crystals. You can tell if a
rock is granite or rhyolite by looking at its texture. Both rocks have the same composition (minerals), but cooled in
different ways changing the texture of the minerals. Granite has large crystals that you can feel and see. Rhyolite
has very small crystals that are hard to see. Extrusive igneous rocks will have a fine texture. Intrusive igneous rocks
will have a coarse texture. Igneous rocks that do have have crystals, like obsidian, have a glassy texture.
FIGURE 1.3
FIGURE 1.4
Pictured above are two pairs of igneous rocks ( Figures above and above). The first pair is two rocks that formed
from a felsic magma. The rhyolite cooled rapidly, and the granite cooled slowly. The second pair is two rocks that
formed from mafic magma. The basalt cooled rapidly, and the gabbro cooled slowly. The color of the rocks and
sizes of the crystals are your clues. Try sorting your rocks by texture. Can you correctly identify the extrusive and
intrusive rocks?
Igneous Rocks in Missouri
Giant elephant-shaped granite boulders are the star at Elephant Rocks State Park. These rocks are coarsely crystalline
red granite. Granite and rhyolite can be found in the St. Francios Mountains in southeast Missouri. What can you
infer about Missouri’s geologic past?
Vocabulary
• extrusive: Igneous rocks that form at Earth’s surface from rapidly cooling lava.
3
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FIGURE 1.5
Elephant Rocks State Park in southeast Missouri
• intrusive: Igneous rocks that form inside the Earth from slowly cooling magma.
• felsic: light in color
• mafic: dark in color
Summary
•
•
•
•
Intrusive igneous rocks cool from magma slowly in the crust. They have large crystals.
Extrusive igneous rocks cool from lava rapidly at the surface. They have small crystals.
Texture reflects how an igneous rock formed.
Igneous rocks are classified first by composition. Categories go from from felsic to mafic. Color can indicate
composition.
• Texture is second when classifying igneous rocks. Texture indicates how a rock cooled.
• Igneous rocks are categorized in pairs. The two rocks in a pair have the same composition but different
textures: gabbro-basalt, diorite-andesite, and granite-rhyolite.
Practice
Use the resource below to answer the questions that follow.
• Igneous Rocks at http://www.youtube.com/watch?v=deC5af9AW6w (5:06)
1.
2.
3.
4.
5.
6.
7.
8.
9.
How are intrusive rocks formed?
What size are the crystals in very coarse rocks?
Why are they that size?
What are the most common coarse rocks?
How are extrusive rocks formed?
List the three textures for extrusive rocks.
Describe rhyolite.
Describe pumice.
Explain why obsidian appears black.
Review
1.
2.
3.
4.
5.
6.
4
How do intrusive igneous rocks form?
How do extrusive igneous rocks form?
How can you use texture to determine whether an igneous rock is intrusive or extrusive?
How does the composition of a rock affect its color?
What are mafic rocks?
What is their color?
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Chapter 1. Characteristics of Igneous Rocks
Missouri Standards
• 5.1.A.d
• 5.2.C.b
• 5.2.C.c
References
1. (A) Courtesy of J.D. Griggs, US Geological Survey; (B) Flickr:SonoranDesertNPS. Lava cools to form
extrusive igneous rock, and rhyolite formations in Chiricahua National Monument.
2. (a) Kevin Walsh (Flickr: kevinzim); (b) User:deltalimatrieste/Wikimedia Commons; (c) Courtesy of the
National Oceanic and Atmospheric Administration. Obsidian, pumice, and basalt are extrusive igneous rocks
that cool at different rates.
3. . Comparison between rhyolite and granite.
4. . Comparison between rhyolite and granite.
5. . Comparison between basalt and gabbro.
6. . Comparison between basalt and gabbro.
5