MAGMA, IGNEOUS ROCKS, AND 4
INTRUSIVE ACTIVITY
INTRODUCTION
Magma, Igneous Rocks, and Intrusive Activity
CHAPTER OUTLINE
1.
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3.
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4.
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7.
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LEARNING OBJECTIVES/FOCUS ON CONCEPTS
4.1 List and describe
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4.3 Identify and describe
4.5 Summarize
4.7 Describe
TEACHING STRATEGIES
Clicker Questions:
Muddiest Point:
A.
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B.
C.
TEACHER RESOURCES
Web Resources:
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o
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Imagery
ANSWERS TO QUESTIONS IN THE CHAPTER:
CONCEPT CHECKS
4.1 MAGMA: PARENT MATERIAL OF IGNEOUS ROCK
1. Magma is material formed by partial melting that occurs at depth within Earth. Lava is
molten rock that reaches Earths surface.
2. Magma consists of a liquid portion (melt), a solid component, and a gaseous component
(volatiles). The liquid portion is mobile ions of the common Earth elements (Si, O, Al, K, Ca, Na,
3. Crystallization occurs as the temperature of a magma drops. As the temperature cools, mobile
4. Extrusive igneous rocks are those rocks that form from the solidification of lava at Earths
surface. Intrusive igneous rocks are those rocks that form at depth in the Earth as a magma
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4.2 IGNEOUS COMPOSITIONS
1. Igneous rocks are composed mainly of silicate minerals.
2. Light-colored igneous rocks are composed primarily of quartz and feldspar with lesser
3. Felsic (granitic) rocks are composed primarily of quartz and feldspar with lesser amounts
(about 10percent) of dark silicate minerals. Intermediate (Andesitic) rocks contain at least
4. The silica content of granitic magma is very high and thus creates a viscous (thick) magma
4.3 IGNEOUS TEXTURES: WHAT CAN THEY TELLS US?
2. Magmas that cool over long time periods create rocks with large crystals. In magmas that cool
3. The six major igneous rock textures are: aphanitic, phaneritic, porphyritic, glassy, pyroclastic,
and pegmatitic.
4. Pegmatites form late in the crystallization of a magma as fluid-rich portions of the melt allow
4.4 NAMING IGNEOUS ROCKS
1. Classification of igneous rocks is based on mineral composition and texture. Texture is defined
as the size, shape, and arrangement of mineral grains.
2. Granite and rhyolite are similar in that they both have a felsic compositionthey are
composed of mineral grains of quartz and potassium feldspar with lesser amounts of
3. Gabbro is mafic; obsidian has a felsic composition although it is dark in color; granite is felsic;
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4. Diorite is phaneritic (coarse-grained) and is composed of relatively equal amounts of
plagioclase feldspar and amphibole. Rhyolite is aphanitic (fine-grained) and is composed
5. The terms and only indicate texture, not composition. Tuff is composed of
cemented ash particles and breccia is composed of cemented particles larger than ash size.
4.5 ORIGIN OF MAGMA
1. The geothermal gradient is the increase in Earths temperature with depth. In the upper crust,
the geothermal gradient is 25°C per kilometer of depth. The melting point of peridotite is
2. Magma melts at higher temperatures at greater depths in the Earth because of increased
3. Water and other volatiles lower the melting temperature of rock, and can cause partial
melting in the mantle where water-rich plates are subducted.
4. Magma is generated at divergent plate boundaries where reductions in confining pressure
4.6 HOW MAGMAS EVOLVE
1. As crystallization progresses, a magma becomes more silica-rich. This is due to the early
2. Bowens reaction series demonstrates the sequence in which minerals will crystallize from a
mafic magma and how that magma changes composition (differentiates) as it gradually
3. Crystal settling occurs as early-formed minerals settle out of the liquid portion of a magma
4. Assimilation and magma mixing both result in magmas of different composition than the
original parent magma. Assimilation occurs through the incorporation of foreign material
4.7 PARTIAL MELTING AND MAGMA COMPOSITION
1. Partial melting is the incomplete melting of rock; this is caused by the differing melting
temperatures of minerals as depicted in Bowens reaction series. Rocks that undergo
2. Most basaltic magmas form from partial melting of ultramafic mantle rocks. These are
3. Primitive magmas are considered un-evolved because they have not undergone extensive
4. Most granitic magmas are generated when ponding basaltic magma partially melts overlying
4.8 INTRUSIVE IGNEOUS ACTIVITY
2. Dikes are tabular, discordant structures. They cut across bedding structures and represent
3. Batholiths are massive structures, typically felsic to intermediate igneous rocks, that represent
large, solidified magma chambers later exposed by erosion and weathering. Stocks are
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EYE ON EARTH
EOE #1 PELES HAIR AND PELES TEARS
1. Peles tears have a pyroclastic texture because they are fragments that solidify after
EOE #2 HOLE IN THE WALL, CALIFORNIA
1. This rock is called a welded tuff because it is composed of tiny ash fragments.
EOE #3 THE PALISADES
2. An escarpment is a steep slope, or cliff, separating two areas of very different elevation.
EOE #3 SHIPROCK, NEW MEXICO
1. Shiprock is a volcanic neck; the remnant of the vent of a former volcano.
GIVE IT SOME THOUGHT
1. Although the crystals would be visible, you would not expect crystals in an intrusive igneous
2. Rock A (rhyolite) had a very quick cooling history because it is fine grained (cannot see all
individual grains). Further, its glassy inclusions also indicate a very quick cooling. Rock B
3. Yes, two igneous rocks can have the same mineral composition but be different rocks. Intrusive
rocks have extrusive equivalents with the same mineralogy and vice versa. Further, the
4.
a. An aphanitic rock containing about 30 percent calcium-rich plagioclase feldspar, 55
percent pyroxene, and 15 percent olivine would be called basalt.
5.
a. Vesicles
b. Glassy
6.
a. The small, rounded, glassy green crystals are olivine.
b. The magma for this rock likely formed in the mantle because it has a mafic
7. Earths mantle has a composition of peridotite. When we look at the melting curve for
peridotite at various pressures, we see that its melting point is always higher than that of the
8. Mantle rock can melt through decompression melting, and by the incorporation of fluids or
volatiles. Decompression melting occurs when confining pressures are dropped enough to
9. Partial melting can generate magmas with different compositions because partial melts are
enriched in ions of minerals with lower melting temperatures. The smaller the percentage of
10. The suggestion that this layer is a sill is plausible, but inaccurate with the evidence given. Here,
we see vesicles in the top of the basalt layer; these vesicles suggest a very fast cooling of the top
11.
a. Laccolith
12. According to Figure 4.32, Mt. Whitney is part of a granitic batholith complex. Because this is a
continental setting, the rock of this batholith probably formed as hot basaltic magma ponded
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