chapter 4 give it some thought
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Geology
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Apr 3, 2024
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Abigail Walden
Give It Some Thought 1)
Would you expect all the crystals in an intrusive igneous rock to be the same size? Explain why or why not. No I would not expect all of the crystals in an intrusive igneous rock to be the same size because the rock has many different ways to cool underground in the
magma chambers. 2)
Apply your understanding of igneous rock textures to describe the cooling history of each of the igneous rocks pictured here. a)
This igneous rock has a coarse-grained texture. This indicates that the rock formed slowly
and cooled slowly beneath the surface. This rock is likely an intrusive rock formed from magma.
b)
This igneous rock has a fine-grained texture. This indicates that the rock cooled rapidly on the surface. This rock is likely an extrusive rock formed from lava.
c)
This igneous rock has a vesicular texture. This indicates that the rock formed from lava that had gas bubbles in it. The bubbles were trapped in the rock as it cooled and solidified. This rock is likely an extrusive rock formed from lava.
d)
This igneous rock has a glassy texture. This indicates that the rock cooled very rapidly, too quickly for crystals to form. This rock is likely an extrusive rock formed from lava that cooled quickly on the surface.
3)
Use Figure 4.5
to classify the following igneous rocks:
a)
An aphanitic rock containing about 30 percent calcium-rich plagioclase feldspar, 55 percent pyroxene, and 15 percent olivine. This can be classified as a mafic igneous rock, specifically a basalt. b)
A phaneritic rock containing about 20 percent quartz, 40 percent potassium feldspar, 20 percent sodium-rich plagioclase feldspar, a few percent muscovite, and the remainder dark-colored silicate. This can be classified as a felsic igneous rock, which is granite. c)
An aphanitic rock containing about 50 percent plagioclase feldspar, 35 percent amphibole, 10 percent pyroxene, and minor amounts of other light-colored silicates. This can be classified as an intermediate igneous rock, an andesite. d)
A phaneritic rock made mainly of olivine and pyroxene, with lesser amounts of calcium-rich plagioclase feldspar. This can be classified as an ultramafic igneous rock, this is a peridotite. 4)
Identify the igneous rock textures described by each of the following statements.
a)
Openings produced by escaping gases. Vesicles b)
The texture of obsidian. Glassy(not crystalline)
c)
A matrix of fine crystals surrounding phenocrysts. the finer-grained matrix is the groundmass and the texture is porphyritic
d)
Consists of crystals that are too small to be seen without a microscope. aphanitic texture
e)
A texture characterized by rock fragments welded together. a porphyritic texture (phenocrysts and groundmass)
f)
Coarse grained, with crystals of roughly equal size. phaneritic texture; the equal-size or equigranularity idea helps students understand the difference between phaneritic and porphyritic-phaneritic textures
Abigail Walden
5)
During a hike, you pick up the igneous rock shown in the accompanying photo.
a)
What is the mineral name of the small, rounded, glassy green crystals?
That stone would be olivine. b)
Did the magma from which this rock formed likely originate in the mantle or in the crust? Explain. The magma from which this rock formed likely originated in the mantle due to the dark silicate.
c)
Was the magma likely a high-temperature magma or a low-temperature magma? Explain. The magma was a high-temperature magma due to the crystallization
d)
Describe the texture of this rock. The texture is granular. 6)
A common misconception about Earth’s upper mantle is that it is a thick shell of molten
rock. Explain why Earth’s mantle is actually solid under most conditions. It is a solid layer that goes through a slow continuous convection motion with iron and nickel. The increase in temperature with depth and pressure is between solid rock and a hundred percent melted.
7)
Describe two mechanisms by which mantle rock can melt without an increase in temperature. How do these magma-generating mechanisms relate to plate tectonics? Mantle rock can melt without an increase in temperature if there is a decrease in pressure exerted by overlaying rock layers. This is called decompression melting. Rocks
can also melt in the mantle due to dehydration. A subducting slab carries with its water
as it is pulled into the mantle. The gradual release of the water contained in the slab is the mechanism that causes melting.
8)
Use your understanding of Bowen’s reaction series (see Figure 4.20
) to explain how partial melting can generate magmas that have different compositions. In Bowen’s reaction series the minerals all melt at different times, mixing with the magma.
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