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Discuss on Intrusive Rock Types
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Intrusive rocks crystallize from magmas that have been intruded into the earth's
crust at depths far below the surface. These intrusions are then usually exposed
millions or billions of years later through the processes of uplift,
mountain‐building, and erosion. Other intrusive rocks are discovered through
deep‐drilling programs.Country rock is the surrounding rock that the magma
invades. A contact then separates the cooled intrusive rock from the country rock.
Contacts are rarely straight lines, are quite irregular, and mark the change in rock
type. The edge of the intrusive rock is usually very fine grained because it is here
where the most rapid cooling took place. This edge of the intrusion is called
the chill zone. The grain size in the intrusion increases away from the chill zone
toward the center, where it remained the hottest for the longest time. The intrusive
rock often containsxenoliths—fragments of the country rock that were torn away
during the emplacement of the magma and that are generally most abundant near
the contact with the country rock.
Plutonic rocks. Intrusive rocks that were formed deep in the earth's crust are
called plutonic rocks and are generally coarse grained (mineral grains greater than
1 millimeter in diameter), large, and often associated with mountain‐building.
Mafic, felsic, and intermediate intrusive rocks. Intrusive rocks are classified the
same way extrusive rocks are—according to the relative amounts of feldspars,
quartz, and ferromagnesian minerals. Gabbro is a mafic rock and has the same
chemistry and mineralogy as basalt; diorite is an intermediate rock equivalent to
andesite; and granite is a felsic rock equivalent to rhyolite. For example, a magma
that would form rhyolite if it vented at the surface would crystallize into a granite
in a subterranean chamber kilometers below the surface. Granite is the most
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common intrusive rock on the continents; gabbro is the most common intrusive
rock in oceanic crust.
Ultramafic intrusive rocks. Ultramafic intrusions are almost completely
composed of ferromagnesian minerals, mostly olivine and pyroxene. They contain
less than 45 percent silica and are thought to be derived from the mantle. A typical
ultramafic intrusion is called a peridotite.
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