Which of the following textures indicates that a rock is of intrusive origin?
answer
Phaneritic
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Which of the following textures indicates that a rock is volcanic in origin?
answer
Aphanitic
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Which igneous texture has visible crystals that are a few millimeters across?
answer
Phaneritic
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What is the geologic definition of texture?
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Size, shape, and arrangement of mineral grains in the sample
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What creates the small holes found in a vesicular texture?
answer
Volatiles
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What substance makes ion migration easier in the later stages of magma crystallization, which allows abnormally large mineral crystals to form and create pegmatites?
answer
water
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Which of the following best describes a pegmatitic texture?
answer
Large crystals that form in a fluid-rich environment late in crystallization
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has a large percentage of voids and is lightweight, so therefore has the ability to float in water.
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pumice
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An igneous rock with a ______ texture solidified deep within Earth.
answer
Phaneritic
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What factors influences the texture of an igneous rock?
answer
-the rate at which the rock cools,
-the amount of silica that is present in the magma from which it forms, and
-the amount of dissolved gases present in said magma
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How do crystals in pegmatite grow so large?
answer
Large crystals in pegmatites are the result of a high percentage of the melt being composed of water, carbon dioxide, and other materials. These materials enhance ion migration, allowing the crystals to grow larger than usual.
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What does a porphyritic texture indicate about the cooling history of an igneous rock?
answer
The rock began cooling under the Earth's surface, but then was moved to above the Earth's surface where it finished cooling.
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What is one way a glassy texture forms?
answer
Lava cools rapidly.
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Which of the following textures describes an igneous rock with anomalously large crystals?
answer
Pegmatitic
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What does an igneous rock with a pyroclastic texture tell a geologist?
answer
The rock was likely formed by a violent volcanic eruption.
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Which of the following textures indicates two stages of cooling and crystallization?
answer
Porphyritic
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What large, overall igneous feature makes up the Sierra Nevada Mountains in California?
answer
batholith
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What is a roof pendant?
answer
older rock capping the batholith. This rock would have been the roof of the magma chamber.
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How do sheet joints and exfoliation domes form in granite plutons?
answer
Confining pressure on the plutons is removed through erosion of surrounding rock. Plutons then expand.
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Why is the monolith El Capitan in Yosemite National Park more resistant to erosion than other igneous plutons in the park?
answer
El Capitan is unfractured.
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Ultramafic rocks contain __________ and are commonly found in __________.
answer
olivine; mantle
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What is an accessory mineral?
answer
A mineral that makes up a relatively small portion of the total rock composition
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Which of the following best describes the difference between granite and rhyolite?
answer
Granite is phaneritic and rhyolite is aphanitic.
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What is the rock name of an intermediate rock with two distinct grain sizes?
answer
Andesite porphyry
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What do pumice and scoria have in common?
answer
They both exhibit a vesicular texture.
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A ________ is a tabular, concordant pluton that is nearly horizontal and forms when magma exploits a weakness between sedimentary layers.
answer
sill
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A ________ is a mushroom-shaped pluton that forms by injecting magma between sedimentary strata, forcing the upper layers to arch upward.
answer
laccolith
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Which of the following is a characteristic of an igneous rock found in a batholith?
answer
phaneritic texture
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What is the name of the preexisting rock through which intrusive igneous bodies intrude?
answer
Country rock
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A dike is mostly likely to be composed of which of the following igneous rocks?
answer
basalt
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Select the correct description of dikes and sills.
answer
Dikes are discordant igneous intrusions; sills are concordant igneous intrusions; both are tabular.
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How do batholiths, stocks, and laccoliths differ?
answer
Batholiths are the largest type of igneous bodies and occur in a linear fashion with a distance of 100km or more; stocks are smaller than batholiths; laccoliths bend the sedimentary layers above them, whereas the sedimentary layers below remain relatively undeformed.
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To what does the term "country rock" refer?
answer
the crustal rocks into which igneous bodies intrude
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Which of the following is true based off your observation of intrusive igneous rocks on Earth's surface?
answer
Intrusive igneous bodies are formed by magma intruding and crystallizing in country rock below Earth's surface. Therefore, if we observe these features at the surface today, they must be exposed by uplift and erosion.
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What is the best interpretation for how the porphyritic texture of these rocks formed?
answer
The magma cooled slowly at first, forming large crystals, and then quickly, forming small crystals from the remaining magma.
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What is the difference between a dike and a sill?
answer
Dikes crosscut sedimentary layers; sills pry between horizontal layers.
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Why are the dikes and sills so easily distinguished in Sinbad Country?
answer
Basalt and the hardened, baked zones are more resistant to erosion than the sedimentary rocks.
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Why are sills only located fairly close to the Earth's surface?
answer
Sills form as magma intrudes with enough force to overcome the weight of the rocks above.
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Why don't the baked zones along the edges of the dikes and sills show evidence of metamorphism?
answer
They weren't under enough pressure and didn't remain hot for a long enough period of time.
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Approximately how long did it take for the magma that fed each dike in Sinbad Country to be injected?
answer
The magma was injected in a matter of hours to days.
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A_____________has broad and moderately steep flanks and a steep summit. It results from the eruption of a mixture of both lava flows and pyroclastic deposits that contain less fluid materials and travel shorter distances.
answer
composite volcano
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A wide volcano that produces mainly basaltic lava flows is called a
answer
shield volcano
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A very steep-sided volcano built primarily from ejected lava fragments is called a
answer
cinder cone
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Broad sheets of fluid ________ lava travel long distances and give shield volcanoes their distinctive shape.
answer
basaltic
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Moderately viscous and gas-rich________ lavas can erupt as lava flows or pyroclastic materials, leading to the steepening slope of composite volcanoes.
answer
andestic
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Explosive eruptions of ________________create steep-sided cinder cone volcanoes.
answer
pyroclastic materials
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Which of the following scenarios best describes how the Hawaiian Islands formed in the Pacific Ocean?
answer
Magma generated from a hot spot burned through the overlying plate to create volcanoes.
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Which materials in Hawaii's lava help to keep it fluid for longer at the surface (as opposed to cooling and solidifying quickly), thus allowing extensive lava flows to develop?
answer
magnesium and ion
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Which of the following has the single most immediate effect on lava's ability to solidify?
answer
temperature
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What is piled up along the cone of SP Crater that causes it to be so steep?
answer
cinders
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What causes vesicles to form in lava?
answer
trapped gas bubbles
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What is the difference between aa lava and pahoehoe lava?
answer
a lava has a prickly texture; pahoehoe has a smooth, ropy texture.
question
What is one way that cinder cones differ from composite and shield volcanoes?
answer
Cinder cones are smaller than composite and shield volcanoes.
question
Which one of the following statements is NOT true about andesite and basalt?
answer
Basaltic lava is thicker than andesitic lava.
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