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Igneous Rocks

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Form as molten rock (magma) cools and solidifies

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Magma

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Parent material of igneous rocks Forms from partial melting of rocks Magma at surface is called Lava

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Lava

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Magma at surface

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Volatiles

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Dissolved gases in the melt that vaporize at surface pressure

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Crystallization

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The cooling of magma which results in the systematic arrangement of ions into orderly patterns

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Plutonic (Intrusive Igneous Rocks)

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Magma that crystallizes at depth These rocks are observed at the surface following periods of uplifting and erosion of overlying rocks. ** Inside the earth**

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Volcanic (Extrusive Igneous Rocks)

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The solidification of lava or volcanic debris **Outside the Earth**

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Granitic (Felsic Composition)

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Light-colored silicates Composed almost entirely of quartz and potassium feldspar Termed felsic (feldspar and silica) in composition High silica (SiO2) content Major constituent of continental crust

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Basaltic (Mafic Composition)

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Dark silicates and calcium-rich felspar Termed mafic (magnesium and ferrum, for iron) in composition Higher density than granitic rocks Comprise the ocean floor and many volcanic islands

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Andesitic (Intermediate Composition)

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Contain 25% or more dark silicate minerals (amphibole, pyroxene, and biotite mica)

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Ultramafic Composition

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Rare composition of mostly olivine and pyroxene Composed almost entirely of ferromagnesium minerals Peridotite is an example

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Texture

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The overall appearance of a rock based on the size, shape, and arrangement of mineral grains.

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Aphanitic (Fine-Grained) Texture

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Rapid rate of cooling Microscopic crystals

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Phaneritic (Coarse-Grained) Texture

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Slow cooling Large, visible crystals

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Porphyritic Texture

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Some minerals can grow large before others form from the magma The magma can move to a different environment which causes the remaining minerals to form quickly Large crystals (phenocrysts) are embedded in a matrix of smaller crystals (groundmass)

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Phenocrysts

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Large crystals

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Groundmass

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Smaller crystals

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Vesicular Texture

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Rocks contain voids left by gas bubbles in the lava Common feature of an extrusive igneous rock

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Glassy Texture

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Very rapid cooling Ions are frozen in place before they can unite in an orderly crystalline structure Once used as arrowheads, they used in scalpels (less scarring)

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Pyroclastic (fragmental) Texture

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Forms from the consolidation of individual rock fragments ejected during explosive eruptions

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Pegmatitic Texture

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Exceptionally coarse-grained Form in late stages of crystallization of magmas Rocks with this texture are called pegmatites

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Granite

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Coarse-grained (phaneritic) One of the best known igneous rocks Very abundant Natural beauty, especially when polished 10% - 20% quartz, roughly 50% potassium feldspar Small amounts of dark silicates Some granites have a porphyritic texture Contain elongated feldspar crystals

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Rhyolite

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Extrusive equivalent of granite Composed essentially of light-colored silicates Typically pink or light gray in color Less common and less voluminous than granite

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Obsidian

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Dark-colored, glassy rock Forms when silica-rick lava cools quickly at Earth's surface Usually black to reddish-brown Similar chemical composition of granite Dark color is the result of small amounts of metallic ions in an otherwise clear, glassy substance

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Pumice

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Glassy textured rock that forms when large amounts of gas escape from lava Voids are quite noticeable Resembles fine shards of intertwined glass Typically found in deposits with obsidian Will float when placed in water

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Andesite

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Medium-gray, fine-grained rock Volcanic origin Commonly exhibits a porphyritic texture

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Diorite

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Intrusive equivalent of andesite Coarse-grained rock Looks like gray granite, but lacks visible quartz crystals Can have a salt-and-pepper appearance

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Basalt

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Very dark green to black, fined-grained rock Composed mostly of pyroxene and calcium-rich plagioclase feldspar When porphyritic, contains small, light-colored feldspar phenocrysts Most common extrusive igneous rock Upper layers of oceanic crust are composed of basalt

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Gabbro

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Intrusive equivalent of basalt Very dark green to black, coarse grained rock Composed mostly of pyroxene and calcium-rich plagioclase feldspar Uncommon on the continental crust, but makes up a significant portion of the oceanic crust

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Geothermal Gradient

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Temperatures in the upper crust increase about 25 degree C per kilometer

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Decompression Melting

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Melting occurring at higher temperatures with increasing depth Reducing pressure lowers the melting temperature Solid, hot mantle rocks will ascend to regions of lower pressure, inducing melting

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Bowen's Reaction Series

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Minerals crystallize in a systematic fashion based on their melting points As minerals crystallize, the composition of the liquid portion of the magma continually changes

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Crystal Settling

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Earlier-formed minerals and denser than the liquid portion of the magma and sink to the base of the magma chamber

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Magmatic Differentiation

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The formation of one or more secondary magmas from a single parent magma

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Assimilation

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As magma migrates through the crust, it may incorporate some of the surrounding rock

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Magma Mixing

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During the ascent of two chemically different magma bodies, the more buoyant mass may overtake the slower-rising body

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Pluton

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Cooled, emplaced magma into preexisting rocks Classified by their orientation to the surrounding rock

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Tabular

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Table-like

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Discordant

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Cut across existing structures

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Concordant

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Parallel to features like sedimentary strata

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Massive

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Irregularly shaped

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Dike

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a tabular, discordant pluton Serves as tabular conduits to transport magma Parallel groups are called dike swarms

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Sill

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A tabular, concordant pluton Tend to accumulate magma and increase in thickness Closely resembles buried lava flows

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Batholith

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Largest intrusive body Surface exposure of 100+ square kilometers (smaller bodies are termed stocks) While expansive, most are less than 10km thick

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Xenoliths

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Suspended blocks of country rocks found in plutons

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Laccoliths

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Forcibly injected between sedimentary strata Causes the overlying strata to arch upward

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How are igneous rocks formed?

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Igneous rocks are formed by the cooling and crystallization of molten rock.

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Intrusive and extrusive igneous rocks are classified based on what criteria?

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How quickly the molten rock cools and crystallizes, and where this occurs with respect to Earth's surface.

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What is required to generate an igneous rock with a fine-grained texture?

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A fine-grained texture is the product of rapid cooling and crystallization of lava.

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A phaneritic texture is characteristic of a(n) ______ igneous rock that cooled ________.

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Intrusive Below the surface

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What does it mean if an igneous rock has an aphanitic texture?

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The igneous rock exhibits mineral crystals too small to see with the naked eye.

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What three components make up most magmas?

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Liquid Portion Solid Portion Gaseous Portion

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Based on the diagram shown in the video, name two minerals that are unlikely to coexist in the same igneous rock?

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Muscovite and Pyroxene

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Igneous rocks of felsic composition have ______ and _____ relative to mafic igneous rocks.

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Greater silica Lower melting temperature

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Which of the following is true about mafic igneous rocks?

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Mafic igneous rocks are rich in Fe, Mg, and Ca.

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You have found a mafic rock with a phaneritic texture. What is a potential rock name?

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Gabbro

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Based on what you learned in this video, what minerals make up rhyolite?

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Quartz, K-feldspar, and plagioclase feldspars, with relatively fewer biotite and amphiboles.

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Which mineral class makes up most igneous rocks?

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Silicates

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Peridotite is the main constituent of which part of the Earth?

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Upper Mantle

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Which two minerals define a felsic composition?

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Quartz and Potassium Feldspar

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What is one way a glassy texture forms?

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Lava cools rapidly.

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Which of the following describes a texture containing bubble-shaped cavities?

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Vesicular

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Which of the following textures describes an igneous rock with anomalously large crystals?

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Pegmatitic

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What does an igneous rock with a pyroclastic texture tell a geologist?

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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?

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Porphyritic

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Which of the following igneous rocks contain no mineral crystals?

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Obsidian

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What is the term used to describe a very large, widespread, and deep intrusive body of igneous rock?

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Batholith

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In what type of igneous feature would you find rocks with a vesicular texture?

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The surface of a lava flow

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Working in the field, you see a thin, dark-colored igneous sill across the valley. Without seeing the sill up close, what texture would you expect it to possess?

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Fine-grained

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Working in the field, you come upon a light-colored, fine-grained igneous rock that contains some very large dark-colored crystals. How do you account for this complex mineralogy?

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The dark crystals formed first, undergoing slow cooling, and the remainder cooled quickly.

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Ultramafic rocks contain ______ and are commonly found in _______.

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Olivine Mantle

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What is an accessory mineral?

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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?

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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?

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Andesite Porphyry

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What do pumice and scoria have in common?

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The both exhibit a vesicular texture.

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At a subduction zone, melting is triggered by ______.

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The introduction of water

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Which is the following statement best describes the relationship between pressure and melting point in the Earth's Interior?

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Higher melting points are determined by higher pressures

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What is the term used to describe increased temperature with depth in the Earth?

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Geothermal Gradient

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How can a magma body change its composition?

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Assimilation Magma mixing Crystal settling Magmatic differentiation

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_______ describes how an entire suite of silicate minerals can form a single basaltic magma as it cools and crystalizes.

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Bowen's Reaction Series

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Which mineral on the Bowen's Reaction Series will be the last to melt if an igneous rock is heated?

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Olivine

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Which mineral on the Bowen's Reaction Series will be the first to melt if an igneous rock is heated?

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Quartz

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Once a source rock partially melts, what does it produce?

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Felsic magma and mafic residue

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According to Bowen's Reaction Series, _______ is one of the first minerals to melt, but last to crystallize.

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Quartz

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How is basaltic magma transformed into felsic magma?

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Basaltic magma partially melts the continental crust, which is more felsic in composition.

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What causes an elevated geothermal gradient in a divergent plate settling?

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Decomposition melting of the mantle

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In a subduction zone, water driven from subducted oceanic crust causes _______.

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The melting temperature of mantle rocks to decrease.

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Which process is thought to generate most felsic magmas?

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Heat from basaltic magma partially melting overlying crust

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Which is the term for a tabular igneous pluton that occurs in an orientation that is discordant with the bedding surfaces of adjacent sedimentary rocks?

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Dike

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What is the term for a tabular igneous pluton that occurs in an orientation that is concordant with the bedding surfaces of adjacent sedimentary rocks?

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Sill

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Which of the following terms is best describes as a massive pluton?

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Batholith

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Batholiths are known to form the cores of mountains ranges such as the Sierra Nevada. What is the implication of this observation?

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Because batholiths form deep within Earth, the occurrence of one at the surface implies that if has since been uplifted and/or the rocks above have been removed.

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While working in the field, you find a dike cutting across some sedimentary rocks. What grain size would you expect to find in this pluton?

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Fine-grained

GEOL 101: Magma, Igneous Rocks and Intrusive Activity – Flashcards

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