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chapter 4 igenous rocks


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Volcanic, p 110
another name for extrusive
Volatiles, p 110
The gaseous component of the parent material for igneous rocks which is formed by the partial melting of solid rock in the crust and upper mantle. The most common volatiles dissolved in magma are water vapor, carbon dioxide, and sulfur dioxide. Volatiles disappear quickly and easily when magma reaches the surface (as lava) because they vaporize.
Vesicular texture, p 114
a frothy/bubbly texture in igneous rocks . . . the vessicles are small holes left by escaping gas bubbles
Ultramafic, 117
a rock type such as peridotite that rarely occurs at the earth’s surface. Ultramafic rocks contain ferromagnesian minerals, exclusively (minerals with iron and magnesium). They contain absolutely no silica-rich minerals.
a microscope with two eyepieces allowing you to look at a specimen with both eyes open. This gives a three-dimensional view.
Porphyry, p 114
a rock with a porphyritic texture
Porphyritic texture, p 114
a texture of igneous rocks where large crystals (phenocrysts) are embedded in a matrix of smaller crystals (groundmass)
Plutonic, p 110
means the same as igneous (in the realm of Pluto – the god of the underworld)
Phenocryst, p 114
large crystals in a porphyritic rock occuring in a groundmass of smaller crystals
Phaneritic texture, p 114
an igneous rock with
a relatively coarse-grained texture where mineral crystals are visible in a hand-sample
Pegmatitic texture, p 115
an ingeous rock with especially large crystals has a pegmatitic texture.
Petmatite, p 115
igneous rock with especially course textures (large crystals that are larger than a sugar cube)
Melt, p 110
The liquid component of the parent material for igneous rocks which is formed by the partial melting of solid rock in the crust and upper mantle.
Magma, p 110
molten rock in the earth's crust
Mafic, p 117
A igneous rock rich in dark-colored minerals such as olivine, pyroxene, amphibole, and biotite. Mafic rocks (gabbro and its extrusive equivalent basalt) have a low silica content.
Intrusive rock, p 110
igenous rock is rock that has formed by the cooling of intrusive magma, either below the earth's surface or on the earth's surface
Intrusive magma
The parent material for igneous rocks which is formed by the partial melting of solid rock in the crust and upper mantle. Magma has three components . . . solid, liquid, and gas.
Intermediate, p 117
a rock composition type in between granitic and basaltic. See andesitic
Igneous rock, p 110
rocks forming as magma or lava cools and solidifies
Granitic, p 116
Granite is a common and widely-occurring group of intrusive felsic igneous rocks forming at great depths and pressures under continents. Granite can contain two types of feldspars (orthoclase and plagioclase), quartz, hornblende, and two types of mica (biotite and muscovite). Minor accessory minerals such as magnetite, garnet, zircon and apatite can also appear in granite.
Groundmass, p 114
the matrix of fine-grained crystalline material in which larger crystals are embedded in coarse-grained igneous rocks
Glassy texture, p 114
a texture in igneous rocks where atoms are “frozen” in an unordered structure with no definite crystalline structure.
Glass, p 113
The word “Glass“ refers to the solid phase of a material with no overall molecular order. Glass can be described as “amorphous,” “non-crystalline,” and “vitreous.” Because glass is a disordered structure, it is very different from any crystalline material that exhibits a symmetrical, ordered structure such as the minerals found in rocks.
Geothermal gradient, p 124
the change in temperature within the earth that comes with increasing depth below the surface. If you dug a tunnel toward the center of the earth, the temperature of the rocks would increase about 25 degrees C for every kilometer you descended into the earth in the crust.
Pyroclastic texture (fragmental texture), p 114
an ingeous rock formed of ash, hardened “blobs” of once-molten lava, or large angular blocks torn from underground has a pyroclastic texture.
Felsic, p 116
An igneous rock rich in light-colored minerals such as feldspar and quartz. Felsic rocks (granite and it’s extrusive equivalent . . . rhyolite) have a high silica content.
Extrusive rock, p. 110
igneous rock that has formed by the cooling of extrusive lava
Extrusive lava
magma that has been erupted onto the land surface as lava
Decompression melting, p 126
Melting always involves an increase in volume. As confining pressure increases with depth, the melting point of rocks below the earth’s surface rises. On the other hand, as confining pressures in buried rock decrease (as rocks above them are uncovered by erosion), the melting point of the buried rock will decrease. When the confining pressure drops enough, decompression melting can occur underground.
Crystallization, p 111
The process of formation of a crystal (an ordered state) from a disordered (gas) or partially ordered (liquid) state. Examples are the freezing of liquid water, the deposition of water vapor (frost), and crystal formation in magma as it cools.
Basaltic, p 117
A fine grained igneous rock composed of calcium-rich plagioclase feldspar and dark silicate minerals such as pyroxene. Basaltic rocks contain no quartz.
Aphanitic texture
a fine-grained texture where mineral crystals are not visitble in a hand-sample without the aid of a stereoscope.
a rock that is halfway between granitic and basaltic in compostition. Andesitic rocks have 25% dark minerals while basaltic rocks have 50 % dark minerals. They have 40% silica-rich minerals, while many granitic rocks can be 80% silica-rich or more.
In Polynesian mythology (specifically: Hawaii), Pele is a goddess of fire, lightning, dance, volcanoes and violence. She lives on Kilauea, the largest volcano on the big island of Hawaii.
Katia and Maurice Krafft
Katia Krafft (17 April 1942 – 3 June 1991) and her husband, Maurice Krafft ( 25 March 1946 – 3 June 1991) were French volcanologists who died in a pyroclastic flow on Mount Unzen, in Japan, on June 3, 1991.
David A. Johnston
David A. Johnston, a volcanologist killed in the 1980 eruption of Mount St. Helens
the closest of Jupiter's moons; has active volcanoes
rock texture
The general appearance of a rock as shown by the size, shape, and arrangement of the materials composing it
ferromagnesian minerals
silicate rock- forming minerals containing iron and magnesium, applied to the mafic minerals
A fine-grained silica-rich igneous rock, the extrusive equivalent of granite
Obsidian is the result of volcanic lava coming in contact with water. Often the lava pours into a lake or ocean and is cooled quickly. This process produces a glassy texture in the resulting rock. Iron and magnesium give the obsidian a dark green to black color. Obsidian has been used by ancient people as a cutting tool, for weapons, and for ceremonial purposes and is sometimes found by archaeologists in excavations
the commonest type of solidified lava; a dense dark gray fine-grained igneous rock composed chiefly of calcium-rich plagioclase feldspar and pyroxene
A fine-grained volcanic rock of intermediate composition, consisting largely of plagioclase and one or more mafic minerals. Andesite composition is intermediate between gabbro and granite.
A coarse-grained igneous rock, chemically equivalent to a basalt
clastic materials distributed on the earth’s surface by volcanic explosion or aerial expulsion from a volcanic vent.
volcanic breccia
Rock formed from relatively large blocks of cooled lava embedded in mass of ash
hard volcanic rock composed of compacted volcanic ash
plutonic igneous rock having visibly crystalline texture; generally composed of feldspar and mica and quartz
a light glass formed on the surface of some lavas; used as an abrasive

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