Term
| Small crystals usually indicate __________ cooling |
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Definition
rapid
*Such a texture is characteristic of a magma that has reached Earth’s surface (called lava). Such rocks are referred to as “volcanic” or “extrusive.” |
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Term
| Course crystals means cooled ______, deep within the earth's crust |
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Definition
slowly
*These rocks are referred to as “intrusive” or “plutonic.” |
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Term
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Definition
| amorphous (cryptocrystalline to noncrystalline). Such a texture occurs when the cooling of magma is so rapid that crystals either do not have time to form (therefore no true minerals are present) or are so small that an electron microscope is required to see them. Such a rock forms when magma reaches the surface as lava and cools very rapidly (within a matter of hours or minutes). |
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Definition
| fine-grained and require a light microscope to be seen. Such a texture occurs when a magma reaches the surface as lava or intrudes into shallow, relatively cold rocks and cools rapidly (commonly within a matter of weeks). |
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Term
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Definition
| —relatively large crystals in a groundmass of smaller crystals (see Figure 4.5 on page 85 in Exploring Earth). Such a texture forms when magma begins to cool slowly, forming large crystals. Then it cools rapidly and forms the fine, crystalline groundmass. This might happen if the magma begins to cool within the crust and then completes cooling after extruding on the surface as a lava. |
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Term
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Definition
| consist of broken fragments of glassy material and crystals that are compacted into rock. Such a texture results from the violent extrusion of magma onto the surface as volcanic ash and cinder. |
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Term
| Two primary types of igneous rock; those that are rich in silicon and aluminum, called ________ |
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Definition
Felsic
*felsic is derived from the minerals feldspar and quartz (silica), which are two of the most common minerals in this type of rock. |
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Term
| Compositionally, there are two primary types of igneous rocks: those that are rich in silicon and aluminum, called felsic, and those containing abundant iron and magnesium, referred to as ________. |
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Definition
Mafic
Mafic stands for magnesium and iron (also called ferrous and ferric). |
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Term
| ________is another common element in felsic rocks, whereas ________ is more likely to be found in rocks of mafic composition. |
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Definition
Sodium, Calcium
*Felsic rocks, also called granitic, are found almost exclusively in continental crust. Mafic or basaltic rocks can be found either in continental or oceanic crust but are more indicative of an oceanic setting. Because of the high iron and magnesium content, mafic minerals tend to have a higher specific gravity than felsic mineral, making those rocks feel heavier. |
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Term
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Definition
| The chart above shows how igneous rocks are classified according to their crystal size and mineral composition (see also Chart 4.4 on page 84 in Exploring Earth). Rocks may be aphanitic (fine-grained) and classified as rhyolite, andesite, or basalt on the basis of their mineral composition. Phaneritic rocks are classified as granite, diorite, gabbro, or peridotite according to their observed mineral content. Plagioclase feldspar is found mostly in igneous rocks and ranges from varieties that are high in sodium (Na) in felsic rocks to varieties that are rich in calcium (Ca) in mafic rocks. |
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Term
| The oceanic crust created at the mid-ocean ridge is _______because that is the composition of the asthenosphere where the magma originates. Continental crust, on the other hand, is composed mainly of _______ rocks, such as granite. |
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Definition
| The oceanic crust created at the mid-ocean ridge is mafic because that is the composition of the asthenosphere where the magma originates. Continental crust, on the other hand, is composed mainly of felsic rocks, such as granite. |
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Term
| When volcanic and/or plutonic rocks are examined closely, one can see that many different rock types can form from the same parent magma. This is because different minerals in igneous rocks melt and crystallize at different temperatures. |
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Definition
In the early part of the last century, N. L. Bowen, a Canadian geologist, performed a series of laboratory experiments to determine the order in which minerals would crystallize when a melt with the composition of basalt was cooled. In this way, he hoped to show that a felsic rock could be derived from a mafic magma.
*This separation may occur as the magma moves upward through the crust. Thus, several distinctly different rock types can form from a single magma, each progressively more felsic. This process is called fractional crystallization |
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Term
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Definition
| Rock that is intruded by the magma |
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Term
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Definition
| large, irregular mass of igneous rock that may underlie an area of more than 100 square kilometers and does not have a distinct floor. Whole mountain ranges, such as the Sierra Nevada Mountains of the western United States, are exposed batholiths. |
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Definition
| An intrusion of magma into fractures that cut across strata |
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Term
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Definition
| An intrusion of magma between layered rocks |
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Term
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Definition
| May be associated with intrusions but is not an intrusion itself. As magma cools, hot fluids from the cooling body may move into country rock along faults and fractures. As the fluids cool, minerals crystallize into the vein. Many valuable ore deposits are associated with hydrothermal veins, including gold and silver. |
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Term
| _____ or _______ lavas are low in silica; therefore, they tend to be thin and fluid, commonly flowing a great distance from the source before cooling into rock. Mafic volcanic eruptions tend to be less explosive than eruptions of felsic lavas because volatile gasses are more able to escape the thin mafic lavas. |
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Definition
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Term
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Definition
| angular blocks of hardened lava |
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Term
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Definition
| Smooth, ropy-textured lava flows |
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Term
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Definition
| Ellipsoidal or “pillow-shaped” structures in a lava flow that result from the extrusion of lava under water |
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Term
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Definition
| hexagonal columns of basalt that result from the shrinkage of the basalt during cooling |
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Term
Types of Volcanic Cones:
Shield Volcanoes |
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Definition
| composed of the build-up of many basaltic lava flows around a central vent. The shape of the volcano is a broad cone with gently sloping sides. The Hawaiian Islands are composed of large shield volcanoes. |
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Term
Types of Volcanic Cones:
Composite Volcanoes |
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Definition
| —composed of the build-up of lava and pyroclastic deposits. These volcanoes erupt explosively. The shape is the classic steep-sided cone. Mount St. Helens is such a volcano. |
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Term
Types of Volcanic Cones:
Cinder Cones |
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Definition
| steep-sided cones that are formed mainly of volcanic cinder and ash blown from a central vent. Cinder cones are most commonly basaltic in composition and may be associated with flows. Cinder cones range from tens of meters to several thousands of meters in height, such as the mountains north of Flagstaff, Arizona. |
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