Exam 2 - Igneous Rocks Flashcards
the word igneous
The word igneous is derived from the Latin word ignus, meaning fire or fiery
The term igneous applies to natural process relating to the formation, movement, and cooling of molten materials—magma (hot, molten material underground), and lava (molten material flowing on or near the surface)
In geology, the word igneous applies to materials that have solidified from molten rock material, it also applies to the processes associated with the movement of molten material underground or erupting or the surface
how are igneous rocks formed?
Igneous rocks are classified by their unique properties and characteristics which are related to composition of their host melt and the environmental setting where they form, underground or on the surface
volcanoes
Volcanoes form where molten material erupts on the surface
Regions where volcanoes occur, both modern (active or dormant) or ancient (extinct) have a unique variety of landforms. There are at least 500 active volcanoes around the world, of which about 25 are actively erupting, spewing out lava, rock, ash, and noxious gases. It is estimated that nearly 600 million people around the world live within volcanic hazard zones
what causes rocks to melt?
Rocks will melt (generating magma) if heat flow increases in an area to the point that minerals reach their melting points. Other factors that cause melting include the introduction of hot volatile fluids (water and gases) into rocks under pressure, or if there is a decrease in pressure confining hot rocks (such as the release of pressure caused by a great earthquake)
Melting ranges from only partial melting of select low-temperature minerals to complete melting under higher temperature conditions. As heat flow increases, more materials will potentially melt
intrusive igneous rocks
Rocks formed from magma cooling and crystallizing underground are called intrusive igneous rocks
extrusive igneous rocks
Rocks formed from cooling lava on or very near the surface are called extrusive igneous rocks
When lava cools rapidly it crystallizes quickly, preventing visible crystals from forming. Extrusive rocks include lava flows and pyroclastic material such as volcanic ash, cinders, etc.
plutonism
Plutonism refers magma moving, cooling, and crystallizing underground. A pluton is a body of igneous rock formed underground
Plutonic rock is a rock formed at considerable depth by crystallization of magma and/or by chemical alteration. It is usually medium- to coarse-grained with a granitic (phaneritic) texture
volcanism
Volcanism is any of various processes and phenomena associated with the surface discharge of molten rock or hot water, steam, or gases. Volcanic rock is any rock formed by volcanism
A volcanic eruption occurs when molten material under pressure is expelled on the surface. Some may be discharged into the atmosphere or oceans producing a variety of rock fragments including large blocks, volcanic bombs (blobs that may have a hard crust and a partially molten interior), cinders, and ash
phaneritic texture
Phaneritic texture is a term usually used to refer to igneous rock with a larger crystal grain size and texture. It means that the size of matrix grains in the rock are large enough to be distinguished with the unaided eye as opposed to aphanitic (which is too small to see with the naked eye)
Rocks with phaneritic texture are usually intrusive igneous rocks. Magma underground cools and crystallizes very slowly allowing large crystals to form
Aphanitic texture
Aphanitic texture applies to dense, homogeneous rock with constituents that are so fine grained that they cannot be seen by the naked eye
Rocks with aphanitic texture are usually extrusive igneous rocks. Magma or lava at or near the surface tends to cool very quickly, preventing large crystals from forming, so the rock typically has a very fine-grained texture
classification of igneous rocks
1) mineral composition,
2) crystalline sizes,
3) general color, and
4) textural characteristics.
felsic rock
Felsic is a term used to describe molten material (magma), minerals, and rocks which are enriched in the elements such as silicon, oxygen, aluminum, sodium, and potassium. The term combines part of the words “FELdspar” and “SILica”. Most felsic minerals are light in color and have a density less than 3 grams per cubic centimeter (g/cm3) . Felsic minerals produce felsic rocks. Common felsic minerals include quartz, muscovite, and feldspars. Granite and rhyolite are common felsic rocks
malfic rock
Mafic is an adjective describing molten material (magma), minerals, or rocks that are enriched in magnesium and iron;. The term combines part of the words “magnesium” and “ferric” (ferric iron are compounds with the Fe+3 ion). Most mafic minerals are dark in color and the relative density is greater than 3 grams per cubic centimeter (g/cm3). Common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include basalt and gabbro
felsic rock examples
granite - a common, coarse-grained (crystalline), light-colored, hard plutonic (intrusive igneous) rock consisting chiefly of quartz, orthoclase or microcline (feldspars), and mica. Granite is found in plutonic rocks that have been exposed by erosion
rhyolite - a pale fine-grained volcanic (extrusive igneous) rock of granitic composition. Rhyolite is common in continental volcanic regions with notable deposits around Yellowstone and volcanic centers throughout the Great Basin region extending from Nevada to New Mexico
intermediate rocks
refers to rocks that are in-between felsic and mafic in mineral composition. Intermediate rocks tend to be pale to medium gray in color and subdivided into two groups, dacitic rocks and andesitic rocks
intermediate rocks examples
Granodiorite—a coarse-grained (crystalline) plutonic igneous containing quartz and plagioclase, intermediate between granite and diorite in composition. The rocks exposed in the Sierra Nevada Range (including Yosemite National Park) are mostly granodiorite
Dacite—an extrusive igneous (volcanic) rock with an aphanitic to porphyritic texture and is intermediate in composition between andesite and rhyolite. Lassen Volcano in northern California and many volcanoes in the Cascade Range are dacitic in composition
intrusive rock examples
Diorite—a crystalline intrusive igneous rock intermediate in composition between granite and gabbro, consisting essentially of plagioclase and hornblende or other mafic minerals; having a “salt and pepper”-like appearance. Diorite is found in mountain ranges throughout the Pacific Northwest
Andesite—A fine-grained, brown or grayish volcanic rock that is intermediate in composition between rhyolite and basalt, dominantly composed of plagioclase feldspar. Andesite is the most abundant rock found in the volcanic rocks of the Cascade Range extending from northern California into British Columbia
AA and pahoehoe rock
Materials ejected from volcanic eruptions have some unique characteristics. Some are of interest to the gem community, not that they are gems or considered precious, but they can be cut or shaped into interesting variety of uses for jewelry and art. Visitors to Hawaii are introduced to the terms pahoehoe (pronounced “pā-hoi’hoi’) and a’a (ä’ä) (Figures 7-27 and 7-28). Pahoehoe has a ropey fluid texture formed when hot basaltic lava cools quickly. A’a is lava rock with a rough, blocky surface when a lava flow continues to move slowly as it cools, and congealed rock breaks into rough pieces
cinder cone
A cinder cone is a cone-shaped hill formed around a volcanic vent by fragments of lava (blocks, cinder, ash) blown out during eruptions (Figures 7-42 and 7-43). Cinder cones and lava flows can form from a vent or series of vents associated with an eruption
shield cone
A shield cone is broad, domed volcano with gently sloping sides, characteristic of the eruption of fluid, basaltic lava. The large volcanoes on Hawaii are shield cones (Figure 7-44), including Kilauea and Mauna Loa volcanoes. Shield cones are formed from very hot lavas
composite cone
A composite cone (also called stratovolcano) is a typically tall and large, steep volcanic cone built up of many layers of both lava and pyroclastics (tephra, pumice, and volcanic ash), often created by a series of cyclic eruptions in which pyroclastics are created by explosive eruptions until the vent is open, then lava flows occur (Figure 7-45). Most large continental volcanic cones are this type
dome volcano
A dome volcano is a volcano composed of lava domes; a lava dome is a roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano. Lava domes can vary from basalt to rhyolite in composition although most preserved domes tend to have high silica content
dike
A dike in a vertical or near vertical wall of igneous rock formed where magma squeezed into a fault zone or fracture before cooling and crystallizing. Dike form in volcanic regions, and often appear as dark castle wall-like features on landscapes where the host rock surrounding the intrusion have eroded away
sill
abular, typically more horizontal than vertical, sheet of intrusive igneous rock that has intruded between layers of older rocks. Dikes and sills can form simultaneously of at different stages in igneous activity in an area
