PE Rocks Igneous

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Information about PE Rocks Igneous

Published on September 20, 2007

Author: Peppar


Planet Earth: Rocks:  Planet Earth: Rocks © M. L. Anderson, 2005 The Rock Cycle:  The Rock Cycle Igneous Rocks & Activity:  Igneous Rocks andamp; Activity The early crust consisted entirely of igneous rocks. Later geologic activity modified the original igneous crust to form sedimentary, metamorphic and younger igneous rocks. Today, 95% of the Earth’s crust is still igneous rock or metamorphosed igneous rock. Most of this igneous rock is buried under a thin layer of sediments and sedimentary rock. Igneous rocks from magma:  Igneous rocks from magma Igneous rocks form when molten rock, magma, cools andamp; solidifies. This molten rock originates in the Aesthenosphere, through cracks in the lithosphere at @ 200 kilometers within the earth. The magma consists largely of elements found in silicate minerals, gasses and water vapor. Because the magma is lighter then surrounding rock it makes it's way to the surface. Magma is produced when rock is heated to it's melting point.:  Magma is produced when rock is heated to it's melting point. As magma moves into a near surface environment (i.e.: volcanoes) the pressure in the uppermost portion of the magma body is greatly reduced. This allows the gasses, which had been dissolved when they were at greater depths, to be released suddenly. Magma to Lava:  Magma to Lava These gasses expand to occupy hundreds of times their original volume. Lava is magma that has lost most of it's gases. Lava is extrusive or volcanic. Very fluid basaltic magmas allow these gasses to migrate up and escape from a vent on the surface with relative ease. Slide7:  Slide8:  BASALTIC MAGMA:  BASALTIC MAGMA As the gasses escape, they will often carry incandescent lava hundreds of meters into the air, producing lava fountains. Due to their low silica content, basaltic lava's are usually very fluid and flow in thin, broad sheets of stream-like ribbons. Eruptions of basaltic lava's, as in Hawaii, are relatively quiet. GRANATIC MAGMA:  GRANATIC MAGMA Highly viscous magmas impede the upward migration of the gasses. Gasses collect and build in pressure until they explosively eject the lava from the volcano. (Mt. St. Helen's) Only in the uppermost portion of the magma body, where confining pressure is low, does the gas pressure build up to explosive levels. After an explosive event, a quiet emission of gas-free lava occurs. Andestic & Granatic Lava's:  Andestic andamp; Granatic Lava's Most volcanoes that extrude andestic andamp; granatic lava's are confined largely to continental margins or volcanic island chains located adjacent to deep ocean trenches. Krakatau off the Java Trench, 1997 eruption Rate of Cooling:  Rate of Cooling The rate of cooling affects the size of the crystals. Slow cooling produces fewer growth centers, thus larger crystals. Rapid cooling results in the development if a large number of nuclei which compete for the available ions. This results in a solid mass of small inter-grown crystals. When magma cools, the silica and oxygen atoms link together first. TEXTURE:  TEXTURE The overall appearance of the rock is based on the size and arrangement of it's interlocking crystals. The environment during crystallization can be inferred from the size and arrangement of the mineral grains, or it's texture. Aphanitic - Fine grained texture:  Aphanitic - Fine grained texture Formed on the earth's surface or within the upper crust. The grains are too small to be seen by the naked eye. Light colored - nonferromagnetic silicate materials. (Rhyolite) Dark colored - ferromagnetic materials. (Basalt) Common are the voids, or vesicles, left by escaping gases. These are found at the outer portions of lava flows. Rhyolite Phaneritic - course grained :  Phaneritic - course grained Phaneritic - large masses of rock formed far below the surface forming a course grained texture. A mass of inter-grown crystals, roughly equal in size. The crystals are large enough to see with the eye. (Granite) A large mass of magma deep within the crust could take tens of thousands or millions of years to cool. Porphyritic Texture:  Porphyritic Texture If partially crystallized magma begins to cool suddenly, later crystals will be smaller, creating a porphyritic texture. The large crystals are the phenocrysts, the smaller ones create the ground mass. This rock is a porphyry. Glassy Texture:  Glassy Texture Rocks that are cooled instantly, have an unordered mass of atoms. The ions were not permitted the time to unite into an orderly crystalline structure. (Obsidian) Basaltic magma will usually generate crystalline rocks when cooled quickly in a thin lava flow. Obsidian: Volcanic glass:  Obsidian: Volcanic glass Granatic magma will produce a glassy textured rock under similar conditions. Obsidian: Volcanic glass. (Glass Mt., Calif.) Pyroclastic Rocks:  Pyroclastic Rocks Pyroclastic rocks - come from individual rock fragments that are fused together during an eruption. Tuff: tiny ash sized fragments welded together), welded tuff and Volcanic breccia : composed of particles larger than ash. Bowen’s Reaction Series:  Bowen’s Reaction Series The sequence in which minerals crystallize from a magma. Minerals crystallize from magma in a systematic fashion. During the crystallization process the solid and liquid components may separate and migrate either up or downward. If the earlier formed minerals are heavier then the liquid, they will sink to the bottom of the magma chamber. Intrusive Igneous Sequence:  Intrusive Igneous Sequence Above: Gabbro Right: Diorite Top R: Granite Slide22:  The first minerals to crystallize:  The first minerals to crystallize They are high in iron, magnesium, calcium and low in silicon. (Calcium feldspar, pyroxene, and olivine). Since basalt has this mineral makeup, all minerals with similar mineral makeup are called basaltic. Since basaltic rocks contain a high % of ferromagnetic minerals they are called mafic: (magnesium and Fe: Iron). Mafic rocks are darker and heavier than other igneous rocks, due to their iron content. Grand Ronde Basalt Mafic:  Mafic Basalt - dark green to black, fine grained. Most common extrusive igneous rock. Volcanic islands and upper layers of the oceanic crust. Gabbro - the intrusive equivalent to basalt. Makes up a significant portion of the oceanic crust. Ultramafic:  Ultramafic Peridotite - contains mostly olivine and is at the very beginning of the Bowen's reaction series. It is composed almost entirely of ferromagnetic minerals. Granatic and Felsic Rocks:  Granatic and Felsic Rocks The last minerals to crystallize are potassium feldspar and quartz - the primary components of granite. Rocks in which these two minerals predominate are granatic. Granatic rocks are Felsic (from feldspar and silica [quartz]). Intermediate Igneous Rocks :  Intermediate Igneous Rocks Minerals located in the center of Bowens Reaction series. Amphibole and intermediate plagioclase feldspars. These rocks are andesitic: from andesite. Diorite - course-grained intrusive rock. Absence of crystals. Andesite: Andes Mts.:  Andesite: Andes Mts. Andesite - Medium gray, fine-grained rocks of volcanic origin . Below: Nevado Ojos, Chile, Andes Mts. Granatic Rocks:  Granatic Rocks Granite is a Phaneritic rock composed of up to 25% quartz and over 50% potassium feldspar and sodium rich feldspar. The quartz crystals are mostly clear or gray and spherical in shape. The feldspar crystals are rectangular in shape, not as glossy, and are salmon pink to white. May contain muscovite and dark silicates - biotite and amphibole. Above: Granite; Below: Rhyolite. Rhyolite:  Rhyolite Rhyolite is the volcanic equivalent to granite. Watchung Mountains, N. J.; Columnar Jointing:  Watchung Mountains, N. J.; Columnar Jointing The rocks of the Watchung Mountains formed about 201MYA. Fluid basaltic lava poured out of a fissure in the bedrock (the Brunswick Formation). of the Newark Basin. As the lava cooled, it solidified, and in many places the basalt formed columnar joints. Located on either side of Route 280 in West Orange, NJ. Watchung Mountains, NJ; Columnar Jointing:  Watchung Mountains, NJ; Columnar Jointing Located off the westbound side of route 78 in New Providence, Scotch Plains and Watchung. Sheepeater Cliff, Yellowstone, Wy.:  Sheepeater Cliff, Yellowstone, Wy. Yellowstone had 3 very large eruptions in the last 2MY. During each event, a large volume of rhyolite magma was erupted from a shallow level in the crust and a large caldera formed. These eruptions occurred 2.0, 1.3, and 0.6 MY ago. The volume of lava erupted makes the 1980 eruption of Mount St. Helens look very small. Granatic Rocks in Sierra Nevada Batholith:  Granatic Rocks in Sierra Nevada Batholith Sierra Nevada Batholith:  Sierra Nevada Batholith The Sierra Nevada Batholith is a plutonic mass of granatic rocks that underlies 40,0002 kilometers of eastern California and Nevada. The main mass of the batholith is exposed in the Sierra Nevada as an irregular, ellipsoidal mass nearly 650 km. long and 100 km. wide. The rocks of the batholith age from about 220 to 75 myo. Sierra Nevada Batholith:  Sierra Nevada Batholith The batholith is magmatic. It is composite, consisting of a large number of plutons belonging to various intrusive suites. For example, in the area of Yosemite Valley alone, there are 10 different plutonic units. Sierra Nevada Batholith &Yosemite:  Sierra Nevada Batholith andamp;Yosemite The batholith as exposed in Yosemite Valley and regions west, contains mostly Cretaceous rocks with the rocks in general becoming younger towards the east. Yosemite National Park, Calif.:  Yosemite National Park, Calif. Yosemite Valley Map:  Yosemite Valley Map Slide40: 

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