GLG111-igneous - Chapter 5 Igneous Environments What...

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Chapter 5: Igneous Environments What Textures Do Igneous Rocks Display? -Large crystals, typically centimeters long called pegmatite . -Crystals larger than several mllimeters called coarsely crystalline -Crystals that are easily visible to the unaided eye are Medium-grained -Too small to see without a magnifying glass is called fine-grined -Some consist of glass rather than minerals called volcanic glass or glassy -Larger crystals in finer grained matrix is called porphoritic -Volcanic rocks with small hoes known as vesicles caused by gas released -Volcanic ashe and pumice can become compacted called being welded -Angular gragments in a finer matrix can be called a volcanic breccia In What Settings Does Each Igneous Texture Form - Vesicles : form when gases dissolved in the magma accumulate as bubbles. They can form only under low pressures on or very near the surface . - Breccia can form in many ways, including explosive eruptions of ash and rock fragments, a lava flow that breaks apart as it solidifies and flows. - Glass forms when magma erupts on the surface and cools so quickly that crystals do not have time to form. - Porphoritic forms when some crystals must have time to grow in magma chambers at depth. Magma rises closer to the surface where the remaining magma solidifie s into the fine-grained matrix around the larger crystals. - Pyroclastic flows rapidly downhill and is welded. - Coarse-grained form at depth, where the magma cools at a rate that is slow enough to allow large crystals to grow. - Pegmatite forms when water dissolved in the magma helps large crystals grow. This occurs near the sides and top of a magma chamber and in local pockets within the mama. Most occurs deep within the Earth’s crust . How Does the Composition of Igneous Rocks Vary? Felsic Intermediate Mafic Ultramafic Coarsely Crystalline Granite: light- colored rock Diorite: more mafic minerals Gabbro: consists of Peridotite: more potassium and
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consisting of fledspar and quartz with some mica. than granite. It is between felsic and mafic compositions. mafic minerals such as dark- colored pyroxene and calciumrich feldspar. iron minerals and dark as gren olivine and dark pyroxene. Finely Crystalline or Glassy Rhyolite: fine- grained equivalent of granite. Pieces of pumice and ash, glass, crystals. . Andesite: fine- grained euivalent of diorite. It is commonly gray or greenish and may have phenocrysts. Basalt: common and familiar dark laa rock. Most samples are dark gray to nearly black and some have vesicles. Unusually long
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GLG111-igneous - Chapter 5 Igneous Environments What...

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