It is very hot, very thin and free of gases. Rock Texture 1a. How do composition and texture relate to igneous rocks? Specimen is about 4.5 x 3.5 x 3.5 centimeters. Igneous Rocks study guide by hcps-pencecs includes 24 questions covering vocabulary, terms and more. Basalt forms at or near the surface as mafic magma cools, typically on oceanic plates. Basalt is a common form of extrusive igneous rock and forms lava flows, lava sheets and lava plateaus. Depending on the type of plate tectonic environment that a magma or lava is from effects its composition. The difference in elemental composition causes mafic rocks to be denser than felsic rocks. Igneous rocks are called intrusive or plutonic when they cool and solidify beneath the surface.Because they form within the Earth, cooling occurs slowly.Such slow cooling allows time for large crystals to form, therefore, intrusive or plutonic igneous rocks have relatively large mineral crystals that are easy to see. The photo below shows a close-up of the texture of this pink granite. These rocks form a series that varies in composition, from felsic to mafic because of an increasing fraction of iron+magnesium rich minerals that darkens the tone. 9. Where do igneous rocks form? Because divergent plates move apart from each other at these mid-oceanic ridges, magma flows upwards from the mantle beneath.. How would you identify an igneous rock as having phaneritic texture? Mafic rocks include basalt, its intrusive equivalent of gabbro, and a variety of less common rock types with higher or lower contents of Na and K. Andesite and its intrusive equivalent, diorite, with slightly higher silica content, are intermediate between mafic and felsic. The specimen shown here is about two inches (five centimeters) across. Mineral cleavage and hardness are additional features to help discern between minerals. Mafic magmas are ___ _____ than felsic magmas because silicon-oxygen tetrahedra tend to link together in magma to create long melecular chains that can't move past each other easily and there are more of these chains in a felsic magma than in a mafic magma. Where do glassy and frothy glass textured igneous rocks form? The Earth's mantle is composed of ultramafic rocks. Igneous rocks are formed by the cooling and crystallization of magma. A light-colored intrusive or extrusive felsic rock, the shallow version of granite, is called felsite or rhyolite. 2a. Basalt is dark and finely-grained, a mafic rock type rich in iron … Form by the solidification of molten rock or melt 2. Mafic rock, in geology, igneous rock that is dominated by the silicates pyroxene, amphibole, olivine, and mica. extrusive igneous rock. Pegmatite: Pegmatite is an igneous rock composed almost entirely of crystals that are over one centimeter in diameter. 7. You may have noticed that there are many kinds of rocks, from the soft sandy rocks that form the cliffs at Scripps beach to the hard rocks that form … Where do ophanitie rocks form? Mafic (high iron/magnesium content) at <50% silica; Denser at 2.8 - 3.0 g/cm 3; Rocks typically dark in color; Solidifies/melts at high temperatures: 1000-1250 °C; Low viscosity of 10 1-2 Pa s (similar to ketchup) allows flows to travel long distances; Effusive eruptions flows typical (lava seeps out). Basaltic rocks are found in association with rift zones, hot spots (like Hawaii), spreading centers (like exposed on Iceland), and are the dominant rock exposed along mid-ocean ridges and makes up the basement bedrock on the seafloor. Basalt is a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and pyroxene minerals. 8. Basalt and granite are the two most common forms of igneous rock. 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