Porphyritic texture indicates the magma body underwent a multi-stage cooling history, cooling slowly while deep under the surface and later rising to a shallower depth or the surface where it cooled more quickly. The large crystals are called phenocrysts and the fine-grained matrix is called the groundmass or matrix. Some igneous rocks have a mix of coarse-grained minerals surrounded by a matrix of fine-grained material in a texture called porphyritic. This is a common component of volcanic ash and rocks like obsidian. This non-crystalline material is not classified as minerals, but as volcanic glass. In some cases, extrusive lava cools so rapidly it does not develop crystals at all. These tiny crystals can be viewed under a petrographic microscope. The fine-grained texture indicates the quickly cooling lava did not have time to grow large crystals. Extrusive igneous rocks have a fine-grained or aphanitic texture, in which the grains are too small to see with the unaided eye. When lava is extruded onto the surface, or intruded into shallow fissures near the surface and cools, the resulting igneous rock is called extrusive or volcanic. This sample is mostly fine groundmass with a few small green phenocrysts that are the mineral olivine. The individual crystals in phaneritic texture are readily visible to the unaided eye.īasalt is a classic fine-grained (aphanitic) extrusive igneous rock. The slow cooling process allows crystals to grow large, giving intrusive igneous rock a coarse-grained or phaneritic texture. If magma cools slowly, deep within the crust, the resulting rock is called intrusive or plutonic. The black colors are likely two or three different minerals. The different colors are unique minerals. 4.1.1 Texture Granite is a classic coarse-grained (phaneritic) intrusive igneous rock. Cooling history is also related to changes that can occur to the composition of igneous rocks. Composition refers to the rock’s specific mineralogy and chemical composition. This relates to the cooling history of the molten magma from which it came. Texture describes the physical characteristics of the minerals, such as grain size. Igneous rocks are classified based on texture and composition. This relationship between cooling rates and grain sizes of the solidified minerals in igneous rocks is important for interpreting the rock’s geologic history. These are known as coarse-grained intrusive, or plutonic, igneous rocks. In contrast, magma that cools slowly below the earth’s surface forms larger crystals which can be seen with the naked eye. Half Dome, a mass of intrusive igneous rock in Yosemite National Park, now exposed by erosion. This leads to different types of volcanoes and different volcanic hazards. Depending on the properties of the lava that is erupted, the volcanism can be drastically different, from smooth and gentle to dangerous and explosive. Volcanism is the process in which lava is erupted. Extrusive rocks are often vesicular, filled with holes from escaping gas bubbles. These are known as fine-grained extrusive, or volcanic, igneous rocks. Lava flow in Hawaii Lava cools quickly on the surface of the earth and forms tiny microscopic crystals. This chapter will describe the classification of igneous rocks, the unique processes that form magmas, types of volcanoes and volcanic processes, volcanic hazards, and igneous landforms. It is this magma that becomes the source for volcanoes and igneous rocks. However, there are a few minor pockets of magma that form near the surface where geologic processes cause melting. Only the Earth’s outer core is liquid the Earth’s mantle and crust is naturally solid. This molten material is called magma when it is in the ground and lava when it is on the surface. Igneous rock is formed when liquid rock freezes into a solid rock.
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