Igneous rocks classification igneous rocks are

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Igneous Rocks Classification Igneous rocks are typically classified on the basis of their texture (crystal size and arrangement) and chemical composition (minerals present). The texture of an igneous rock reflects its cooling history. The composition of igneous rock, to a large degree, reflects its plate tectonic setting during formation. I. Igneous Textures The texture of an igneous rock reflects how the magma cooled and crystallized to form minerals. The size of the crystals depends on the cooling rate. Coarse-grained textures indicate slow cooling (tens of thousands of years), whereas fine-grained and glassy textures indicate fast cooling (months to hours). Texture is used to indicate whether the magma cooled at the surface ( volcanic ) or deep underground ( plutonic ). The following are igneous textures: Coarse-grained: Interlocking crystals (typically 1-10 mm) that can be seen with the naked eye. Great thickness of overlying rock insulated the magma, so that it cooled slowly to form large crystals. Igneous rocks exhibiting this texture cooled deep underground. Fine-grained: Small interlocking crystals (typically <1 mm, which are too small to see with the naked eye). Most fine-grained igneous rocks cooled at the Earth’s surface after being erupted from a volcano. Fine-grained textures can also result from shallow intrusions, or if magma is injected into fractures in cooler rock. These injections are called dikes or sills. Porphyritic: An igneous rock texture that is composed of two different distinct crystal sizes (see Fig. 1). Specifically, crystals >2mm in size are called phenocrysts , and they are embedded in a groundmass made of fine-grained crystals (often called a matrix ). Porphyritic rocks are interpreted to have undergone two stages of cooling: the phenocrysts would form while the magma is slowly cooling deep under ground, and, when the magma erupts, the groundmass cools quickly when exposed at the surface. Figure 1. A volcanic rock with a porphyritic texture. Crystal (phenocryst) Groundmass (matrix)
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Lab #2: Igneous Rocks 33 Glassy: Volcanic glass is called obsidian and lacks crystals. Rocks that are very glassy show a characteristic conchoidal (curved) fracture pattern (see Fig. 2). Glassy texture forms when lava cools so quickly, or is so viscous that ions can not migrate through the melt and become arranged in an ordered pattern to form crystals. Pyroclastic: When molten material is erupted from a volcano it can solidify before it hits the ground, yet still be hot enough to weld to other erupted material (Fig. 3). This forms a rock called tuff , which is composed of fragments (of other rocks) and crystals, all embedded in a matrix of ash. Vesicular: Vesicles are holes in volcanic rock that form as lava solidifies around gas bubbles. These are good indicators of volcanic rocks (plutonic rocks don’t have vesicles). This term can be used as an adjective (e.g. this is a vesicular basalt). The size of the bubbles is often dictated by the viscosity of the magma, with bigger bubbles forming in less viscous magma because they can coalesce more easily.
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