Igneous Rocks
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- Formed by solidification of molten magma or lava
- Most common type of rock
- Scattered crystals of different minerals may or may not be visible to the naked eye
- Do not contain fossils
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Outcrops of granite rock. The dark color is due to desert varnish, not the rocks themselves. Piestewa Peak, AZ
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A red Granite.
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Granite. Note patches of different colored crystals. Usually light colored
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A basalt lava flow at Sunset Crater, AZ
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Basalt. Often has numerous small cavities
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Rhyolite. Rhyolites are lighter colored. Cavities may be few or absent in some
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Andesite (left) and Obsidian (right)
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Peridotite
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Gabbro (left) and Diorite (right)
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Metamorphic Rocks
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- Rocks that have been distorted by high heat and pressure
- Variable, but often contain layered patterns of minerals
- Rarely contain fossils
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Gneiss. Note the layers of different colored minerals
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Marble. An often whitish, soft rock (derived from limestone)
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Slate. A lightly transformed from shale; it can still contain fossils. Texture is fine-grained; hard and splits horizontally
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Sedimentary Rocks
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- Form when sediments are fused together
- Or form through the precipitation of dissolved minerals
- Characteristics reflect the sediments on which it is based (such as grain size)
- Contain the vast majority of fossils
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Layered sandstone cliffs. Petrified Forest N.P., AZ.
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Sandstone. The texture is relatively coarse with sand-sized particles. The colored layers result from accumulations of different types of sand.
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Shale. Composed of much finer particles (mud and silt) than sandstone. Often splits horizontally. Often gray.
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Limestone cliffs. The rounded edges and pitted surface are typical of limestone because it dissolves easily. Natural Bridges S.P., KY.
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Coquina Limestone. Limestone that has a high concentration of visible shelly material is called coquina.
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Limestone. The texture of limestone is quite variable, and can resemble shale or sandstone. Fizzes when exposed to acid
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Chert. Chert is often associated with limestone but is impregnated with silica rather than calcite. Forms curved surfaces when split
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Conglomerate. Contains various sized, rounded rocks cemented with finer particles
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Breccia. Differs from conglomerate in that the embedded rocks are angular rather than rounded
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