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Igneous rocks are named by the latin word ignis which means "fire." These rocks are formed from a molten silicate material called magma. |
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Minerals crystalize at different temperatures and different rates, but they do it in a systematic manner. Bowen's reaction series shows the sequence in which minerals crystallize from a magma to form the many different igneous rocks. |
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Igneous rocks can be classified into two categories; plutonic and volcanic. Plutonic rocks are crystallized at depths below the surface and have coarser sized grains which can be seen by the naked eye. They have a phaneritic texture. Examples include Granite, Granodiorite, Diorite, and Gabbro. Volcanic rocks are fine grained, extrusive rocks. They cooled very quickly at the surface. You can't see the individual crystals. Examples include Rhyolite, Andesite, and Basalt. In the picture to the right, you can see the individual crystals in the piece of granite at Enchanted Rock.
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Igneous rocks are often in the form of intrusive bodies. They are classified according to their shapes and their relationships to structures in the country rock. A dike is an igneous rock mass that cuts across the country rock. Dikes usually cut across planar cracks and zones of weaknesses. |
At Enchanted Rock, you can see many examples of pegmatic dikes cutting across the granite batholith. |
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| GRAPHIC | Metamorphic rocks are exposed on every continent in relatively flat regions known as shields. They are an important part of mountain belts and a significant part of the earth's continental crust. Metamorphism is a transformation of pre-existing rocks. They can form igneous and sedimentary rocks. |
How do these rocks change form? They change by agents of heat, pressure, and chemically active fluids. Sometimes they change due to all three agents. Heat from below the earth's surface causes extreme temperatures which can make cetain minerals unstable and therefore recrystallize. Pressure increases with and causes stress. The heat and pressure cause the rocks to bend and fold during mountain building. Chemically active fluids ,like water, also causes changes. The water helps the ions of the crystals move and recrysatllize into more stable forms. |
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| GRAPHIC | In the diagram to the left, note how evenly the crystals line up after stress. |
There are two types of metamorphic rocks; foliated and nonfoliated. Foliated rocks have a layered or banded appearence. The minerals are in parellel alignment. Examples include: slate, phyllite, schist, and gneiss. Nonfoliated rocks are composed of only one mineral that forms equidimensional crystals. Examples include: marble, quartzite, and hornfels. |
In the Texas Hill Country, metamorphic rocks can be seen near Inks Lake State Park. Valley Spring Gneiss was buried by sediments and metamorphosed during Precambrian mountain building. You can see the dark grey weathering gneiss in the picture to the left. |