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Types of Rock and Slate

The formation and combination of minerals and organic matter makes thousands of different types of rocks. Petrologists are geologists who study these rocks. Location and time of creation can make even more distinctions among rocks of the same type. For categorization, petrologists use the ways rocks form to divide them into three main groups.

Sedimentary Rocks
- Sedimentary rocks are called secondary rocks, because they form when minerals, calcium and current rocks break down into minute pieces called sediment. These pieces then drift on wind, water, glaciers or animals until they gather with other bits of sediment. After thousands of years, the sediment compacts into rocks. Sedimentary rocks also form when water repeatedly hydrates and evaporates off minerals for thousands of years. Sandstone is compacted sand and shale is compacted clay.
Igneous Rocks
- Igneous rocks form when minerals cool from a molten state. They are further divided into extrusive rocks or intrusive rocks. Extrusive rocks, including basalt and andesite, form when lava flows above ground where it is exposed to the atmosphere. These rocks have a grainy texture because the lava cools too fast for the minerals to form crystals. Intrusive rocks, such as granite, form when magma flows underground. These rocks cool slower without atmosphere exposure, so the minerals can form crystals.
Metamorphic Rocks
- Metamorphic rocks form when any rock is in a stressful environment so that it undergoes a change to maintain equilibrium. Stressors include chemicals, extreme pressure or temperature. Metamorphic rocks are either foliated or nonfoliated. Foliated rocks, including slate and gneiss, are flat with a defined shape. Nonfoliated rocks are much larger and have no structured shape. The most commonly known nonfoliated rock is marble, which is transformed limestone.
Slate
- Slate forms when shale enters a stressful environment. Different chemical and mineral compositions in the shale create the different colors of slate. Petrologists can use the color to determine where a piece of slate originated. For example, bright red slate is most commonly found in Washington County, NY. Slate is further divided into weathering, semi-weathering and unfading. Weathering and semi-weathering slates change colors or fade when exposed to the elements. Unfading slates stay hardy and vibrant throughout years of use.

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