Sedimentary rocks are formed by the accumulation of sediments. There are three basic types of sedimentary rocks.

Clastic (fragmental) sedimentary rocks- Inorganic land derived sediments classified according to grain size. These are formed by the compaction and cementation of the fragments deposited usually by running water entering calm water. The fragments are deposited in horizontal layers called beds or strata. Clastic sedimentary rocks such as breccia, conglomerate, sandstone, siltstone, and shale are formed from mechanical weathering debris.

Breccia Sandstone

Chemical sedimentary rocks- Precipitation (setting out) of minerals from a solution. They are referred to evaporate and precipitate. These are formed when the minerals in the water settle and then the water evaporates, leaving behind the rock. Sedimentary rocks formed by this process are usually called monominerallic. Monominerallic means composed of one mineral. Chemical sedimentary rocks such as rock salt, iron ore, chert, flint, some dolomites, and some limestones, form when dissolved materials precipitate from solution.

Rock salt Flint rock

Organic sedimentary rock- Formed from the remains of a once living thing. They are referred to bioclastic. Organic sedimentary rocks such as coal, some dolomites, and some limestones, form from the accumulation of plant or animal debris.

Coal Limestone

Metamorphic rocks- They form from already existing rocks. They can form from igneous, sedimentary and existing metamorphic rocks. They are formed by extreme heat and/or pressure. The rocks do not melt they recrystallize. There are two ways metamorphic rocks are formed:

1. Regional Metamorphism- This happens over a large area deep inside the earth. A. Layers of rock become Distorted (layers become wavy) B. Minerals to in bands (minerals separate out according to density creating bands) Metamorphic rocks that are distorted and/or have bands are Foliated. These rocks have Recrystallized to form bands of minerals. The minerals were rearranged without melting.Foliated metamorphic rocks such as gneiss, phyllite, schist, and slate have a layered or banded appearance that is produced by exposure to heat and directed pressure.

Gneiss Slate

2. Contact Metamorphism- This occurs when a rock is altered by coming into contact with melted rocks. Rocks formed in this way are considered Nonfoliated metamorphic rocks. These rocks are compose of crystals.Non-foliated metamorphic rocks such as hornfels, marble, quartzite, and novaculite do not have a layered or banded appearance. Pictures and brief descriptions of some common types of metamorphic rocks are shown on this page.

Marble Hornfels

Igneous means "fire formed". Rocks that form from molten rock that solidifies (hardness) are called Igneous. Igneous rocks are formed from the solidification of molten rock material. There are two basic types.

Intrusive igneous rocks- Crystallize below Earth's surface, and the slow cooling that occurs there allows large crystals to form. Examples of intrusive igneous rocks are diorite, gabbro, granite, pegmatite, and peridotite.

Gabbro Granite

Extrusive igneous rocks- Erupt onto the surface, where they cool quickly to form small crystals. Some cool so quickly that they form an amorphous glass. These rocks include andesite, basalt, obsidian, pumice, rhyolite, scoria, and tuff.

Scoria Basalt


Created with images by James St. John - "Arkose"

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