Rock Classification Victoria villano

Sedimentary rocks are types of rock that are formed by the deposition and subsequent cementation of that material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles (detritus) to settle in place.

There are three different types of Sedimentary Rocks.

Inorganic sedimentary rocks such as breccia, conglomerate, sandstone, silt-stone, and shale are formed from mechanical weathering debris.

Chemical sedimentary rocks, such as rock salt, iron ore, chert, flint, some dolomites, and some lime-stones, form when dissolved materials precipitate from solution.

Organic sedimentary rocks such as coal, some dolomites, and some lime-stones, form from the accumulation of plant or animal debris.

These are the two most common Sedimentary Rocks, and a little bit about each.

Breccia consists of angular rock fragments cemented together. The angular shape implies that the fragments have not moved far from their source.
Conglomerate is made of rounded or semi-rounded rock fragments cemented together. The rounding of the fragments implies that the fragments were transported a substantial distance from their source and were abraded in contact with other moving fragments.

Where these Rocks are Found.

Sedimentary rocks form when mud and sand are deposited in layers on the Earth's surface. The layers are deposited in many environments including oceans, rivers and deserts. These layers of sand and mud are later buried. The weight of overlying layers compresses the mud and sand to form solid rock.

Sedimentary Rocks are the most common rocks in America.


Igneous rock (derived from the Latin word ignits meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava.

Two types of Igneous Rocks

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.

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.

Andesite is a fine-grained, extrusive igneous rock composed mainly of plagioclase with other minerals such as hornblende, pyroxene, and biotite. The specimen shown is about two inches (five centimeters) across.
Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende, and sometimes quartz. The specimen shown above is about two inches (five centimeters) across.

Where They Are Found

Igneous rocks can be found on oceanic plates at divergent plate boundaries.

Metamorphic Rocks

Metamorphic rocks have been modified by heat, pressure, and chemical processes, usually while buried deep below Earth's surface. Exposure to these extreme conditions has altered the mineralogy, texture, and chemical composition of the rocks.

There are two basic types of metamorphic rocks.

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.

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.

Amphibolite is a non-foliated metamorphic rock that forms through recrystallization under conditions of high viscosity and directed pressure. It is composed primarily of hornblende (amphibole) and plagioclase, usually with very little quartz.
Gneiss is a foliated metamorphic rock that has a banded appearance and is made up of granular mineral grains. It typically contains abundant quartz or feldspar minerals. The specimen shown above is about two inches (five centimeters) across.
Rocks can be metamorphosed simply by being at great depths below the Earth's surface,


Created with images by James St. John - "Jasper breccia" • yaybiscuits123 - "Breccia" • James St. John - "Black chert nodules in limestone (Chepultepec Formation, Lower Ordovician; Natural Bridge State Park, Virginia, USA) 4" • James St. John - "Auriferous sulfidic hydrothermal quartz vein (gold ore) (Agnesa Vein, central zone of the Stiavnica Volcano, Central Slovakia Volcanic Field, Middle Miocene, 13-15 Ma; ~10 meters above the XIV Level, Rozalia Mine, Stvnicke Vrchy Mountains, Slovakia) 2" • James St. John - "Garnet schist (Late Cretaceous, 89-90 Ma; Garnet Ledge, about 12 km north of Wrangell, southeastern Alaska, USA)" • James St. John - "Garnet-biotite rock (alteration halo around magnetite-rich meta-iron formation) (Archean, metamorphism at about 3.4 Ga & about 2.8 Ga; Quad Creek section - roadcut along the Beartooth Highway, Beartooth Mountains, southern Montana, USA)" • James St. John - ""Rockville White Granite" (porphyritic granite, Rockville Granite, late Paleoproterozoic, 1.73 to 1.78 Ga; quarry near Rockville, Minnesota, USA) 2"

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