Types of Rock

Rocks are any naturally occurring solid mass of one or more minerals, mineraloids, or organic materials. Rocks are classified into three main types based on how they were formed and their composition. They include igneous, sedimentary, and metamorphic.

Igneous rock
Igneous rock is rock that is formed by cooling and solidification of molten material, such as magma or lava (examples of igneous rock are granite, basalt, pumice and gabbro).

Sedimentary rock
Sedimentary rock is rock that is formed by the accumulation of mineral particles and/or organic matter that has been deposited in an area and then consolidated and/or cemented. So, these are rocks that are formed from pre-existing rocks that have been broken down into smaller rocks or smaller particles, such as sand, pebbles, or mud, and once-living plants or animals or parts of them. (ex: limestone, sandstone, shale. Siltstone, conglomerate, chalk, chert, mudstone, rock salt, and breccia)

Metamorphic rock
Metamorphic rock is rock that has been altered or transformed from its original form by high heat and pressure in a process called metamorphism. Metamorphic rocks can also be created by chemical processes, which cause changes in their mineral composition or texture. (ex: marble -from limestone, slate-from shale) slate, marble, gneiss, schist, quartzite, phyllite

Rocks come in all sorts of shapes and sizes but are generally differentiated by particle or grain size. The size classes are, from largest to smallest:

Boulder
Boulder is a large rock, typically one that has been worn smooth by erosion. A boulder is too large for a person to move. Smaller pieces are called cobbles, pebbles. Smaller boulders are usually called rocks or stones.

Cobble
Cobble is larger than a pebble and smaller than a boulder.

Pebble
Pebble is very coarse gravel, coarse gravel, and fine gravel. Generally larger than granules and smaller than cobbles.

Granule
A granule is larger than sand and smaller than pebbles.

Sand
Sand is very coarse sand, coarse sand, medium sand, fine sand, and very fine sand. A granular material composed of finely divided rock and mineral particles. It is finer than gravel and coarser than silt.

Silt
Granular material between sand and clay. It usually has a floury feel when dry and slippery when wet. Easily transported in water and fine enough to be carried long distances by air int form of dust.

Clay
Clay is finely grained natural rock or soil material. It contains water trapped in the mineral structure.

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