Types of Rocks

Three Basic Rock Types.
                            The earth’s crust is composed of three basic rock types: igneous, sedimentary and
metamorphic.

Igneous rocks.

 Igneous Rocks are formed from the crystallization of molten rock (magma or lava) from
within the earth’s mantle. Common igneous rocks include granite, basalt, and gabbros.

 

 Metamorphic rocks.

 Metamorphic Rocks are formed from pre-existing rocks by mineralogical, chemical
and/or structural changes in response to marked changes in temperature, pressure,
shearing stress, and chemical environment. These changes generally take place deep
within the earth’s crust. Examples of common metamorphic rocks include slate, marble
and schist.



Sedimentary rocks.

 Sedimentary Rocks are formed as sediments, either from eroded fragments of older rocks
or chemical precipitates. Sediments lithify by both compaction, as the grains are
squeezed together into a denser mass than the original, and by cementation, as minerals
precipitate around the grains after deposition and bind the particles together. Sediments
are compacted and cemented after burial under additional layers of sediment. Thus
sandstone forms by the lithification of sand particles and limestone by the lithification of
shells and other particles of calcium carbonate. These types of rocks are typically
deposited in horizontal layers, or strata, at the bottom of rivers, oceans, and deltas.
Limestone, sandstone, and clay are typical sedimentary rocks.

            

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