GEOL 0101/0102: Physical Geology
GEOL 0106: Historical Geology
GEOL 0124: Geologic Hazards

Common Rocks
Igneous Rocks

Igneous rocks form by the cooling and crystallization of molten rock and are often characterized by their random interlocking crystals. Cooling of the molten rock may take place at/near the Earth's surface, resulting in mineral crystals that are small (generally too small to see with the unaided eye) OR may take place deep below the surface, resulting in larger mineral crystals.

granite
Sedimentary Rocks

Sedimentary rocks form by the burial and cementation of loose sediments near the Earth's surface and are often characterized by their layered particles. Many of the sediments from which these rocks form were originally deposited by water, and fossils are nearly always found in sedimentary rocks.


conglomerate
Metamorphic Rocks

Metamorphic rocks form when heat and/or pressure are applied to earlier rocks. This happens most commonly along convergent plate boundaries, where some rocks are squeezed and pushed inside the Earth, resulting in metamorphic rocks that are often characterized by a non-random arrangment of mineral crystals.



gneiss
GRANITE

GABBRO

BASALT

ANDESITE
CONGLOMERATE

SANDSTONE

MUDSTONE

LIMESTONE
GNEISS

SCHIST

SLATE

MARBLE

QUARTZITE


ROCKS: Rocks are made up of one or more minerals and form in one of three characteristic ways. The different kinds of rocks and the processes that transform one kind into another are summarized by the "rock cycle", which is often illustrated by diagrams like the one below:

rock cycle


Igneous rocks form by cooling from molten rock (magma or lava). The cooling process results in the crystallization of minerals from the molten rock, giving igneous rocks a distinctive texture of randomly oriented interlocking mineral crystals. When exposed at the surface rocks of all types are subjected to weathering processes which break them down both physically (into smaller pieces) and chemically (into new minerals and/or individual ions that can be carried off in solution). The weathered debris is then subject to erosion by water, wind, or ice - which eventually deposit the material as sediment, commonly along stream valleys or in the oceans. As the sediment is buried by later deposits it undergoes lithification (compaction, and cementation by the growth of new minerals in the spaces between sediment grains) to become sedimentary rock . If caught between colliding plates, the sedimentary rock may be pushed deep into the Earth and subjected to pressures and temperatures that are great enough to cause changes in the rock (formation of new minerals, alignment of "platy" minerals), generating a metamorphic rock . Even deeper burial may eventually raise the temperature to the melting point of the material, generating magma and beginning the cycle again. Although this sequence is a common one, some variations are possible as well - for example all types of rocks can be weathered and eroded if exposed at the Earth's surface.


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Please forward any suggestions or corrections to Mike Young at myoung@wsc.ma.edu