APHANITIC - a texture of an igneous rock in which the crystals are too small to be seen with the naked eye.

CLASTIC - (or DETRITAL) a sediment composed of fragments of pre-existing rock, weathered at the earth's surface.

CRYSTALLIZATION - the formation of crystals, resulting from the cooling and solidification of molten rock (as applied to igneous rocks); or from chemical precipitation (solids formed from a dissolved state - as applied to sedimentary rocks).


EXTRUSIVE (or VOLCANIC) - a descriptive term for igneous rocks that formed through the crystallization of lava, which is molten rock that reaches the earth's surface. Because of the rapid rate of cooling, extrusive rocks have tiny crystals, or if cooled very rapidly, have no crystals at all (as in a volcanic glass, or obsidian).

FELSIC - igneous rocks composed primarily of feldspar ("fel") and quartz or silica ("sic").  Because felsic minerals are light in color, felsic igneous rocks are also light in color.

FOLIATION - the layered texture found in metamorphic rocks, caused by great pressure; elongated or platy minerals such as amphiboles and micas respond to this pressure by aligning into layers or bands.

FOSSIL - the evidence of remains of prehistoric life.

IGNEOUS - rocks which formed by the cooling and solidification (crystallization) of molten rock which originated from below the earth's surface.

INTRUSIVE (or PLUTONIC) - a descriptive term for igneous rocks that formed through the crystallization of magma deep within the earth. Because the cooling rate of deeply buried magma is slow, large crystals are found in intrusive rocks.

LAVA - molten rock which reaches the surface of the earth, commonly seen as a volcano.

LITHIFICATION - the formation of a rock due to cementation and/or compaction; usually applied to sedimentary rocks.

MAFIC - rocks composed primarily of minerals containing magnesium and iron (Fe); usually applied to igneous rocks. Because mafic minerals tend to be dark, mafic rocks are mostly dark in color.

MAGMA - molten rock found below the earth's surface, often containing dissolved gases which later escape if the magma reaches the surface (which then becomes lava).

METAMORPHIC - rocks that have been changed due to heat and/or pressure.

NON-CLASTIC - sediments composed of chemical precipitates, biochemical sediments, and organic sediment.

PHANERITIC - an igneous rock composed of crystals large enough to be seen with the naked eye.


PORPHYRITIC - a textural term for igneous rocks that underwent a 2-stage cooling history; initially, coarse crystals  (called "phenocrysts") cooled below the ground, but were dragged to the surface by a volcanic eruption which later produced a larger proportion of fine-grained aggregate of crystals (called the "groundmass").

ROCK - an aggregate or mixture of one or more minerals; each mineral retains its individual properties.

SEDIMENT - solid particles of minerals or organic material, found at the earth's surface. When cemented, sediments become sedimentary rock.

SEDIMENTARY - rocks which form at the earth's surface, made up of either particles of pre-existing rock or chemical precipitates (minerals solidified from a dissolved state).

SORTING - the degree of similarity in the grain sizes of sediments or sedimentary rock; for example, well-sorted rocks would include shale, siltstone, and quartz sandstone; poorly-sorted rocks would include conglomerate, breccia, arkose sandstone, and glacial till.

TEXTURE - the size, shape, and orientation of crystals in a rock; when analyzed properly, they help to determine the rock's origin.

VESICULAR - a textural term for a volcanic rock which contains many holes or cavities as a result of escaped gases.

VOLCANIC - an igneous rock originating from lava erupted at the earth's surface (see EXTRUSIVE).

WEATHERING - physical and chemical processes which operate at the earth's surface, which ceaselessly break down rocks into fragments, which when deposited, are called sediment.