absolute age dating

Assigning units of time, usually years, to fossils, rocks, and geologic features or events.  The method utilizes mostly the decay of radioactive elements, but ages may also be obtained by counting tree rings or annual sedimentary layers.

alpha decay

The transformation that occurs when the nucleus of an atom spontaneously emits a helium atom, reducing its atomic mass by 4 units and its atomic number by 2; for example,

.  amorphous

Descriptive term applied to glasses and other substances that lack an orderly internal crystal structure.

A fine-grained igneous-rock texture resulting from rapid cooling.  Aphanites are fine-grained igneous rocks.

The smallest particle of an element that can exist either alone or in combination with similar particles of the same or a different element.

sum of the number of protons and neutrons (uncharged particles) in the nucleus of an atom.

:   The number of protons (positively-charged particles) in an atomic nucleus.   Atoms with the same atomic number belong to the same chemical element.

A large, discordant intrusive body greater than 100 km²  (40 mi²) in surface area with  no known bottom.  Batholiths are granite in composition.

The radioactive emission of a nuclear electron (beta-ray, B¯ ) that converts a  neutron to a proton.  The element is changed to the element of next highest atomic number; for example,

Rocks resulting directly from the activities of living organisms, such as a coral reef, shell limestone or coal.

:   Rock resulting from precipitation of chemical compounds from a water  solution.  See evaporate.

The breaking of minerals along certain crystallographic planes of weakness that reflects their    internal structure.

Rocks composed of fragments of minerals or other rock materials.

The angle that an inclined geological planar surface (sedimentary bedding plane, fault, joint) makes with the horizontal plane.

Igneous rocks that formed at or near the surface of the earth.  Because they cooled rapidly, they generally have an aphanitic texture.

:  Descriptive of magma or rock with abundant light-colored minerals and a high silica content.  The term is derived from feldspar + silica + c.

The planar or wavy structure that results from the flattened growth of minerals in a metamorphic rock.

Break in a rock caused by tensional, compressional or shearing forces (see also joints).

The period of time during which one-half of a given number of atoms of a radioactive element or isotope will disintegrate.

Rocks that crystallize from molten material at the surface of the earth (volcanic) or within the earth (plutonic).

Igneous rocks that have “intruded” into the crust, hence they are slowly cooled and generally have phaneritic texture.

An atom with a positive or negative electrical charge because it has gained or lost one or more electrons.

One of two or more forms of an element that have the same atomic number but different atomic masses.

Separation or parting in a rock that has not been displaced.  Joints usually occur in groups (“sets”), the members of a set having a common orientation.

The manner in which a mineral reflects light using such terms as metallic, resinous, silky, and glassy.

Descriptive of a magma or rock rich in iron and magnesium.  The mnemonic term is derived from magnesium + ferric + ic.

Molten material within the earth that is capable of intrusion or extrusion and from which igneous rocks form.

metamorphic rocks

Preexisting rocks that have been altered by heat, pressure, or chemically active fluids.

Naturally occurring crystalline substance with well-defined physical properties and a definite range of chemical composition.

A scale that indicates the relative hardness of minerals on a scale of 1 (talc, very soft) to 10 (diamond, very hard).

The positively charged central core of an atom containing protons and neutrons that provide its mass.

A coarse-grained igneous-rock texture resulting from slow cooling.  Phanerites are coarse-grained igneous rocks.

Pertaining to rock formed by any process at depth, usually with a phaneritic texture.

(“fire broken”) Descriptive of the fragmental material, ash, cinders, blocks, and bombs ejected from a volcano.  (See also tephra).

The formation in the solid state of new mineral grains in a metamorphic rock.  The new grains are generally larger than the original mineral grains.

The chronologic ordering of geologic strata, features, fossils, or events with respect to the geological time scale without reference to their absolute age.

The cycle or sequence of events involving the formation, alteration, destruction, and reformation of rocks as a result of processes such as erosion, transportation, lithification, magmatism, and metamorphism.

:  A layer of rock deposited from water, ice, or air and subsequently lithified to form a coherent rock.

(bedding)  The arrangement of sedimentary rocks in strata or layers.

The direction west or east of north of the trace of the inclined plane (sedimentary bedding plane, fault, or joint surface) on the horizontal plane.