San Diego Natural History Museum--Your Nature ConnectionSDNHM Field Guide

Glossary

Arc
See volcanic arc.
Asthenosphere
The weak layer in the earth’s mantle that sits below the lithosphere. It deforms plastically when stressed and is the layer in the earth on which the lithospheric plates ride.
Absolute age
Age as measured in a specific quantity such as years, millions of years (Ma), or billions of years (Ba) before present.
Basalt
A fine-grained (volcanic) igneous rock composed of olivine, pyroxene, and plagioclase. See also gabbro.
Batholith
An exposed, irregular mass of igneous rock greater than 100 kms2 in total area and composed of individual plutons.
Continental crust
A large mass of lower density material, consisting of granitic igneous and/or sedimentary rocks, that sits on top of the earth’s mantle. Continental crust varies from about 70 to 30 kms in thickness.
Dike
A planar or tabular mass of igneous rock that formed by the injection of melt into a preexisting fracture that cuts across older rock structures.
Emplacement age
The time at which an igneous body solidifies in the Earth’s crust.
Gabbro
A coarse-grained (plutonic) igneous rock composed of various amounts of olivine, pyroxene, and plagioclase. See also basalt.
Gneiss
A metamorphic rock with elongate clusters of dark minerals.
Grus
Deeply weathered, disintegrated igneous rock.
Igneous
A category of rock formed by the crystallization of molten material. See also magma.
Lithosphere
The outer, rigid shell of the earth that consists of continental and oceanic crust as well as approximately 80-100 kms of underlying mantle.
Lithospheric plate
A segment of the earth’s lithosphere that is overlain by either oceanic crust or continental crust. Same as tectonic plate. See oceanic crust, continental crust.
Magma
Molten rock material consisting of melt and any contained solids.
Mantle
That portion of the earth that lies between the crust and the earth’s outer core.
Melt
The molten portion of a magma.
Metamorphic
A category of rock formed deep within the earth from pre-existing rock material as the result of high temperatures and pressures, or by reaction with chemically active fluids.
Metasedimentary
Metamorphosed sedimentary rocks.
Oceanic crust
A large mass of medium density material, consisting of basaltic igneous and/or sedimentary rocks, that sits on top of the earth’s mantle. Oceanic crust typically underlies ocean basins and varies from about 5 to 10 kms in thickness.
Paleomagnetism
The study of fossil magnetic fields as recorded in iron-bearing minerals.
Paleolatitude
The initial latitude of a rock mass prior to its movement via plate tectonic.
Pluton
An intrusion of igneous material that solidified at depth. See also batholith.
Rift zone
See spreading center.
Schist
A metamorphic rock characterized by an abundance of platy minerals such as mica or chlorite.
Sedimentary
A category of rock formed by the accumulation and lithification of weathered rock debris.
Spreading center
A linear zone in the earth’s crust whose opposite sides are moving away from one another.
Spreading rate
The speed at which two sides of a spreading center are moving apart. Generally given in inches or centimeters/year.
Strike-slip fault
A vertical fault in the earth’s crust whose sides are moving in opposite directions.
Subduction
The process whereby one tectonic plate is forced beneath another.
Volcanic arc
An arcuate band of volcanic rocks formed at approximately the same time from a single subduction event.