Definitions modified from the Glossary of Geology (5th Ed., American Geological Institute, 2005) and Wikipedia.


andesite     A dark-colored, fine-grained extrusive (volcanic) rock; 57%–63% SiO2; intermediate in composition between basalt and dacite; often porphyritic.

anticline     A convex-upward fold; the core of the fold contains the stratigraphically older rocks.

anticlinorium     A composite anticlinal structure of regional extent composed of smaller folds.

arête     A narrow mountain crest or rocky, sharp-edged ridge or spur; carved by glaciers, and resulting from the headward migration of the walls of adjoining cirques.

basement rock     The crust of the Earth below sedimentary deposits; typically igneous or metamorphic and of Precambrian age.

biscuit-board topography     A glacial landscape characterized by a rolling upland on the sides of which are cirques that resemble the bites made by a biscuit cutter in the edge of a slab of dough.

butte     An isolated hill having steep, often vertical sides and a small, relatively flat top; a butte represents an erosional remnant carved from flat-lying layers of rock. The area at the top of a butte is smaller than that of a mesa.

cinder cone     A conical hill formed by the accumulation of volcanic cinders and other pyroclastic deposits; typically of basaltic or andesitic composition.

cirque     A steep-walled, half-bowl-like hollow situated high on a mountain at the head of a glacial valley, formed by the erosive activity of an alpine glacier.

convergent plate boundary     A boundary between two lithospheric plates that are moving toward each other.

dacite     A fine-grained extrusive (volcanic) rock; 63%–77% SiO2; intermediate in composition between andesite and rhyolite; often porphyritic.

extrusive     Said of igneous rock that has been erupted onto the surface of the Earth; extrusive rocks include lava flows and pyroclastic material such as volcanic ash.

foliation     The planar arrangement of textural or structural features in a rock, such as a preferred orientation of crystal planes in mineral grains, a preferred orientation of grain shapes, or compositional banding.

footwall     The area of rock lying beneath the plane of an inclined fault.

glacier     A large mass of ice formed, at least in part, on land by the compaction of recrystallization of snow; moves slowly downslope or outward in all directions due to the stress of its own weight; includes small mountain (alpine) glaciers and continental ice sheets.

gneiss     A foliated rock formed by regional metamorphism; foliation is characterized by alternating lighter and darker colored bands.

granite     An intrusive igneous rock having a granular texture and composed mostly of quartz (at least 20% by volume), mica, and feldspar;  broadly applied to any quartz-bearing intrusive rock (i.e., “granitic rock”).

hanging wall     The area of rock lying above the plane of an inclined fault.

igneous     Said of a rock or mineral that solidified from molten or partially molten material (i.e., magma or lava).

lahar     A mudflow composed primarily of volcanic material on the flank of a volcano.

lava     A general term for a molten extrusive; also, for the rock that is solidified from it.

lava dome     A dome-shaped mountain of solidified lava comprising many individual flows; formed by highly viscous lava.

magma     Molten or partially molten rock material beneath the surface of the Earth.

massif     A massive topographic and structural feature commonly formed by rocks more rigid than those of its surroundings; used to describe a large mountain mass or compact group of connected mountains forming an independent portion of a range.

mesa     An elevated area of land with a flat top and sides that are usually steep cliffs; rock layers capping a mesa are typically flat lying and resistant to erosion. The area at the top of a mesa is larger than that of a butte.

metamorphic     Said of a rock derived from pre-existing rocks by mineralogical, chemical, and/or textural changes, essentially in the solid state, in response to marked changes in temperature, pressure, or chemical environment.

moraine     A mound, ridge, or other distinct accumulation of sediment deposited by direct action of glacier ice.

normal fault     A fault characterized by a mostly vertical component of movement, in which the hanging wall has moved downward relative to the footwall. The Wasatch fault in Utah is an example of a normal fault.

orogeny     The process of formation of mountains. Includes faulting, folding, and metamorphism.

porphyritic     Said of the texture of an igneous rock in which larger crystals (phenocrysts) are set in a finer-grained groundmass.

pyroclastic     Pertaining to clastic (fragmental) rock material formed by volcanic explosion or aerial expulsion from a volcanic vent. Literally means “fire-broken.”

rhyolite     A silica-rich (>69% SiO2) volcanic rock; can be considered the extrusive equivalent of granite.

rift zone     A long, narrow, continental trough formed by extension of the crust and bounded by normal faults; some rift zones eventually fill with very thick sequences of sediment.

sedimentary     Said of a rock resulting from the consolidation of loose sediment that has accumulated in layers; sediment includes solid fragmental (clastic) material that originates from weathering of rocks and is transported and deposited by water, ice, or air, or that accumulates by other natural agents such as chemical precipitation.

stratovolcano     A volcano that is constructed of alternating layers (strata) of lava and pyroclastic deposits.

strike-slip fault     A fault on which the movement occurs in a horizontal direction. When looking across the fault, if the block of crust on the opposite side of the fault moves to the left, the fault is called a left-lateral strike-slip fault; if the opposite side moves to the right, the fault is a right-lateral strike-slip fault. The San Andreas fault in California is an example of a right-lateral strike-slip fault.

subduction zone     A long, narrow belt along which one lithospheric plate converges upon and descends beneath another.

syncline     A concave-upward fold; the core of the fold contains the stratigraphically younger rocks.

talus     Rock fragments, usually coarse and angular, derived from and lying at the base of a cliff or on a steep, rocky slope; formed by the rocks falling, rolling, or sliding to their present positions.

terrane     A fault-bounded body of rock of regional extent, whose geologic history differs from that of adjacent terranes.

thrust (fault)     A fault in which the hanging wall has moved upward relative to the footwall, and horizontal compression rather than vertical displacement is its characteristic feature.

transform plate boundary     A boundary between two lithospheric plates that are sliding past one another horizontally (i.e., along a strike-slip fault).

transpression     Crustal deformation characterized by oblique compression.

volcano     A vent in the surface of the Earth through which magma and associated gases erupt; also, the form or structure, typically conical, that is produced by the ejected material.

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