Definitions and images to illustrate geological terms, links to images and website articles


chonolithsclayscrack-seal filling (veins) ▪ cratonconcordantconformablecryoseismcumulates

Cratons ▫ Laurentian Craton

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The term chonolith is used to describe intrusive igneous bodies with a nonspecific, irregular shape that does not fit into other categories of plutonic structure (such as dike, sill, or laccolith).

A cactolith is a quasi-horizontal chonolith composed of anastomosing ductoliths, whose distal ends curl like a harpolith, thin out like a sphenolith, or bulge discordantly like an akmolith or ethmolith.

Ductoliths are horizontal plugs of teardrop cross section, or a headed dike.

Harpoliths are large, sickle-shaped intrusions injected into previously deformed strata, intruding horizontally in the direction of maximum orogenic displacement.

A sphenolith us a partly concordant, partly crosscutting injected body of igneous rock, in which country rocks are overturned in some sections.

Akmoliths are intrusive bodies injected along a decollement, which send numerous tongues into the overlying folded rocks.

Ethmoliths are cross-cutting bodies of plutonic rock that narrow downward and are thus funnel-shaped.

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Concordant or conformable, when referring to plutonic bodies, indicates that the intruding magma of sills and laccoliths lies parallel to rather than cutting across country strata, as do discordant structures such as veins, dikes, bysmoliths, and batholiths.

A concordant coastline comprises bands of different rock types that run parallel to the shore. The rock types are typically of alternating resistance, so that the coastline forms distinctive landforms, such as coves. A discordant coastline comprises rock types of alternating resistance that run perpendicular to the shore, creating distinctive landforms when the rocks are eroded by ocean waves. Less resistant rocks erode faster, creating inlets or bays; more resistant rocks erode more slowly, remaining as headlands or outcroppings.

Concordant flows at different points in a river system have the same recurrence interval, or the same frequency of occurrence. The term is most often applied to floodflows.

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cratons are orange areas in image of global geological provinces Cratons, or continental platforms are the ancient, stable geological provinces at the core of continents (orange in image at left). Cratons have peristed for more than 500 million years (some over 2 billion years).

A shield is defined as a craton in which basement rocks have been exposed by erosion at the surface; and such shields typically comprise Precambrian basement rocks.

Cratons characteristically consist of ancient felsic igneous rocks in the crystalline basement at the center of continents, as distinguished from the linear belts of geosynclinal troughs at the margins of continents. Although the oldest rocks and proto-continents date from the Hadean, the earliest large cratonic landmasses formed during the Archean eon when radioactive decay maintained heat flow at three times current levels.

Cratons have a thick continental crust and deep roots that extend into the mantle beneath to depths of 200 km. Cratons sit on anomalously cold mantle that is more than twice the approximately 100 km thickness of mature oceanic or noncratonic continental lithosphere.

Slave Craton

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Cryoseisms, frost quakes, or ice quakes are non-tectonic seismic events caused by the expansion of water that has frozen in rock cracks or soils. The expansion of water on freezing generates stresses in the surrounding rock, and this stress may be released explosively in a cryoseism. By a different mechanism, cryoseims can accompany glacial surges when ice at the base of a glacier, which had been frozen to bedrock moves suddenly when released by a thin layer of lubricating meltwater.

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