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assimilation-fractional crystallization

Overview page. Subjects: Earth Sciences and Geography.

(AFC)

An important process in igneous petrology whereby melts with widely differing isotopic and trace elements can be produced. When a primitive magma, such as a basalt, invades...

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batholith

Overview page. Subjects: Environmental Science — Earth Sciences and Geography.

A massive, frequently discordant, intrusion of coarsely textured plutonic rocks, at least 100 km2 in area and extending 20–30 km down into the layer of magma, which may be composed of...

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bomb sag

Overview page. Subjects: Earth Sciences and Geography.

The deformation of primary, unconsolidated, volcaniclastic bedding structures by the impact of a large ballistic block. The block, which can be a volcanic bomb or a fragment of crystalline...

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concordant

Overview page. Subjects: Earth Sciences and Geography.

Complying with. In geomorphology, relief and drainage may be concordant with geological structure. A concordant coast is parallel to the ridges and valleys of the country.

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contact

Overview page. Subjects: Earth Sciences and Geography.

The depositional, intrusive, or faulted surface along which two different rock types are juxtaposed. The term is especially applicable to situations where plutonic igneous rocks intrude...

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contact aureole

Overview page. Subjects: Earth Sciences and Geography.

A region in which country rocks surrounding an igneous intrusion have been recrystallized in response to the heat supplied by the intrusion. The widths of contact aureoles are quite...

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contact metamorphism

Overview page. Subjects: Earth Sciences and Geography.

The metamorphism of rock that is caused mainly by heat and pressure from pockets of magma, which bakes and hardens the surrounding pre‐existing rocks. Contrast regional metamorphism.

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dyke

Overview page. Subjects: Earth Sciences and Geography.

Discordant, or cross-cutting, tabular intrusion. Most dykes are vertical or near vertical, having pushed their way through the overlying country rock. See also dyke set; dyke swarm; and...

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dynamic metamorphism

Overview page. Subjects: Earth Sciences and Geography.

Fragmentation and recrystallization of rocks in narrow zones such as faults or thrusts where strong deformation has occurred. Rocks are ground to a fine powder in the zone of deformation...

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erratic

Overview page. Subjects: Earth Sciences and Geography.

Glacially transported rock whose lithology shows that it could not have been eroded from the local country rock. An example is the occurrence near Snowdonia, Wales, of granites derived from...

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fenite

Overview page. Subjects: Earth Sciences and Geography.

A rock that was first described from the Fen carbonatite intrusion in southern Norway. It is sodium-rich, metasomatic, orthoclase-nepheline-arfvedsonite- aegirine-bearing, and developed...

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ghost stratigraphy

Overview page. Subjects: Earth Sciences and Geography.

The alignment of country rock xenoliths within large granite bodies such that they reflect a continuation of the stratigraphy and structure of the country rocks surrounding the intrusion....

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hornfels

Overview page. Subjects: Earth Sciences and Geography.

(pl. hornfelses)

A massive, fine-grained, granoblastic contact metamorphic rock (see contact metamorphism), commonly displaying conchoidal fracture and splintery debris. Heat from...

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lithic fragment

Overview page. Subjects: Earth Sciences and Geography.

The dense or crystalline components of a pyroclastic deposit. Three types are recognized. Cognate lithics are fragments of non-vesiculated, juvenile, magmatic material, e.g. dense, angular,...

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maar

Overview page. Subjects: Ecology and Conservation — Earth Sciences and Geography.

A crater, often occupied by a shallow lake, which is produced by an explosive volcanic eruption. Normally it is surrounded by a low rampart or ring of ejected material. Typically, maars are...

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metasomatism

Overview page. Subjects: Earth Sciences and Geography.

Kind of metamorphism that involves the introduction of chemical constituents into a rock, or their removal from it (or both), via a volatile phase. Complete mineral transformations may...

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phyllic alteration

Overview page. Subjects: Earth Sciences and Geography.

A type of alteration that is found in country rocks of copper and molybdenum porphyry deposits.

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roof pendant

Overview page. Subjects: Earth Sciences and Geography.

A downward projection of country rock into a batholith. The removal of roof rocks by erosion may leave remnants of roof pendants which resemble xenoliths in the intruding rock.

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stock

Overview page. Subjects: Earth Sciences and Geography — Ecology and Conservation.

1 An igneous intrusion that has steep contacts with the surrounding country rocks. It is approximately circular in plan and has an area of 20 km2 or less.

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stoping

Overview page. Subjects: Earth Sciences and Geography.

1 The method of emplacement of an igneous intrusion in which percolating magma detaches blocks of country rock which sink, allowing the magma to move upwards.

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