geomaps.wr.usgs.gov

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Definitions (307)

1

5   10

parent rock


The preexisting rock from which a metamorphic rock forms.
Source: geomaps.wr.usgs.gov

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disappearing stream


In karst areas, streams often disappear into the ground usually at a sinkhole.
Source: geomaps.wr.usgs.gov

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2   2

sheetwash


Overland flow of water in thin sheets
Source: geomaps.wr.usgs.gov

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1   1

freeze-thaw cycle


In colder temperate regions, water trapped in fractures and between grains of rocks repeatedly freezes, then thaws during the winter months. In some areas this occurs on a daily basis as water freezes at night, then melts in warmer daytime temperatures. Only in the coldest regions does water remain frozen throughout the winter.
Source: geomaps.wr.usgs.gov

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frost wedging


A process that mechanically breaks apart rock caused by expansion of water as it freezes in cracks and crevices.
Source: geomaps.wr.usgs.gov

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phenocryst


A term used to describe large crystals embedded in a mass of finer crystals (groundmass) in an igneous rock. See 'porphyritic'.
Source: geomaps.wr.usgs.gov

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tectonically active


A term used to describe regions that are strongly affected by movement of Earth's tectonic plates. Earthquakes and volcanoes are common features in these regions. more details...
Source: geomaps.wr.usgs.gov

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1   1

thrust plate


Slab of rock, generally on the scale of a mountain or more, bounded by two thrust faults.
Source: geomaps.wr.usgs.gov

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b


banded gneiss See gneiss.
Source: geomaps.wr.usgs.gov

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basalt


A dark, fine-grained, extrusive (volcanic) igneous rock with a low silica content (40% to 50%), but rich in iron, magnesium and calcium. Generally occurs in lava flows, but also as dikes. Basalt makes up most of the ocean floor and is the most abundant volcanic rock in the Earth's crust. more details...
Source: geomaps.wr.usgs.gov


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