Rocks

A rock is defined as a naturally occurring aggregate of minerals and/or mineraloids. The three classes of rocks: the igneous, the sedimentary and the metamorphic — are subdivided into many groups. There are, however, no hard and fast boundaries between allied rocks. By increase or diminution in the proportions of their constituent minerals they pass by every gradation into one another, the distinctive structures also of one kind of rock may often be traced gradually merging into those of another. Hence the definitions adopted in establishing rock nomenclature merely correspond to selected points in a continuously graduated series.

Classifications

Igneous Rocks
Rock Color Structure
Basalt Dark Greenish-gray to black. Dense, microscopic crystals, often form columns.
Gabbro Greenish-gray to black. Coarse crystals
Granite White to gray or Pink to red. Tightly arranged, medium-to coarse crystals
Obsidian Black sometimes with brown streaks. Glassy, no crystals, breaks with a shell-like fracture
Peridotite Greenish-Gray. Course crystals.
Pumice Grayish-White Light, glassy, frothy, fine pores, floats in water.
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Igneous rocks are formed from molten magma and are divided into two main categories: plutonic rock and volcanic. Plutonic or intrusive rocks result when magma cools and crystallises slowly within the Earth's crust, while volcanic or extrusive rocks result from magma reaching the surface either as lava or fragmental ejecta.


Sedimentary Rocks
Rock Color Structure
Breccia Gray to black, tan to red. Angular pieces of rock, held together by natural cement.
Coal Shinny to dull black. Brittle, in seams or layers.
Flint Dark gray, black, brown. Hard, glassy, breaks with a sharpe edge.
Limestone White, gray, and buff to black and red. Dense, forms thick beds and cliffs. May contain fossils.
Sandstone White, yellow, gray, red. Fine or coarse grains cemented togetherin beds.
Shale Yellow, red, gray, green, black. Dense, fine particles, soft, splits easily, smells like clay.
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Sedimentary rocks are formed by deposition of either clastic sediments, organic matter, or chemical precipitates , followed by compaction of the particulate matter and cementation during diagenesis. Sedimentary rocks form at or near the Earth's surface. Mudrocks comprise 65% (mudstone, shale and siltstone); sandstones 20 to 25% and carbonate rocks 10 to 15% (limestone and dolostone)


Rock Color Structure
Amphibolite Light green to black. Fine-to-coarse grains, hard, often sparkles
Gneiss Gray and pink to black and red. Medium to coarse crystals arranged in bands.
Marble Many colors, often mixed. Medium to coarse crystals. May be banded.
Quartzite White, gray, pink buff. Massive, hard, often glassy.
Schist White, gray, red, green, black. Flaky particles, finely banded, feels slippery, often sparkles with mica.
Slate Black, red, green. purple. Fine grains, dense, splits into thin, smooth slabs.
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Metamorphic rocks are formed by subjecting any rock type to different temperature and pressure conditions than those in which the original rock was formed. These temperatures and pressures are always higher than those at the Earth's surface and must be sufficiently high so as to change the original minerals into other mineral types or else into other forms of the same minerals