Metamorphic and sedimentary rocks are two of the three classifications of rocks found on the earth, the third being extrusive igneous rocks. The word "metamorphic" is derived from the Greek roots “meta,” meaning “change,” and “morphe,” meaning “shape." Thus metamorphic rocks are those that have changed their shape. "Sedimentary" is derived from the Latin “sedimentum,” meaning “sitting," and means rocks that are layered.
Metamorphic rocks were originally igneous, sedimentary or other metamorphic rocks. Over millions of years, these rocks have been changed by heat, pressure or by the permeation of gases or liquids. The resulting metamorphic rock is usually crystalline, with interconnected mineral particles. Metamorphic rocks can have colored streaks. They comprise less than 1 percent of the earth’s crust. Other than meteorites, metamorphic rocks are the oldest known rocks on the earth, some being 3.8 billion years old. These rocks form up to 40 miles deep within the earth’s crust under immense pressure and heat.
Types of Metamorphic Rocks
Examples of metamorphic rocks include anthracite (metamorphosed bituminous coal), gneiss (metamorphosed granite and other igneous rocks), marble (from limestone or dolomite), schist, serpentinite, and slate (derived from shale). Many mines and quarries are found in the zone where hot magma touches preexisting rocks to form what is called contact metamorphism, as new crystals and hence gemstones form in the contact layer.
The formation of sedimentary rocks starts with the weathering of bedrock outcrops. The weathering eventually forms small fragments of rock that are transported to their final resting place by rivers, wave action, the wind and glaciers. Over millions of years, the sediments undergo lithification, which is the hardening of the layer of sediment, by pressure from increasing overlaying layers. Sedimentary rocks comprise more than 75 percent of the earth’s crust. Examples of sedimentary rocks can easily be identified by the different layers that can be seen. The Grand Canyon is a good example of this layering.
Types of Sedimentary Rocks
Sedimentary rocks started off as clay, sand, gravel, lime from skeletons of sea creatures, precipitates, or volcanic sediments such as ash and cinders. Common sedimentary rocks include coal, gypsum, halite or rock salt, amber, limestone, dolomite, sandstone, shale, and conglomerate, which is a sedimentary rock made from pebbles and boulders that are cemented together with calcium carbonate, iron oxide, silica or sometimes clay. Conglomerate is often found at the bottoms of canyons or the mouths of fast rivers.
- "Mineral Collector's Handbook"; Barry Krause; 1996
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