The Grand Canyon in Arizona is composed of several types of sedimentary rock.
As bits and pieces of rock and coral erode and are borne by wind and water, these pieces of the earth find their way to the bottoms of bodies of water, such as rivers, lakes, oceans and seas. The microscopic remains of the calcite skeletons of ancient microorganisms as well as that of ancient plants accumulate to form sediments. As layers of sediment of various kinds continue to build up over millions of years and as pressures bear down on them, they turn into sedimentary rock, one of the three major categories of rock on earth.
Melting Point
The rock formations of North Coyote Buttes in Arizona are composed of sandstone.
Sedimentary rocks, such as sandstone, limestone, shale, conglomerate and gypsum, are formed on the earth's surface but may find their way into the mantle, deep below the crust. The temperature range of the earth's mantle varies from 900 degrees Fahrenheit to more than 7,000 degrees (500 to 4,000 degrees Celsius). When temperatures reach 2,000 to 3,000 degrees, sedimentary rock will melt.
Subduction
The geological pressures that contribute to volcanoes also melt sedimentary rock.
At the joints of tectonic plates, there are subduction zones, where one plate is pushed deep underground by the other. Sedimentary rock layers are driven downward along these subduction zones into the depths of the outer and inner mantle. When temperatures in the mantle are hot enough, the sedimentary rock melts.
Lithosphere and Inner Mantle
Marble is a metamorphic rock created when limestone is exposed to heat and pressure.
The lithosphere is the stiffer part of the outer mantle and crust, and is 62 miles thick. It sits on the asthenosphere and floats on it like ice on water. Such forces as friction and pressure can increase the temperature at the lower regions of the lithosphere sufficiently enough to melt sedimentary rocks, resulting in a plastic flow of these partially melted rocks.
Most of the rocks in the upper mantle, 62 to 416 miles below the surface of the earth, are hard and brittle, but temperatures rise as the depth approaches 416 miles. The lower mantle, at depths of 416 to 1,800 miles, is hot and plastic. Sedimentary rocks can begin to melt as they approach 416 miles, becoming a part of the viscous mass that forms the inner mantle.
Magma Chambers
Magma chambers exist close to the surface of the earth, at depths of less than one mile to about seven miles. A buildup of pressure within such magma chambers can result in volcanic activity. Surrounding sedimentary and other rock may collapse into magma chambers and melt, becoming part of the magma that volcanoes spew to the surface.
Aboveground
Extreme heat aboveground also can melt limestone.
Sedimentary rock can melt when subjected to extreme temperatures on the surface of the earth from raging fires or by man's controlled application of high temperatures for experimentation. The melting point for sedimentary rock aboveground is the same as below the surface of the earth, 2,000 to 3,000 degrees.
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