Sedimentary Rocks are formed from the consolidation of layers of sediment. Picture below shows Limestone and the manner of its sedimentation.
Sedimentation begins when existing rock at the Earth's surface is broken physically and decomposed chemically from exposure to the atmosphere, and then eroded by streams that flow through the area which transport the debris to the sea, dropping the sediment as the river currents die. Sediments are mineral or organic matter deposited by water, air or ice. When settled on the ocean floor, they are known as Clastic (fragmental or broken) sediments. Dissolved mineral matter may be extracted by plants or animals and accumulate on the sea floor as organic sediments, or it amy be physically precipitated on the floor as chemical sediments. The loose sediment consolidates forming sedimentary rock. Sedimentary rocks can be thus classified as Clastic, Organic, or Chemical.
Sedimentary rocks compose only 5% of the Earth's crust, but cover 75% of the continents and even more of the oceans basin. They are like clothing covering the surface of the Earth, thus making them conspicuous. These rocks are used in construction and also contain abundant petroleum, coal and metal deposits. They also form interesting landscapes. Colorado river's Grand Canyon is made of bright colored sedimentary rocks. For photographs of the Grand Canyon, click below:
The origin and history of sedimentary rocks are known from their color, rounding, sorting, bedding, fossil content, ripple marks, mud cracks, footprints, and raindrops prints, which are common characteristics that indicate the conditions under which the rocks were formed.
Gray, the most common color, reflects the rock's origin in shallow,
well-aerated marine water. Red-brown or yellow-brown, caused by iron
oxides and found in many western states, are rocks deposited above sea
level in the presence of oxygen. Dark gray to black, not common,
contain carbon from organic matter that accumulated in the stagnant water
where it was deposited.
If the fragments are angular, they were not carried far. on the
other hand, round grains of quartz sandstone tell of their long journey
downstream and of many hours that they were shifted and rolled by the waves
and currents of the sea A current or wave that can keep small grains
afloat may not be able to move large grains. So particles are sorted
or separated according to size. The greater the distance of transportation,
the more effectively the grains are sorted. In a lake or ocean, the
largest particles come to rest near the shore, but the finer grains are
carried to quieter water farther from shore, eventually settling to the
bottom.
Bedding, or stratification, is the layer that develops at the time the
sediment is deposited. It may be apparent or difficult to see.
Cross-bedding is sediments that accumulate in tilted layers. This
is found where rivers empty into a lake, where sediments fill in depressions
scoured by floods along river channels, or where wind drapes sand down
the flanks of a dune.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|