Figure 4: 7K
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Read These Notes: Although igneous rocks make up 95% of the Earth's crust, approximately 75% of the Earth's surface is covered with sedimentary rocks. Their distribution alone tells us that sedimentary rocks form under different conditions from igneous rocks. Whereas igneous rocks frm as a consequence of internal processes, sedimentary rocks form as a result of surface processes.
A sedimentary rock can form from any pre-existing rock that is exposed
to the surface of the Earth where minerals within the rock are subjected
to processes which can eventually produce a sedimentary rock. These processes
Mechanical weathering does not change the chemical make-up of a mineral, it merely changes the grain size. The fine grains of sand that we find at a beach are examples of mechanical weathering. Most beach sand is composed of the mineral quartz (Figure 3). It is generally derived from quartz grains found in pre-existing rocks such as granite (Figure 4) that have be mechanically broken up into smaller sand-sized grains. Throughout this process, however, it remains its chemical identity. This difference in minerals susceptibility to weathering is referred to as differential weathering.
Once the minerals within a rock are weathered they become susceptible to erosion and transportation. Often the sediments bear the marks of whatever agent transports them. For example, transportation by water will frequently causes sediments to become rounded and well sorted. Sediments that are transported a short distance frequently are larger and less rounded than those transported over a long distances. Sediments transported by wind and glaciers tend to be less rounded than those transported by water.
Clearly, the differences in minerals' susceptibility to weathering will also results in differences in their susceptibility to erosion. Such differential weathering and erosion (def) are responsible for producing a variety of landforms. Mountains and other elevated features such as Devils Tower in Wyoming (Figure 5) will naturally form where rocks contain erosion resistant minerals. On the other hand, rocks made of minerals that are susceptible to weathering and erosion will tend to form valleys and other low lying areas.
Sediments that are been eroded and transported are eventually deposited. Deposition of sediments many times occurs hundreds or even thousands of miles from their place of origin. Sediments originating in the Appalachian mountains end up as sands on Atlantic beaches. Sediments originating in the heartland of North American end up in the Gulf of Mexico. Eventually deposited sediments are lithified (def) into a sedimentary rock. (Review this Slide Show on Sedimentary Rocks. There are approximately 14 seconds between each slide.)
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Updated: May 17, 2004