Sedimentation
weathering> erosion > transport > deposition
WEATHERING
Rocks change slowly... but conditions at the surface of the earth and in the earth are always acting to change rocks. Sometimes the changes take hundreds of thousands of years. For example, 2 or 3 miles below the surface of Salt Lake Valley, the sedimentary rocks are becoming more firm and coherent with time... very slowly. The crustal stresses under Salt Lake Valley are not particularly great. Fluids move through the sediments but slowly. In contrast, the rocks on the face of Mt. Olympus experience relatively rapid change. They get weathered, eroded, transported, and deposited in a twinkle of geologic time.
Weathering includes several physical and chemical processes that work at or near the surface of the earth to break down rocks. Most weathering is water-dependent. Fog, rain, snow, runoff from precipitation, ice and even the humidity in soils work to dissolve and dislodge minerals and pieces of rock. Most weathering in Salt Lake County occurs in the upper 20 feet of the land surface however weathering in the tropics has been documented to depths of several hundred feet.
How fast bedrock weathers depends on local topography, the climate, and the kind of rock. The weathering of sandstone differs from the weathering of granite and both differ from the weathering of limestone. The weathering process in the Wasatch Mountains differs greatly from that on the Great Plains. Weathering in the Arctic differs from that in the tropics, in deserts and in forests. Weathering at Alta differs from weathering at Point of the Mountain or at the south shore of Great Salt Lake.
Weathering occurs by two processes:
- physical processes (sometimes called mechanical weathering) and
- chemical processes (especially solution)
Both these sets of processes work away at Salt Lake County rocks, sometimes simultaneously.
EROSION, TRANSPORT, AND DEPOSITION
Weathering is just one of a set of processes that break up bedrock and make it sediment. Weathering, erosion, transport and deposition attack all three kinds of rock (sedimentary, metamorphic and igneous) and attack sediments, too.
Once the weathered residue of bedrock is moved by water, gravity, wind or ice and it is called sediment. The process is a couplet... erosion picks up the material, sedimentation deposits it.
Sediment can weather just as bedrock weathers... by physical and chemical processes.
This is the situation over most of Salt Lake County.
Weathering is the first of sedimentation’s one - two punch. Erosion is the process of breaking materials away from their bedrock or sediment source. Water, wind and ice transport and deposit materials as big as huge boulders and fine as clay. The deposits are usually layered. Younger sediments bury older sediments. In lakes and oceans these layers start out relatively horizontal... a sheet of clay, or a sheet of sand, or a sheet of limestone.
The sedimentation sequence for Salt Lake County can be summarized as:
- Bedrock in the mountains or outside the county weathers and erodes.
- Streams, lake waters, ice, and, to a lesser extent, wind transports the sediments and
- Deposits them.
- The fresh sediments bury the older sediments and bedrock beneath them.
"Erosion and Weathering" used with permission from Teachers Domain see credits
Classroom resources:
The following site is an age appropriate activity for your 4th or 5th grade students:
Erosion and Weathering