Unconformity - Angular Unconformities, Disconformity, Nonconformities, An Ongoing Process
layers rocks rock sedimentary
An unconformity is a widespread surface separating rocks above and below, which represents a gap in the rock record. Unconformities occur when either erosion wears away rocks, or rock deposits never form. Therefore, a time gap exists between when the rocks below the unconformity formed and when those above it formed.
Unconformities are classified as three types. The most easily recognized are angular unconformities, which show horizontal layers of sedimentary rock lying on tilted layers of sedimentary rock. The upper layers may not be perfectly horizontal, but they do not lie parallel to the lower layers. The second type of unconformities are disconformities, which lie between parallel layers of sedimentary rock. The third type are nonconformities, which divide sedimentary layers from metamorphic and intrusive (cooled inside the earth) igneous rocks. Common to all three, erosion causes them to form, and younger rocks sit on older rocks.
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Four basic steps create angular unconformities. In step one, sediment weathered from land and carried to the sea accumulates on the sea floor and over millions of years turns to rock layers. Then the collision of plates, giant sections of the earth's crust that constantly shift, lift and tilt the layers until the layers rise above sea level and then weather and erode. They erode for million…
From a mile away, or perhaps from a few hundred feet away, disconformities can hide. The layers appear regular, all parallel. However, between the layers, a disconformity can lie. Not until a geologist closely examines the fossils in the layers for the presence or absence of certain organisms can he or she recognize the gap in time—an erosional period when sediment accumulation or depositio…
As rocks continue to wear away, more unconformities appear. As road crews cut through mountains, they expose unconformities for the speeding motorist as well as the geologist to enjoy. However, these new exposures and the mountains that contain them will erode flat. The Appalachians, the Himalayas, the Alps, the Rockies, even the Grand Canyon, will die their slow erosional deaths as nature levels …
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