Geology Plate Tectonics and Bedrock Essay

Unconformities

In addition to creating joints and faults, the stresses of tectonic plate movement can also result in other types of strain. This can make it initially more difficult to obtain clear information from a geological record, but ultimately reveals a great deal more about geological history than more easily discernible features. Two primary concepts upon which the science of geology are founded are ideas that layers of rock initially occur in "flat" layers, parallel to the Earth's surface, and that younger layers of rock remain closer to the surface unless otherwise disturbed (Alden 2010). This would produce a highly conformed geological record; the unconformities that exist in reality define the disturbances of geological history.

The most obvious and earliest discovered and described type of unconformity is the angular unconformity, in which the rocks below a certain level have hall been tilted in a similar direction and sheared off at a specific height (Alden 2010). This shows a period in the geological development of the bedrock in which an upheaval or other tilting force occurred, forcing what were parallel rocks into their new angular positions. The top layer of these angled rocks were then eroded down to a level surface by natural forces -- generally wind and/or water -- and subsequent layers of level and parallel rock were laid down in successive geological epochs (Alden 2010). The high degree of persistent regularity in angular unconformities despite the stress to which these areas are subjected is a large part of the reason behind their easily recognized features and the clarity of the geological history they relate.

Disconformities or paraconformities, known as nonsequences and not true unconformities to many British geologists, occur when successive layers of sediment deposit and rock strata buildup occur with an obvious interruption or hiatus in such sedimentation, but no other disturbance exists (Alden 2010). What this essentially represents is a gap in the geological record; a time when sediments did not accumulate to create more layers of rock, and then the start of more deposits that, looked at now, can represent gaps of millions of years between layers.Sudden transitions from fossils of one type to another, instead of the generally steady evolution visible in uninterrupted geological records, can be obvious signifiers of a disconformity or paraconformity (Alden 2010).

Nonconformities are somewhat similar to disconformities, but instead of layers of sedimentary rock strata existing with a jump in time observable in the geological record, a layer or layers of sedimentary strata exists on top of a layer of non-sedimentary rock with a similar gap in the time between which the foundational rock was formed and when the first layer of sedimentary rock formed on top of it (Alden 2010). One example of nonconformity in Colorado represents a gap of more than one-hundred-and-fourteen million years; this is an extreme example of a nonconformity, but all occur along similar principles -- no sedimentary deposits were made (or at least, not in the volumes and rates required) over a layer of newly created igneous or metamorphic rock for some period of time, and then a layer of sediment was deposited and remained long enough to compress into a sedimentary rock layer (Alden 2010). Nothing can be directly known about the events of the intervening time period, but as with paraconformities explanations can be hypothesized (Alden 2010).

Conclusion

The stresses produced by tectonic plate movement can create a variety of strains in the bedrock of crustal material. From producing fractures -- the joins and faults that represent breaks in the Earth's crust -- to readjusting the geological record, disrupting the neat conformity of the record that would otherwise occur, these strains make geology somewhat more difficult but infinitely more interesting and approachable. Though some features are complex, it is their complexity that yields knowledge.

References

Alden, a. (2010). "Unconformities: Gaps in the Record." Accessed 13 May 2010. http://geology.about.com/od/geoprocesses/a/unconformities.htm

Dutch, S. (1999). "Stress and strain: Basic terms and concepts." University of Wisconsin. Accessed 13 May 2010. http://www.uwgb.edu/DutchS/structge/stress.htm

ISU. (2010). "Earthquake fundamentals." Idaho state university. Accessed 13 May 2010. http://imnh.isu.edu/digitalatlas/geo/quakes/text/main.htm.....