Geology Plate Tectonics and Bedrock

Geology Plate Tectonics and Bedrock Responses: The Formation of Unconformities, Stress, Strains, Faults, and Joints Plate tectonics is a relatively recent explanation for many -- arguably most -- of the Earth's geological features. The convergence of two plates of the Earth's crust can cause the thrusting up of mountain ranges, such as the Himalayas and many others, and can also or alternatively result in subduction zones and heavily increased volcanic activity.

Where plates diverge, generally in ocean floors, great canyons and more volcanic activity can appear as the molten rock of the underlying mantle bubbles up to produce new crustal material.

In addition to these large-scale geological features, more minute -- in geological terms -- changes are also produced by the movements of the tectonic plates, creating an abundance of details in the physical geology in the bedrock of any given area that provide clues as to the progression of geological events and the movements of tectonic plates over the course of the Earth's existence.

Stress and Strain A basic definition of terms is necessary to understand the ways in which the forces of plate tectonics create the observable geological features identified above and the many others that exist. Stress, as used in geology, refers to the force(s) applied to an object or geological feature/area; in the context of the scope of this paper, it refers to the various forces arising out of and/or causing the movement of the tectonic plates as they act on bedrock (Dutch 1999).

Though pressure is actually a unique type of stress, it is a useful general term for understanding stress. While stress refers to the actual forces affecting certain geological elements, strain basically refers to the effects of these forces (Dutch 1999). That is, measurements and expressions of strain denote the degree to which a geological feature or area -- in this context, again, the bedrock of a particular area -- is affected, shaped, and warped by the stress to which it is subjected.

The same types and levels of stress can yield very different strains depending on the material upon which the stress is acting, as well as numerous other factors (Dutch 1999). Though the terms stress and strain are very closely related, then, it is important tor recognize the distinctions and differences that exist between these two terms -- the former is a description of force itself, while the latter is a description of a force's effects (Dutch 1999).

Joints and Faults Just as stress and strain are very closely related yet distinct terms, the terms joint and fault refer to highly similar yet fundamentally geological features. Both joints and faults are the result of stress, and shows signs of strain; they are fractures that occur in the Earth's crust due to the movement of the Earth's tectonic plates (ISU 2010).

There are several different kinds of stress that can produce fractures, and this can in large part affect the eventual outcome of the geological forces -- whether the fracture will become a joint or develop into a fault. A joint is essentially a fracture in the Earth's crust along which no movement has occurred, but that remains static after the occurrence of the fraction (ISU 2010). Joints are typically caused by tensional forces, which acts to lengthen or stretch objects (ISU 2010; Dutch 1999).

Faults often begin as joints, but become faults when movement occurs between the sides of the fractured crustal material (ISU 2010). A variety of different stresses can produce faults; tensional stress can result in faulting, as can compression -- stress that acts to shorten or condense an object -- and shear stress, which affects objects laterally (ISU 2010; Dutch 1999). A shear fault is perhaps the easiest to picture -- one side of the fractured structural material will move in a direction following the fault, e.g.

north or south if the fault runs in these directions, while the other side remains still or, less commonly, moves in the opposite direction. Compression stress can also result in lateral strain -- that is, movement along the fault line -- and both compression and tension can result in upward lifts or downward draws of the ground on either or both sides of a fault (ISU 2010; Dutch 1999). 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.