GEO 101 Lecture Notes - Lecture 20: Pleistocene, Structural Geology, Plate Tectonics
23 Jun 2018
School
Department
Course
Professor
Geologic Structures Defined
Geologic structures are usually the result of the powerful tectonic forces that occur
within the earth. These forces fold and break rocks, form deep faults, and build
mountains. Repeated applications of force—the folding of already folded rocks or the
faulting and offsetting of already faulted rocks—can create a very complex geologic
picture that is difficult to interpret. Most of these forces are related to plate tectonic
activity. Some of the natural resources we depend on, such as metallic ores and
petroleum, often form along or near geologic structures. Thus, understanding the origin
of these structures is critical to discovering more reserves of our nonrenewable
resources.
Structural geology is the study of the processes that result in the formation of geologic
structures and how these structures affect rocks. Structural geology deals with a variety
of structural features that can range in size from microscopic (such as traces of earlier
folds after multiple events of deformation have occurred) to large enough to span the
globe (such as midoceanic ridges).
Tectonic Forces
Rocks are under stress when they are subjected to a force at depth. When the rocks
are exposed at the surface after uplift and erosion, the effects of the stress can be
studied. Stressed rocks show varying degrees of strain—the change in the volume
and/or shape of the rock because of that stress. For example, a volcanic agglomerate
may be compacted and its pyroclastic fragments stretched (strained) in response to a
tectonic stress, such as compression.
Stresses. Three kinds of stress can be applied to rocks: tensional, compressive, and
shear. Tensional stress occurs when a rock is subjected to forces that tend to elongate
it or pull it apart; a rock that has experienced tensional stress tends to be narrower and
longer than its original shape, like a piece of gum or taffy that has been pulled.
A compressive stress on a rock is applied from opposite sides and has a tendency to
shorten (compress) the rock between the opposing stresses, which may also stretch it
parallel to the stress free direction. A‐ shear stress results when forces from opposite
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Geologic structures are usually the result of the powerful tectonic forces that occur within the earth. These forces fold and break rocks, form deep faults, and build mountains. Repeated applications of force the folding of already folded rocks or the faulting and offsetting of already faulted rocks can create a very complex geologic picture that is difficult to interpret. Most of these forces are related to plate tectonic activity. Some of the natural resources we depend on, such as metallic ores and petroleum, often form along or near geologic structures. Thus, understanding the origin of these structures is critical to discovering more reserves of our nonrenewable resources. Structural geology is the study of the processes that result in the formation of geologic structures and how these structures affect rocks. Rocks are under stress when they are subjected to a force at depth.
