CSET REQUIREMENTS 2.1 e/f : Relate geologic structures to tectonic settings and forces.
GEOLOGICAL FORCES
Geologic structures are the result of the powerful tectonic forces that occur within the earth. These forces fold and break rocks, form deep faults, and join to build mountains. Most of these forces are related to plate tectonic activity. To understand how geological structures are formed we need to distinguish these forces and how they relate to stress and strain.
Using the diagram on the left to help us visualize stress forces. Stress is defined as force per unit area and responsible for changes in rock shape. The first figure shows the effect of compressional stress where force applied shortens the object. The second figure shows tension stress applied and elongates the object. Skipping to the last figure where strain forces are being applied at different places of the object and in turn creating shear stress. Strain is the deformation in a rock caused by stress. The type of rock, the temperature and pressure, and even the rate of stress all influence how a rock will accommodate strain.
When rocks are subjected to stresses greater than their own strength, they begin to deform usually by folding or fracturing. Types of deformation include elastic and plastic. Elastic is reversible like a rubber band where rocks return to nearly its original size and shape when stress is removed. Plastic results in permanent changes, that is, the size and shape of a rock unit are forever altered through folding and flowing. The process of deformations generate geological structures and features at many different scales. At one extreme are Earth's major mountain systems. At other extremes, are highly localized stress points creating minor fractures in bedrock. These moldings and shaping from the formation of Mount Whitney to small fractures are referred to as rock structures.
ROCK STRUCTURES
Strike and Dip When geologist study regions they identify and describe dominant structures. Normally these settings provide challenges in study where only small portions are visible. They are covered with vegetation, large in size or concealed by sedimentation. However, if the region of study is inclined, bent or broken then it reveals that a period of deformation occurred following deposition. Strike is the geographic pattern or direction in which the surface lies, whereas dip is their angle of inclination downward from the surface.
FOLD During mountain formation, flat lying sedimentary and volcanic rocks are often bent into a series of wavelike undulations called folds. If you were to place a piece of paper on a flat surface and push together from two end you will create this geological effect. the two most common types of folds are called anticline and syncline. Anticlines are mostly formed by upfolding or arching of rock layers and synclines are downfolds. Oldest rocks are found at axial region of the anticline due to erosion taking place.
Using the diagram on the left to help us visualize stress forces. Stress is defined as force per unit area and responsible for changes in rock shape. The first figure shows the effect of compressional stress where force applied shortens the object. The second figure shows tension stress applied and elongates the object. Skipping to the last figure where strain forces are being applied at different places of the object and in turn creating shear stress. Strain is the deformation in a rock caused by stress. The type of rock, the temperature and pressure, and even the rate of stress all influence how a rock will accommodate strain.
When rocks are subjected to stresses greater than their own strength, they begin to deform usually by folding or fracturing. Types of deformation include elastic and plastic. Elastic is reversible like a rubber band where rocks return to nearly its original size and shape when stress is removed. Plastic results in permanent changes, that is, the size and shape of a rock unit are forever altered through folding and flowing. The process of deformations generate geological structures and features at many different scales. At one extreme are Earth's major mountain systems. At other extremes, are highly localized stress points creating minor fractures in bedrock. These moldings and shaping from the formation of Mount Whitney to small fractures are referred to as rock structures.
ROCK STRUCTURES
Strike and Dip When geologist study regions they identify and describe dominant structures. Normally these settings provide challenges in study where only small portions are visible. They are covered with vegetation, large in size or concealed by sedimentation. However, if the region of study is inclined, bent or broken then it reveals that a period of deformation occurred following deposition. Strike is the geographic pattern or direction in which the surface lies, whereas dip is their angle of inclination downward from the surface.
FOLD During mountain formation, flat lying sedimentary and volcanic rocks are often bent into a series of wavelike undulations called folds. If you were to place a piece of paper on a flat surface and push together from two end you will create this geological effect. the two most common types of folds are called anticline and syncline. Anticlines are mostly formed by upfolding or arching of rock layers and synclines are downfolds. Oldest rocks are found at axial region of the anticline due to erosion taking place.
FAULTS are fractures in the crust along which noticeable displacement has taken place.
Before we begin differentiating types of faults, it's important to understand hanging wall vs foot wall. The hanging wall is the side of the fault above the fault surface, and the footwall is the side below the fault surface.
Normal Fault: The hanging wall moves down relative to the foot wall. Normal faults accommodate lengthening, or extension of the crust. Uplifted fault blocks are called horsts and down-dropped blocks are called grabens. Mount Whitney is an example of a mountain formed by normal faults or fault-block which is a large chunk of rock rising up while the east valley sinks giving us both the highest peak in California and the lowest point of the state (Death Valley).
Reverse Fault: In these settings, the hanging wall moves up relative to the foot wall. Compressional tensions generally produce folds as well as faults.
Strike-Slip Fault: Faults in which the displacement is horizontal and parallel to the strike of the fault surface. The concept of hanging wall and foot wall do not apply. The broken rocks produced from strike-slip faults are easily eroded and often form linear valleys and troughs. the San Andreas fault is a transform fault which is a special kind of strike-slip fault which cuts through the lithosphere and accommodate motion between two large crustal plates. It runs about 810 miles and cleanly separates the North American Plate from the Pacific Plate.
Normal Fault: The hanging wall moves down relative to the foot wall. Normal faults accommodate lengthening, or extension of the crust. Uplifted fault blocks are called horsts and down-dropped blocks are called grabens. Mount Whitney is an example of a mountain formed by normal faults or fault-block which is a large chunk of rock rising up while the east valley sinks giving us both the highest peak in California and the lowest point of the state (Death Valley).
Reverse Fault: In these settings, the hanging wall moves up relative to the foot wall. Compressional tensions generally produce folds as well as faults.
Strike-Slip Fault: Faults in which the displacement is horizontal and parallel to the strike of the fault surface. The concept of hanging wall and foot wall do not apply. The broken rocks produced from strike-slip faults are easily eroded and often form linear valleys and troughs. the San Andreas fault is a transform fault which is a special kind of strike-slip fault which cuts through the lithosphere and accommodate motion between two large crustal plates. It runs about 810 miles and cleanly separates the North American Plate from the Pacific Plate.
PLATE TECTONICS
Divergent boundaries, are where the lithosphere is being stretched due to tensile stress, normal faults are common near Earth’s surface, where the lithosphere is cool and brittle.
Convergent boundaries (subduction zones and collision zones), reverse faults and thrust
faults are expected to form at shallow depths, where the rocks are cool and behave in a brittle
fashion under compressional stress. At greater depths, we might expect folding to be common
where the rocks are warmer and under higher pressures.
Transform boundaries, strike-slip faults are common. Where strike-slip faults bend, localized
compression and extension can develop, which may give rise to associated tensile structures
(normal faults) or compressional structures (reverse faults and folds).
Convergent boundaries (subduction zones and collision zones), reverse faults and thrust
faults are expected to form at shallow depths, where the rocks are cool and behave in a brittle
fashion under compressional stress. At greater depths, we might expect folding to be common
where the rocks are warmer and under higher pressures.
Transform boundaries, strike-slip faults are common. Where strike-slip faults bend, localized
compression and extension can develop, which may give rise to associated tensile structures
(normal faults) or compressional structures (reverse faults and folds).