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Flashcards in Ch8 Deck (28):
1

What are Geologic Structures?

Structural Geology is the study of how rock bodies become deformed
-AND the study of the characteristics of mountain building processes, faults, folding, and shearing processes known as deformation.

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Deformation

a general term refers to the changes in the shape or position of a rock body in response to differential stress

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Stress

used to describe the forces that deform rocks

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Compressional stress

often associated with convergent plate boundaries.

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Tensional Stress

associated with divergent plate boundaries.

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Shear stress

associated with transform boundaries

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Types of Deformation

1) Elastic
2) Brittle
3)Ductile

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Elastic deformation

recoverable. When stress increases gradually, rocks initially respond by deforming elastically

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Brittle deformation

rocks fracture of break when the strength of the rock is supposed (low temp, low pressure)

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Ductile deformation

type of solids flow that produces a change in the shape of an object w/out fracturing. (high temp, high pressure)

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Strike

the compass direction of the line produced by the intersection of an inclined rock layer or fault with a horizontal plane at the surface

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Dip

the angle of inclination of the surface of the rock unit or fault from the horizontal plane. Dip includes both an angle of inclination and a direction towards which the rock is inclined

13

Dip-Slip Faults

relative motion of rock units is primarily vertical
-normal faults: hanging wall moves down relative to hanging wall
-reverse fault: hanging wall moves up relative to the foot wall

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Strike-Slip Fault

dominent relative motion of rock units is horizontal; can be described as right-lateral fault or left-lateral fault

15

Geological structures: Fold

1) Anticline
2) Syncline

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Axial Plane

an imaginary plane that divides the fold as equally as possible into two halves
-two halves are called LIMBS

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Plunging folds

are tilted by tectonic forces that cause their hinge lines to slope downward.

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Synclines

-Downward folds, or concave folds
-youngest rock in the middle of the fold

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Anticlines

Upward Folds or Convex Folds
Oldest rocks in the middle

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Trend

The bearing (compass direction) measured in the direction that the axis is inclined downward

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Plunge

The angle between the fold axis and the horizontal. The outcrop pattern of an eroded plunging anticline “points” in the direction it is plunging. The opposite is true of a plunging syncline

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Main Types of Fold

Symmetrical folds
Asymmetrical folds
Overturned Folds(Fold that is titled so that one limb is upside down.)

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Dome

Large, somewhat circular structures formed when strata are warped upward Oldest strata at the center of the dome.

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Basin

Large somewhat circular structures formed when strata are warped downward Youngest strata at the center of the basin

25

Geologic Maps

a two dimensional representation of the location and attitudes of rock formations and contacts. Shows topography of the surface with contour lines.

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Geologic Cross Sections

a drawing of a vertical slice through the Earth – a two dimensional cut-­‐away view.

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Block Diagram

a combination of the geologic map and two cross sections – shows the relationship of what we see on the Earth’s surface and what is taking place below the Earth’s surface.

28

Attitude

: is the orientation of a rock unit, surface (contact), or line relative to horizontal and/or a compass direction