Geologic Structures are features in the earth that are produced by deformation of rock. Folds, faults, domes and basins are all caused by tectonic activity. Geologists can not always see features that occur from the surface. However, cross-sectional views of the interior are not always possible. This is why using block models, knowing the Rules for interpreting features on Geologic Maps and how to use them are important. Chapter 7 in the Introduction to Earth Science Lab book discusses how "block-shaped models are used to assist in visualization and unraveling ages of rock layers and tectonic features" (41).

If you do not understand all of these geologic terms click here for help.

I. Faults

A fault is a fracture in the earth's crust. There are three types of faults.

A. In a normal faults the hanging wall moves down towards the footwall.



B. Opposite to a normal fault, in a reverse fault the hanging wall has moved up toward the footwall.

C. In a srike-slip fault the rock layers move horizontally opposite the other, one to the left and the other to the right.

II. Folding

A fold is a bend in the rock layers.
A. Anticline is a fold with an arch that goes upward

B. Syncline is a fold where the rock dips downward.

C. Axial plane is a plane of symmetry that divides the fold. By doing this one can see if the fold is:

1. Symmetrical

2. Asymmetrical



D. Plunge is the direction of the dip, is it uphill or downhill.



III. Domes

Domes are shaped like upside-down bowel. Here circular rock layers arch in an anticline.
IV. Basins
Basins are circular rock layers that dip ward. They are shaped like a bowel.

Law of Superposition: In a sequence of rock layers, each layer is older than the next layer above.

Principle of Horizontally: All sedimentary rock sequences were originally laid down horizontally.

1. Layers of rock dip downward in the same direction as that which the youngest rock layers are exposed at the surface.

2. The older rocks are exposed in the center of eroded anticlines and domes.

3. The younger rocks are exposed in the center of synclines and basins.

4. Plunging anticlines form "U" shaped outcrop belts that point in the same direction that the fold plunges.

5. Plunging synclines form "U" shaped outcrop belts that point in the opposite direction that the fold plunges.

6. The steeper the dip of the layer, the more narrow the width of its outcrop belt.

7. In compressional faults (two walls are pushed together) the hanging wall tends to move up relative to the footwall.

8. In tensional faults (the two walls are pulled apart) the hanging wall tends to move downward relative to the foot wall.

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     What's that you say?

fault n 5 a: a fracture in the crust of a planet (as the earth) or moon accompanied by displacement of one side of the fracture with respect to the usu. in a direction parallel to the fracture

hanging wall n 1: the upper wall of an inclined fault

foot wall n 1: the lower underlying wall of a vein, ore deposit, or coal seam in a mine 2: the lower wall of an inclined fault

fold n 3 a: a bend or flexure produced in rock by forces operative after the depositing  or consolidation of the rock

anticline n a: a trough of stratified rock in which the bed dips toward each other from either side

syncline n : a arch of stratified rock in which the layers bend downward in opposite direction of the crest

plunge v 2: to become pitched or thrown headlong or violently forward and downward

axial plane n b: a plane extending in a direction essentially perpendicular to the plane of a cyclic structure

basin n 3 a: a large or small depression in the surface of the land or ocean floor c: a great depression in the surface of the lithosphere occupied by an ocean 4: a broad area of the earth beneath which the strata dip usu. from the sides toward the center

dome n 3 a: an upward fold in rock whose sides dip uniformly in all directions
 
 

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Created by Vicki Freeman 5/1/98