Comment: 14:46 - 16:25 (01:39)
Source: Annenberg/CPB Resources - Earth Revealed - 8. Earth's Structures
Keywords: "tectonic activity", fold, structure, "Dee Trent", compression, tension, stress, strain, "elastic strain", "elastic limit", earthquake, deformation
Our transcription: Different types of geologic structures result from different types of stress.
For example, shear stress in which one mass of crust slips laterally past another causes very large strike-slip faults to form.
And most folds are formed by compressive stress.
Where the stress occurs is also significant.
Now, when we talk about stresses and different types of structures, when we're looking at structures forming at depth below the surface of the Earth, just because of the overlying weight of the body of rock, we're almost always dealing with compressive stresses.
So the types of structures we get there really are just a reflection of the different magnitudes of stresses, compressive stresses in different directions.
However, when we get up near the surface of the Earth where we're dealing with a "free surface", with no overlying weight of rock, we can sometimes get tensional stresses, and these stresses will try to pull apart our rock in some direction.
Under those circumstances, we can get things like joints, or fractures, or little veins of different types of minerals that flowed into these fractures that do give us some clues about the directions and magnitude of tensional stresses.
So just by looking at different types of structures, or folds, or joints, or things like that, we can get some clues about what the stress field is like during the time that these structures formed.
Geology School Keywords