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The best direct evidence we have of tectonic forces is in the deformation of rocks.
Even before plate tectonics,the way in which rocks respond to stress was well understood. These results are observed in the folds and faults of mountain belts, such as we discussed in the last lesson.
Folds come in all sizes,their wave life structures that may be hundreds of miles long down to microscopic in size, and all are formed in essentially the same way by compressional forces.
Geologists study rock structures for several reasons. Number l, they help us to understand the geologic history of a region, and they also help us to understand the history of tectonic movements and the plate movements. The analysis of folds, faults, and joints are all in a branch of geology called "structural geology," and it's from these studies that we learned about mountain building forces and were able to interpret them in the light of plate tectonics.
The folds and faults and other geologic structures also help us to make geologic maps, which we use to infer underground structures where we can't see the rocks and to help us to understand the formation of geologic resources to locate and manage them.
Like other geologic structures, folds and faults can be classified, and we learn from the classification that similar structures are formed by similar processes. By classifying the geologic structures and understanding how they were formed, we can learn to interpret the story left behind in the rock. It's actually the geometry, that is the shape, and size, and the orientation of folds, and faults, and joints that holds the key to this interpretation. It's not only the geologic structures themselves, but it's also erosional features, gaps in the geologic record, that also give us information about the geologic history of a particular area.
When erosion was occurring over a particular region, for example, the material that was removed from that region was put some place else, so by studying the sediments in one place and the missing record in another place, we could make connections between the two regions.
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