Background
rock deformation
As strong as they may seem from our puny human perspective, rocks deform when subjected to huge tectonic forces. The type of force acting on them mirrors the types of plate margins - which are after all, the root cause of most of the deformation. Forces can be grouped into
How the rocks respond to these forces depends on many things:
Faulting: faults are where blocks of brittle rock have fractured, and as a result moved with respect to each other across the fault. They can be 'normal' faults, in response to extensional forces; or 'reverse' faults, in response to compressional forces. At shallow depths in Content-Continent Crustal collisions, the compressive forces tend to lead to low angle reverse faults called thrust faults. This type of faulting is the major feature of deformation in southern Spain, although often at too large a scale to be obvious. Folding: Where rocks act in a ductile manner, they tend to bend and fold, rather than break. In response to increasing intensity of deformation, folds can progress from simple symmetrical anticlines and synclines to overturned folds and recumbent folds. The Rock of Gibraltar is an example of an overturned fold (Section 3). Fractures: In geological parlance, fractures are stress cracks in a rock but, unlike faults, across which there has been no appreciable movement. The fractures often occur in sets running at regular angles to each other. One very common form of this type of fracturing are 'joints'. Another form, common in lower grade metamorphic rocks, is called cleavage planes. Veins: Veins are fractures that have been filled with minerals precipitated from groundwaters circulating through them. Many veins contain common minerals such as quartz and calcium carbonate, but some are an important source of metal ores, including gold. Next: Background - Geological Time |
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