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USGS CMG InfoBank: Protolith

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Comment: 03:20 - 04:19 (00:59)
Source: Annenberg/CPB Resources - Earth Revealed - 18. Metamorphic Rocks
Keywords: "Douglas M. Morton", geologist, protolith, metamorphism, limestone, marble, "metamorphic rock", basalt, "volcanic rock", amphibolite, composition, fold, fault, pressure, temperature, mountain

Our transcription: Geologists use the term "protolith" to refer to the original rock existing before metamorphism.
For example, limestone is the protolith of marble, one of the most common metamorphic rocks, and basalt, a volcanic igneous rock, is the protolith of amphibolite.
But geologists have found many more types of metamorphic rocks than protoliths, so factors other than original composition must also play a role in creating these rocks.
Study of geologic structures such as folds and faults suggests that there's a wide range of pressures and temperatures inside growing mountain belts.
Quite likely, this plays a critical role in explaining variations in metamorphic rocks.
Laboratory experiments have helped geologists understand metamorphic conditions.

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