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Metamorphic Rock:
Metamorphic recrystallization may be A thorough and accompanied by marked change chemical compositions, especially if it takes p at depths where high temperatures and pressi prevail. The distinction between igneous metamorphic rocks then becomes vague. Not naturally there are many geological controver concerning the origin and mode of formatior these mixed igneous-metamorphic rocks, or r matites. Granite was formerly regarded a typical igneous rock, but it is now thought some granites may have crystallized from n mas, while others may have been formed by thorough recrystallization of pre-existing ro Distribution of Rocks.A third, rare type of metamorphism is dislocation metamorphism, caused by large areas of rock moving past one another. The pressure shatters the rock and the friction is so great that the rock is partially melted, producing a rock called mylonite. It occurs only in narrow strips, of which the Lizard, in Cornwall, England, is an example. Unlike sedimentary rocks, metamorphic rocks are of no great use to mankind, containing no oil and few useful minerals, and are of little use in building, apart from slate for roofing and marble used in decorative work.
Most great thicknesses of sedimentary rocks accumulate in long, narrow depressions on the sea-floor called geosynclines. These depressions are caused by descending convection currents which, over a period of millions of years, carry the crust of the earth down into the earth's interior where both the pressure and temperature are high. The sedimentary rock in the depression is carried down with the crust. It is folded and squeezed and heated up to between 200°C (392°F) and 500'C (932°F). This changes the sedimentary rock to a metamorphic rock.
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