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Metamorphic Picture: Regional metamorphic picture rocks are classified k accordance with the supposed metamorphic picture intensity to which they responded. This is determined by a set of critical minerals or mineral assero-blages which are believed to be reliable guide-to the metamorphic picture grade. However, there i-some doubt as to the reliability of this test, since it has been shown in the laboratory that all the key minerals can be produced at the same temperature by merely varying the water vapor pressure.
metamorphic picture 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 picture rocks then becomes vague. Not naturally there are many geological controver concerning the origin and mode of formatior these mixed igneous-metamorphic picture 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.
Although the complete cycle is from sedimentary to metamorphic picture to igneous, many rocks short-cut the cycle, usually by missing out the igneous or metamorphic picture stages. For instance, a sediment may be lithified to a sandstone but then may be uplifted out of the sea and eroded. |
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