Read the following text and fill in the chart after the text.
Date: 2015-10-07; view: 443.
INFORMATIVE READING: Metamorphic rocks
Match the definition with the following terms.
| 1. mechanical deformation | a. metamorphic rocks in which particles are reduced to a small size |
| 2. cataclasis | b. developed on a regional scale in response to both deformation and mineral reaction |
| 3. recrystallization | c. readjustment of the material |
| 4. cataclastic rock | d. formed by a significant increase in temperature in the absence of differential stress |
| 5. regional metamorphic rocks | e. mechanical deformation |
| 6. contact metamorphic rocks | f. preexisting mineral assemblages are destabilized by changes in the temperature, pressure or chemical milieu |
The third category is rock that has been changed. The minerals in an igneous rock, or the particles in a sedimentary rock, may look fairly permanent, but in certain circumstances (under extreme conditions of pressure or temperature) they can change and recrystallize into something new. The new rock product is called a metamorphic rock.
There are two types of metamorphic rock. The first is regional metamorphic rock in which the altering force is one of pressure rather than of temperature. These are found deep within the interior of mountain chains and are believed to constitute the lower parts of the crust. Different degrees of pressure produce different grades of metamorphic rock. Slight pressure will produce a low-grade metamorphic rock, in which the only difference will be that the minerals will have been realigned in a different direction. Often this produces flat crystals of mica that are orientated perpendicular to the direction of the applied pressure. The result is a rock that has planes of weakness running in one direction and which can split easily into flat slabs. Slate and phyllite are typical low-grade metamorphic rocks. On the other hand, intense pressure will completely change the mineralogical makeup of the rock and produce a high-grade metamorphic rock. The chemical components may recrystallize into a totally different set of minerals from the original rock and the new minerals may form in distinct bands, often crumpled and contorted as evidence of the great pressures involved. Gneiss is the typical high-grade metamorphic rock showing distinct banding.
A typical sequence of rocks – from unconsolidated sediment, through sedimentary rock, through different grades of metamorphic rock, depending on the depth in the crust at which different conditions are found:
| SURFACE – Mud (sediment) |
| Mudstone (sedimentary rock) |
| 3 miles (5 km) deep |
| Shale |
| 6 miles (10 km) deep |
| Slate (low-grade metamorphic rock) Different kinds of micas develop |
| 9 miles (15 km) deep |
| Schist (medium-grade metamorphic rock) |
| 12 miles (20 km) deep |
| Gneiss (high-grade metamorphic rock) |
| 15.5 miles (25 km) |
| Migmatite (ultrametamorphic rock) |
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| DETAILED READING: Occurrence and classification of metamorphic rocks | | | Fig. 41 Metamorphic facies |