Most metamorphic processes take place deep underground, inside the earth’s crust. These conditions often stretch, twist and fold the rock as it cools. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. • In fine grained – low grade rocks, it is relict bedding or igneous layer • Secondary separation into contrasting layers require diffusion, possible only at high temperature • It is either a secondary layering or poorly developed schistosity. Content Filtration 6. Nappe formation is a major process of crustal thickening during continent-continent collision; emplacement of the nappes results in burial and heating of the underlying rocks. Metamorphism associated with nappes (large recumbent folds) in the Alps and the Appalachians provides strong evidence that the tectonic transport of rocks typically occurs at rates faster than those of thermal equilibration—in other words, that the nappes can transport hot rocks for large distances without significant cooling. Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. Magma metamorphic rock forming here heat Pressure from surface rocks 4. A cataclastic rock is a type of fault rock that has been wholly or partly formed by the progressive fracturing and comminution of existing rocks, a process known as cataclasis.Cataclasis involves the granulation, crushing, or milling of the original rock, then rigid-body rotation and translation of mineral grains or aggregates before lithification. a rock with metamorphic minerals but no foliation or lineation. The rock has split from bedrock along this foliation plane, and you can see that other weaknesses are present in the same orientation. These rocks are identified by their minerals and texture. Metamorphism occurs when solid rock changes in composition and/or texture without the mineral crystals melting, which is how igneous rock is generated. The slaty cleavage may form at any angle to the bedding planes of the shale from which the slaty rock has been derived as shown in 3.19. It forms when a quartz-rich sandstone is altered by the heat, pressure, and chemical activity of metamorphism. Isograds that parallel major structures suggest some cause-and-effect relationship between the structural and metamorphic development of the region. Metamorphic Rocks 5. The most important non-foliated rock is marble. • This can greatly affect the hardness, texture or layer patterns of the rocks. Without careful examination by the Geologist, and the recognition of the subtle changes in mineralogy and texture, much time and money can be wasted by drilling past the economic basement. Copyright 9. Gneissose Structure and Layers • Rocks having alternate light and dark layers. Regional means that the metamorphic conditions occur over large geographic areas and include differential (or shearing) stresses, which help to form the layered structure known as foliation. Classification of Metamorphic Rocks 2. The structures and textures of metamorphic rocks arise during the recrystallization in the solid state of primary sedimentary and magmatic rocks. Engineering Uses. Isograd distributions associated with nappe structures can be either normal or inverted, depending on the relative rates of nappe emplacement and heat transfer. Under greater heat and pressure, as metamorphic minerals such as mica and feldspar begin to form, strain orients them in layers. For example, isograd patterns that are cut by faults clearly indicate that metamorphism predated fault displacement, whereas isograd sequences that overprint structural discontinuities imply the reverse. 3.20. They are not made from molten rock – rocks that do melt form igneous rocks instead.
2020 structure of metamorphic rocks