Choose an RSS feed from the list below. Don't know what to do with RSS feeds? Remember, you can also make your own, personal feed by combining tags from around OpenLearn. For further information, take a look at our frequently asked questions which may give you the support you need. Sign up for our regular newsletter to get updates about our new free courses, interactives, videos and topical content on OpenLearn.
Newsletter sign-up. OpenLearn works with other organisations by providing free courses and resources that support our mission of opening up educational opportunities to more people in more places.
All rights reserved. The Open University is authorised and regulated by the Financial Conduct Authority in relation to its secondary activity of credit broking. Skip to content Study with The Open University. Search for free courses, interactives, videos and more! Free Learning from The Open University. Featured content. Free courses. All content.
Gneiss Updated Thursday, 28th September A brief description of the nature of gneiss. Copyright: The Open University How was it formed? Gneiss displays distinct foliation , representing alternating layers composed of different minerals. Because of the coarseness of the foliation, the layers are often sub-parallel, i. Gneiss is typically associated with major mountain building episodes.
Slate, for example, is characterized by aligned flakes of mica that are too small to see. The various types of foliated metamorphic rocks, listed in order of the grade or intensity of metamorphism and the type of foliation are slate , phyllite , schist , and gneiss Figure 7. As already noted, slate is formed from the low-grade metamorphism of shale, and has microscopic clay and mica crystals that have grown perpendicular to the stress.
Slate tends to break into flat sheets. Phyllite is similar to slate, but has typically been heated to a higher temperature; the micas have grown larger and are visible as a sheen on the surface. Where slate is typically planar, phyllite can form in wavy layers. In the formation of schist, the temperature has been hot enough so that individual mica crystals are visible, and other mineral crystals, such as quartz, feldspar, or garnet may also be visible.
In gneiss, the minerals may have separated into bands of different colours. In the example shown in Figure 7. Most gneiss has little or no mica because it forms at temperatures higher than those under which micas are stable. Unlike slate and phyllite, which typically only form from mudrock, schist, and especially gneiss, can form from a variety of parent rocks, including mudrock, sandstone, conglomerate, and a range of both volcanic and intrusive igneous rocks.
Schist and gneiss can be named on the basis of important minerals that are present. For example a schist derived from basalt is typically rich in the mineral chlorite, so we call it chlorite schist. One derived from shale may be a muscovite-biotite schist, or just a mica schist, or if there are garnets present it might be mica-garnet schist.
Similarly, a gneiss that originated as basalt and is dominated by amphibole, is an amphibole gneiss or, more accurately, an amphibolite. If a rock is buried to a great depth and encounters temperatures that are close to its melting point, it will partially melt.
The resulting rock, which includes both metamorphosed and igneous material, is known as a migmatite Figure 7. JPG] As already noted, the nature of the parent rock controls the types of metamorphic rocks that can form from it under differing metamorphic conditions.
The kinds of rocks that can be expected to form at different metamorphic grades from various parent rocks are listed in Table 7. Some rocks, such as granite, do not change much at the lower metamorphic grades because their minerals are still stable up to several hundred degrees.
0コメント