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. Metamorphic rocks that form under either low-pressure conditions or just confining pressure do not become foliated. In most cases, this is because they are not buried deeply, and the heat for the metamorphism comes from a body of magma that has moved into the upper part of the crust.
Sedimentary rocks were originally sediments, which were compacted under high pressure. Igneous rocks formed when liquid magma or lava —magma that has emerged onto the surface of the Earth—cooled and hardened.
A metamorphic rock , on the other hand, began as a rock—either a sedimentary, igneous, or even a different sort of metamorphic rock. Then, due to various conditions within the Earth, the existing rock was changed into a new kind of metamorphic rock. The conditions required to form a metamorphic rock are very specific. The existing rock must be exposed to high heat, high pressure, or to a hot, mineral-rich fluid. Usually, all three of these circumstances are met.
In order to create metamorphic rock, it is vital that the existing rock remain solid and not melt. If there is too much heat or pressure, the rock will melt and become magma. This will result in the formation of an igneous rock , not a metamorphic rock.
Consider how granite changes form. Granite is an igneous rock that forms when magma cools relatively slowly underground.
It is usually composed primarily of the minerals quartz, feldspar, and mica. When granite is subjected to intense heat and pressure, it changes into a metamorphic rock called gneiss. Slate is another common metamorphic rock that forms from shale. Limestone, a sedimentary rock , will change into the metamorphic rock marble if the right conditions are met. This happens due to geologic uplift and the erosion of the rock and soil above them.
At the surface, metamorphic rocks will be exposed to weathering processes and may break down into sediment. These sediments could then be compressed to form sedimentary rocks, which would start the entire cycle anew. Select personalised ads. Apply market research to generate audience insights. Measure content performance. Develop and improve products.
List of Partners vendors. Share Flipboard Email. Table of Contents Expand. Rock Identification Tips. Rock Identification Chart. Igneous Rock Identification. Sedimentary Rock Identification. Metamorphic Rock Identification. Need More Help? Andrew Alden. Geology Expert. Andrew Alden is a geologist based in Oakland, California. He works as a research guide for the U.
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