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. This is contact metamorphism. Some examples of non-foliated metamorphic rocks are marble , quartzite , and hornfels.
Marble is metamorphosed limestone. When it forms, the calcite crystals tend to grow larger, and any sedimentary textures and fossils that might have been present are destroyed. If the original limestone was pure calcite, then the marble will likely be white as in Figure 7. Quartzite is metamorphosed sandstone Figure 7. It is dominated by quartz, and in many cases, the original quartz grains of the sandstone are welded together with additional silica.
Most sandstone contains some clay minerals and may also include other minerals such as feldspar or fragments of rock, so most quartzite has some impurities with the quartz. Park Passes. Technical Announcements. Employees in the News.
Emergency Management. Survey Manual. Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous , sedimentary , or earlier metamorphic form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors. Conditions like these are found deep within the Earth or where tectonic plates meet. Process of Metamorphism: The process of metamorphism does not melt the rocks, but instead transforms them into denser, more compact rocks.
New minerals are created either by rearrangement of mineral components or by reactions with fluids that enter the rocks. Pressure or temperature can even change previously metamorphosed rocks into new types.
Metamorphic rocks are often squished, smeared out, and folded. Despite these uncomfortable conditions, metamorphic rocks do not get hot enough to melt, or they would become igneous rocks!
Common Metamorphic Rocks: Common metamorphic rocks include phyllite, schist, gneiss, quartzite and marble. Foliated Metamorphic Rocks: Some kinds of metamorphic rocks -- granite gneiss and biotite schist are two examples -- are strongly banded or foliated.
Foliated means the parallel arrangement of certain mineral grains that gives the rock a striped appearance. Foliation forms when pressure squeezes the flat or elongate minerals within a rock so they become aligned.
These rocks develop a platy or sheet-like structure that reflects the direction that pressure was applied. Non-Foliated Metamorphic Rocks: Non-foliated metamorphic rocks do not have a platy or sheet-like structure.
There are several ways that non-foliated rocks can be produced. Some rocks, such as limestone are made of minerals that are not flat or elongate. No matter how much pressure you apply, the grains will not align! Another type of metamorphism, contact metamorphism, occurs when hot igneous rock intrudes into some pre-existing rock. The pre-existing rock is essentially baked by the heat, changing the mineral structure of the rock without addition of pressure.
Learn more: Geologic units containing metamorphic rock. The buildings of our Nation's Capital are constructed with rocks from quarries located throughout the United States and many distant lands. The earliest Government buildings, however, were constructed with stones from nearby sources because it was too difficult and expensive to move heavy materials such as stone any great distance without the aid Ever wondered what the difference between a rock and a mineral was?
This EarthWord should cover it For the first time, geologists have extracted intact rock samples from 2 miles beneath the surface of the San Andreas Fault, the infamous rupture that runs miles along the length of California. Gneiss forms by regional metamorphism from both high temperature and pressure. Quartzite and marble are the most commonly used metamorphic rocks.
They are frequently chosen for building materials and artwork. Marble is used for statues and decorative items like vases Figure 4. Ground up marble is also a component of toothpaste, plastics, and paper.
Quartzite is very hard and is often crushed and used in building railroad tracks Figure 4. Schist and slate are sometimes used as building and landscape materials. Skip to main content. Search for:. Metamorphic Rocks In this lesson you will learn about metamorphic rocks, how they form, and some of their common uses.
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