Lesson 8 Metamorphic Rocks

Here are links to other web sites that will help in recognizing and identifying metamorphic rocks

Of all the rock types metamorphic rocks come in the greatest variety. Metamorphic rocks are the result of changes which take place in other rocks when they are subjected to changes in temperature, pressure, or chemically active fluids.

The changes that take place consist of rearrangements of atoms to form new minerals. Most of the time no atoms are gained or lost from outside the original rock. The notable exception to this is when superheated water from nearby magma reacts with the rocks as they are undergoing changes.

These changes always take place without melting, although the temperature may be high enough to cause the rocks to become quite plastic.

Since rocks of all types are primarily made of the same eight kinds of atoms, the kinds of minerals that form in metamorphic rocks depends mostly on the temperature, the pressure, and the time. Like any chemical process,>  metamorphism will proceed as long as the conditions remain stable. Any change in temperature and pressure will result in new equilibrium conditions.

The common minerals in metamorphic rocks are the same as the rock-forming silicates found in igneous rocks with the addition of a few high-temperature, high-pressure forms. Some different minerals that are common in metamorphic rocks are: kyanite, garnet, sillimanite, andalusite, and corundum. In addition, metamorphic rocks can contain combinations of minerals that would never be found in ingeous rocks, such as quartz and amphibole. Feldspar is also a common mineral in some metamorphic rocks.

Metamorphic rocks are often subjected to DIRECTED PRESSURE. Since pressure depends on depth, the rocks are squeezed vertically,. like smashing a piece of clay between a book and a tabletop. This restricts the movement of atoms in the vertical direction but allows movement horizontally. For this reason many of the common metamorphic minerals are elongated in the direction perpendicular to the directed pressure. These are called FOLITATED rocks. The word foliate comes from the Latin word for ‘sheet’, as in ‘sheets of paper’ or in ‘foliation’, which refers to leaves on trees.

The foliation of metamorphic rocks is due to the presence of sheet silicates such as the micas and chlorite, or minerals that tend to form elongated crystals, such as amphibole (hornblende).

Foliated rocks may contain crystals of mineral that tend for form in rounded or cubic crystals. These appear as ‘chunks’, like nuts in a cookie, called porphyroblasts, which appear like the phenocrysts in igneous rocks.

Because some minerals like calcite and quartz do not easily form elongated crystals under directed pressure, metamorphic rocks which contain them are usually not foliated. These and others which do not show obvious directional characteristics are called NON-FOLIATED rocks.

1. Foliated Rocks

The foliated metamorphic rocks are usually the result of regional metamorphism which takes place in the core of a folded mountain range. They are classified according to the size of the primary mineral grains, similarly to sedimentary rocks. The classifications are as follows:

1.1. Slate: fine-grained, foliated, may have "slaty cleavage" and break easily along nearly parallel planes. Slate is formed from cool brittle rocks which are subject to high pressures. Usually black but may be green, gray or red. Formed from metamorphosis or shales. Many gradations exist between shale and slate and it is not always easy to distinguish them. May contain larger grains of garnet, kyanite or corundum.

1.2. Phyllite slightly coarser grained, otherwise much like slate. Phyllites have a distinctive surface "sheen" which results from the alignment of mica or other platy grains. Grains are generally too small to identify. Like slate, may contain larger cyrstals of garnet, kyanite, corundum or others.

1.3. Schist: Fine to coarse grained. Often rich in mica with larger grains (porphyroblasts) of garnet or other minerals. Individual grains can be seen and identified but they form a dense mass. One common variety is mostly muscovite and quartz, called quartz-sericite schist Here the quartz may be unnoticable except that it makes the mica seem much harder and more difficult to "flake off" than it really is. There are many varieties of schist which are usually named according to the minerals present in them. Other common types are amphibolite (containing amphibole) and chlorite schist (chlorite is a green ferromagnesian mica)

1.4. Gneiss: The most extreme conditions of temperature and pressure can cause almost a complete recrystallization of the minerals. These conditions separate the minerals into felsic and mafic layers.

2. Non-foliated Rocks

The nonfoliated rocks are usually the result of contact metamorphism. This happens when a large body of magma heats up the surrounding rocks at a relatively low pressure. These rocks usually bear characteristics of the original rock (the protolith)..

2.1. Hornfels: a very fine-grained metamorphic rock that may resemble basalt in the hand specimen. Formed from shale or other rocks with slightly different compositions as a result.

2.2. Marble: Contact metamorphism of limestone forms marble, a coarse grained rock consisting of interlocking crystals of calcite. The marbling which is common in marbles is due to the separation of impurities in the original limestone. Black streaks come from carbon, green from chlorite or other ferromagnesians, gold from pyrite and red from iron oxides. Marlbe will fizz with HCl unless it is dolomite marble which fizzes only when powdered.

2.3. Quartzite When quartz sandstone is exposed to high temperatures the quartz grains become plastic and become welded together. In some cases the quartz cement also becomes welded to the grains to form a nearly solid mass of quartz. Calcareous sandstones may have large amounts of wollastinite, a calcium silicate, which forms from the reaction of calcite and quartz at high temperatures.

2.4. Anthracite: is metamorphized coal. During metamorphosis the volatile atoms (water, sulfur, nitrogen, phosphorus) are driven out leaving behind almost pure carbon. Anthracite is shiny and black and often looks like tar only it is much harder.

Metamorphic Links

Cal Poly Pomona

This site takes you through a series of steps of identification.It requires that you can recognize the presence of foliation and mineral grains.

James Madison University

These sites have several different pages. Follow the links to find the particular information needed.

http://csmres.jmu.edu/geollab/Fichter/MetaRx/

http://csmres.jmu.edu/geollab/Fichter/MetaRx/metasimple.html

University of North Dakota

This site is part of the UND Space Studies Virtual Campus and also contains links about volcanoes.