METAMORPHIC ROCKS Formation and Characteristics
Rocks are formed on Earth as igneous, sedimentary, or metamorphic rocks. • Igneous rocks form when rocks are heated to the melting point which forms magma. • Sedimentary rocks are formed from the cementing together of sediments, or from the compaction (squeezing together) of sediments, or from the recrystallization of new mineral grains which are larger than the original crystals. • Metamorphic rocks form from heat and pressure changing the original or parent rock into a completely new rock.
FORMATION OF METAMORPHIC ROCK The word "metamorphic" comes from Greek and means "To Change Form".
HEAT&PRESSURE • Metamorphic rocks form from HEAT and PRESSUREchanging the original or parent rock into a completely new rock. • The parent rock can be either sedimentary, igneous, or even another metamorphic rock.
that cause an increase inPRESSURE that cause an increase inHEAT metamorphic rock HEAT from the core Solid rock can be changed into a new rock by stresses
HEAT • Temperature increases can be caused by layers of sediments being buried deeper and deeper under the surface of the Earth. • As we descend into the earth the temperature increases about 25ºC for every kilometre that we descend. The deeper the layers are buried the hotter the temperatures become. • The great weight of these layers also causes an increase in pressure, which in turn, causes an increase in temperature. • Metamorphism can take millions of years as in the slow cooling of magma buried deep under the surface of the Earth.
The descending of rock layers at subduction zones causes metamorphism in two ways: • the shearing effect of the plates sliding past each other causes the rocks coming in contact with the descending rocks to change. Some of the descending rock will melt because of this friction. • When rock melts it is then considered igneous not metamorphic, but the rock next to the melted rock can be changed by the heat and become a metamorphic rock. • The diagram above shows you where metamorphic rock (yellow) can be produced at a subduction zone.
PRESSURE There are 3 factors that cause an increase in pressure which also causes the formation of metamorphic rocks. These factors are: • The huge weight of overlying layers of sediments. • Stresses caused by plates colliding in the process of mountain building. • Stresses caused by plates sliding past each other, such as the shearing stresses at the San Andreas fault zone in California, or under another, such as the Pacific & Indo-Australian plates beneath New Zealand. Metamormorphism can be instantaneous as in the shearing of rocks at plate boundaries.
There are 2 main ways that metamorphic rocks can form. These are: Contact and Regional metamorphism.
CONTACT METAMORPHISM • Contact Metamorphism occurs when magma comes in contact with an already existing body of rock. • When this happens the existing rocks temperature rises and also becomes infiltrated with fluid from the magma. • The area affected by the contact of magma is usually small, from 1 to 10 kilometres. • Contact metamorphism produces non-foliated (rocks without any cleavage) rocks such as marble, quartzite, and hornfels.
REGIONAL METAMORPHISM • Regional Metamorphism occurs over a much larger area than contact metamorphism. • This metamorphism usually produces rocks such as gneiss and schist. • Regional metamorphism is caused by large geologic processes such as mountain-building. • These rocks when exposed to the surface show the unbelievable pressure that cause the rocks to be bent and broken by the mountain building process.
MOUNTAIN RANGES • Metamorphic rocks are almost always harder than sedimentary rocks. They are generally as hard and sometimes harder than igneous rocks. • They form the roots of many mountain chains and are exposed to the surface after the softer outer layers of rocks are eroded away. • Many metamorphic rocks are found in mountainous regions today and are a good indicator that ancient mountains were present in areas that are now low hill or even flat plains. Investigate the rocks of the South Island.
FOLIATE METAMORPHIC ROCKS • Foliate comes from the Latin word that means sheets, as in the sheets of paper in a book. • Metamorphic rocks are divided into two categories- foliates and non-foliates. • Foliates are composed of large amounts of micas and chlorites. These minerals have very distinct cleavage. • Foliated metamorphic rocks will split along cleavage lines that are parallel to the minerals that make up the rock. Slate, as an example, will split into thin sheets.
SHALE to SLATE Silt and clay can become deposited and compressed into the sedimentary rock shale. The layers of shale can become buried deeper and deeper by the process of deposition. • Deposition is the laying down of rock forming material by any natural agent (wind, water, glaciers) over time. • Because these layers are buried, temperatures and pressures become greater and greater until the shale is changed into slate. • Slate is a fine-grained metamorphic rock with perfect cleavage that allows it to split into thin sheets. • Slate usually has a light to dark brown streak. • Slate is produced by low grade metamorphism, which is caused by relatively low temperatures and pressures.
Slate has been used by man in a variety of ways over the years. • One use for slate was in the making of headstones or grave markers. Slate is not very hard and can be carved easily. The problem with slate though is its perfect cleavage. The slate headstones would crack and split along these cleavage planes as water would seep into the cracks and freeze which would lead to expansion. This freeze-thaw, freeze-thaw over time would split the headstone. Today headstones are made of a variety of rocks, with granite and marble being two of the most widely used rocks. • Slate was also used for chalk boards. The black colour was good as a background and the rock cleaned easily with water. Today, it has been replaced by materials that do not have the disadvantage of slate, i.e. weight and the splitting and cracking over time.
SLATE TO SCHIST • Schist is a medium grade metamorphic rock. This means that it has been subjected to more heat and pressure than slate, which is a low grade metamorphic rock. • Schist is a more coarse grained rock than slate. The individual grains of minerals can be seen by the naked eye. Many of the original minerals have been altered into flakes. Because it has been squeezed harder than slate it is often found folded and crumpled.
Schists are usually named by the main minerals that they are formed from. Bitotite mica schist, hornblende schist, garnet mica schist, and talc schist are some examples of this. GARNET-MICA SCHIST Garnet-mica schist – microscopic view
In this specimen of coarse-grained biotite schist you can see clearly the flakes of dark mica, lined up parallel to each other, and separated by light-coloured material, which is mostly millimetre-sized quartz crystals. In the photo above you see the thin edges of the mica flakes.
PARENT ROCK - sedimentary LOW-GRADE METAMORPHIC ROCK MEDIUM-GRADE METAMORPHIC ROCK
GNEISS • Gneiss is an old German word meaning bright or sparkling – it is pronounced "nice”. • It is a high grade metamorphic rock. This means that it has been subjected to more heat and pressure than schist. • Gneiss is coarser than schist and has distinct banding. This banding has alternating layers that are composed of different minerals.
PARENT ROCK • The minerals that compose gneiss are the same as granite. • Feldspar is the most important mineral that makes up gneiss along with mica and quartz. Gneiss can be formed from a sedimentary rock such as sandstone or shale, or it can be formed from the metamorphism of the igneous rock granite. Gneiss can be used by man as paving and building stone.
METAMORPHOSED into GNEISS • Nearly all traces of the original structures (including fossils) and fabric (such as layering and ripple marks) are wiped out as the minerals migrate and re-crystallise. The streaks are composed of minerals, like hornblende, that do not occur in sedimentary rocks. See the photos on slide 21 for some of the variety to be found in gneiss. • Thicker veins of large-grained minerals form in it, unlike the more evenly layered appearance of schist. • With still more metamorphism, gneisses can turn to migmatite and then totally re-crystallise into granite.
NON-FOLIATE METAMORPHIC ROCKS • Non-Foliates are metamorphic rocks that have no cleavage at all. • Quartzite and marble are two examples of non-foliates . quartzite marble
QUARTZITE • Quartzite is composed of sandstone that has been metamorphosed. • Quartzite is much harder than the parent rock sandstone. • It forms from sandstone that has come into contact with deeply buried magmas.
Quartz sandstone PARENT ROCK • This is a pure sandstone composed of rounded grains of quartz, cemented by further quartz between the grains. This makes the rock very hard and resistant to weathering and erosion, rather like a metamorphic quartzite formed by the action of heat and pressure. • Marble is much harder than its parent rock. This allows it to take a polish which makes it a good material for use as a building material, making sink tops, bathtubs, and a carving stone for artists. Today, headstones are made from marble and granite because both of these rocks weather very slowly and carve well with sharp edges.
METAMORPHOSED into QUARTZITE • Quartzite looks similar to its parent rock. • The best way to tell quartzite from sandstone is to break the rocks. • Sandstone will shatter into many individual grains of sand while quartzite will break across the grains.
MARBLE • Marble is metamorphosed limestone or dolomite. • Both limestone and dolomite have a large concentration of calcium carbonate (CaCO3). • Marble has many different sizes of crystals. • Marble has many colour variances due to the impurities present at formation. Some of the different colours of marble are white, red, black, mottled and banded, grey, pink, and green.
limestone dolomite PARENT ROCKS • Limestone is usually not made of sediment as we think of it—not clay or sand, derived from rocks—but instead is built from the tiny calcite skeletons of microscopic organisms in shallow seas. • Dolomite looks like limestone, but unlike limestone it does not bubble when treated with weak acid. The mineral responsible is also called dolomite.
This fresh boulder shows that much of limestone is like this, with no discernible structure or texture. It looks strong, but the quarry this came from was set in this place not for building stone but because the rock was pure and readily burnt for lime (CaO, calcium oxide) for making cement. There must have been a good wood supply near to make that operation efficient.
Close up, this piece of Hoyt Limestone shows no more detail or structure than the boulder in the previous picture. • Its crystals are small and tightly interlocked, due to the effects of pressure and mild heat that solidifies rock. • The colour of limestone tends to lie in the range of white to dark gray owing to the presence of organic matter, though it also takes warmer hues from clay minerals.
METAMORPHOSED into MARBLE • In this hand specimen of marble, the crystals are large. For fine marble of the sort used in buildings and sculpture, the crystals are small. • Like other metamorphic rocks, marble has no fossils, and any layering that appears in it probably does not correspond to the original bedding of the precursor limestone. And like limestone, marble tends to dissolve in acidic fluids. It is quite durable in dry climates, as in the Mediterranean countries where ancient marble structures survive. • Marble is what happens to fairly pure limestone or dolomite rock after metamorphism. Heat and pressure cause the grains of calcite (in limestone) or dolomite (in dolomite rock) to combine into larger crystals.
MARBLE with ruby • Pure limestones, made of calcium carbonate, are converted into crystalline calcite marble (also calcium carbonate) during metamorphism. • However, if the limestone contains impurities, other minerals may form in the marble. • This rock had aluminium as its principal impurity (probably in the form of clay particles), and high grade metamorphism has produced large crystals of corundum (aluminium oxide). • There must also have been tiny amounts of the metal chromium, which colours the corundum red - this is the gemstone ruby. • The coarse interlocking texture of the calcite crystals can also be made out.
MARBLE with serpentine • This rock formed from a dolomitic limestone (calcium magnesium carbonate) with some silica impurities. • High grade metamorphism formed the mineral olivine (magnesium silicate). • Along bands and fractures, where water got in while the rock was still hot, this has been converted to the distinctive yellow-green colour of serpentine (hydrated magnesium silicate). • Serpentine-rich marbles are commonly quarried, cut into slabs, and used for decorative purposes.
HEAT&PRESSURE METAMORPHIC ROCKS - review • Metamorphic rocks form from HEAT and PRESSUREchanging the original or parent rock into a completely new rock. • The parent rock can be either sedimentary, igneous, or even another metamorphic rock. • Slate, schist, gneiss, and marble are all examples of metamorphic rocks.