Physical Geography - Version 1 Unit 11 Metamorphism & Metamorphic Rocks

. Figure Polished Marbled in Mosaic Canyon , Death Valley California . Image by Jeremy is used under a license . UNIT 11 METAMORPHISM METAMORPHIC ROCKS Goals Objectives of this unit Summarize the factors that influence the nature of metamorphic rocks and explain why each one is important . Describe the mechanisms for the formation of foliation in metamorphic rocks . Classify metamorphic rocks based on their texture and mineral content and explain the origins of these differences . Describe the various settings in which metamorphic rocks are formed and explain the links between plate tectonics and metamorphism . GEOGRAPHY

WHAT IS METAMORPHISM ?

is the change that takes place within a body of rock as a result of it being subjected to conditions that are different from those in which it formed . In most cases , but not all , this involves the rock being deeply buried beneath other rocks , where it is subjected to higher temperatures and pressures than those under which it formed . Metamorphic rocks typically have different mineral and different textures from their parent rocks , but they may have the same overall composition . Most metamorphism results from the burial of igneous , sedimentary , or metamorphic to the point where they experience different pressures and temperatures than those at which they formed . Metamorphism can also take place if cold rock near the surface is intruded and heated by a hot igneous body . Although most metamorphism involves temperatures above , some metamorphism takes place at temperatures lower than those at which the parent rock formed . Controls over Metamorphic Processes The main factors that control metamorphic processes are The mineral composition of the parent rock The temperature at which metamorphism takes place The amount and type of pressure during metamorphism The types of fluids ( mostly water ) that are present during metamorphism The amount of time available for metamorphism PARENT ROCK The parent rock is the rock that exists before metamorphism starts . In most cases , this is a sedimentary or igneous rock , but metamorphic rock that reaches the surface and is then reburied can also be considered a parent rock . On the other hand , if , for example , a mudstone is metamorphosed to slate and then buried deeper where it is metamorphosed to schist , the parent rock ofthe schist is mudstone , not slate . The critical feature ofthe parent rock is its mineral composition because it is the stability of minerals that counts when metamorphism takes place . In other words , when a rock is subjected to increased temperatures , certain minerals may become unstable and start to recrystallize into new minerals . TEMPERATURE The temperature that the rock is subjected to is a key variable in controlling the type of metamorphism that takes place . As we learned in the context of igneous rocks , mineral stability is a function of temperature , pressure , and the presence of fluids ( especially water ) All minerals are stable over a specific range of temperatures . For example , quartz is stable from environmental temperatures ( whatever the weather can throw at it ) all the way to . If the pressure is higher , that upper limit will be higher . If there is water present , it will be lower . GEOGRAPHY

On the other hand , most clay minerals are only stable up to about or above that , they transform into . Most other common minerals have upper limits between and . Some minerals will crystallize into different ( same composition , but different crystalline structure ) depending on the temperature and pressure . Quartz is a good example as slightly different forms are stable between and 1800 The minerals , andalusite , and sillimanite are with the composition . They are stable at different pressures and temperatures , and , as we will see later , they are important indicators of pressures and temperatures in metamorphic rocks . PRESSURE Pressure is important in metamorphic processes for two main reasons . First , it has implications for mineral stability . Second , it has implications for the texture of metamorphic rocks . Rocks that are subjected to very high confining pressures are typically denser than others because the mineral grains are squeezed together , and because they may contain mineral in which the atoms are more closely packed . Because of plate tectonics , pressures within the crust are typically not applied equally in all directions . In areas of plate convergence , the pressure in one direction ( perpendicular to the direction of convergence ) is typically greater than in the other directions . In situations where different blocks of the crust are being pushed in different directions , the rocks will be subjected to sheer stress . One ofthe results of directed pressure and sheer stress is that rocks become foliated , meaning that the ricks will have a directional fabric . FLUIDS Water is the main fluid present within rocks ofthe crust , and the only one that is considered here . The presence of water is important for two main reasons . First , water facilitates the transfer of ions between minerals and within minerals and therefore increases the rates at which metamorphic reactions take place . So , while the water does necessarily change the outcome of a metamorphic process , it speeds the process up so metamorphism might take place over a shorter time period , or metamorphic processes that might not otherwise have had time to be completed are completed . Secondly , water , especially hot water , can have elevated concentrations of dissolved substances , and therefore it is an important medium for moving certain elements around within the crust . So not only does water facilitate metamorphic reactions on a basis , it also allows for the transportation of ions from one place to another . This is very important in hydrothermal processes , which are discussed toward the end ofthis chapter , and in the formation of mineral deposits . GEOGRAPHY

Most metamorphic reactions take place at very slow rates . For example , the growth of new minerals within a rock during metamorphism has been estimated to be about per million years . For this reason , it is very difficult to study metamorphic processes in a lab . While the rate of metamorphism is slow , the tectonic processes that lead to metamorphism are also very slow , so in most cases , the chance for metamorphic reactions to be completed is high . For example , one important metamorphic setting is many kilometers deep within the roots of mountain ranges . A mountain range takes tens of millions of years to form , and tens of millions of years more to be eroded to the extent that we can see the rocks that were metamorphosed deep beneath it . CLASSIFICATION OF METAMORPHIC ROCKS There are two main types of metamorphic rocks those that are foliated because they have formed in an environment with either directed pressure or shear stress , and those that are not foliated because they have formed in an environment without directed pressure or relatively near the surface with very little pressure at all . Some types of metamorphic rocks , such as quartzite and marble , which also form in situations , do not necessarily exhibit foliation because their minerals ( quartz and calcite respectively ) do not tend to show alignment . When a rock is squeezed under directed pressure during metamorphism it is likely to be deformed , and this can result in a textural change such that the minerals are elongated in the direction perpendicular to the main stress . This contributes to the formation of foliation . Figure The Effects During Metamorphism . image by Steven is used under a Creative Commons Attribution international License . GEOGRAPHY

When a rock is both heated and squeezed during metamorphism , and the temperature change is enough for new minerals to form from existing ones , there is a likelihood that the new minerals will be forced to grow with their long axes perpendicular to the direction of squeezing . After both heating and squeezing , new minerals have formed within the rock , generally parallel to each other , and the original bedding has been largely obliterated . 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 . As already noted , slate is formed from the 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 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 colors . In the example shown in Figure , the dark bands are largely amphibole while the bands are feldspar and quartz . Most gneiss has little or no mica because it forms at temperatures higher than those under which are stable . Unlike slate and phyllite , which typically only form from , schist , and especially gneiss , can form from a variety of parent rocks , including , sandstone , conglomerate , and a range of both volcanic and intrusive igneous rocks . Schist and gneiss can be identified based on both the unique and important minerals that are present . As an example , schist derived from basalt is typically rich in the mineral chlorite , so we call it chlorite schist . One derived from shale may be schist orjust mica schist , or if there are garnets present it might be schist . Similarly , a gneiss that originated as basalt and is dominated by amphibole , is an amphibole gneiss or , more accurately , an amphibolite . GEOGRAPHY

15 lust . 19 ' Figure Examples Metamorphic Rocks . Image by Steven Earl and Michael used under a 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 . 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 the table below . Table Metamorphic Rocks That Form from Different Parent Rocks APPROXIMATE VERY LOW LOW GRADE MEDIUM HIGH GRADE TEMPERATURE GRADE GRADE RANGES PARENT ROCK 150 300 45 Above Slate Phyllite Schist Gneiss GRANITE No change No change No change Granite gneiss BASALT Chlorite schist Chlorite schist Amphibolite Amphibolite SANDSTONE No change Little change Quartzite Quartzite LIMESTONE Little change Marble Marble Marble GEOGRAPHY

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 conditions orjust 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 metamorphic rocks are marble , quartzite , and hornfels . Marble is a 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 . Ifthe original limestone was pure calcite , then the marble will likely be white , but if it had various impurities , such as clay , silica , or magnesium , the marble could be marbled in appearance . Figure Example of Marble . Image is in the public domain . Quartzite is metamorphosed sandstone . 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 . GEOGRAPHY

Figure AN Example of Quartzite . Image is in the public domain . Hornfels is another metamorphic rock that normally forms during contact metamorphism of rocks like mudstone or volcanic rock . In some cases , hornfels has visible crystals of minerals like biotite or andalusite . If the hornfels formed in a situation without directed pressure , then these minerals would be randomly orientated , not foliated as they would be if formed with directed pressure . Figure Example of Hornfels . Image is in the public domain . GEOGRAPHY

PLATE TECTONICS METAMORPHISM All of the important processes of metamorphism that we are familiar with can be directly related to geological processes caused by plate tectonics . Most regional metamorphism takes place within continental crust . While rocks can be metamorphosed at depth in most areas , the potential for metamorphism is greatest in the roots of mountain ranges where there is a strong likelihood for the burial of relatively young sedimentary rock to great depths . An example would be the Himalayan Range . At this convergent boundary , sedimentary rocks have been both thrust up to great heights ( nearly above sea level ) and also buried to great depths . Considering that the normal geothermal gradient ( the rate of increase in temperature with depth ) is around per kilometer , rock buried to below sea level in this situation could be close to 18 below the surface ofthe ground , and it is reasonable to expect temperatures up to . Metamorphic rocks formed there are likely to be foliated because of the strong directional pressure of converging plates . Figure Regional Metamorphism beneath a Mountain Range . Image by Karla ( 2018 ) modified after Steven ( 2015 ) GEOGRAPHY

At an oceanic spreading ridge , recently formed oceanic crust of gabbro and basalt is slowly moving away from the plate boundary . Water within the crust is forced to rise in the area close to the source of volcanic heat , and this draws more water in from farther out , which eventually creates a convective system where cold seawater is drawn into the crust and then out again onto the seafloor near the ridge . The passage of this water through the oceanic crust at to promotes metamorphic reactions that change the original pyroxene in the rock to chlorite and serpentine . Because this metamorphism takes place at temperatures well below the temperature at which the rock originally formed ( it is known as retrograde metamorphism . The rock that forms in this way is known as greenstone if it is foliated , or if it is . Chlorite ( Ol ) and serpentine ( Fe ) OH ) are both hydrated minerals meaning that they have water ( as OH ) in their chemical formulas . When metamorphosed ocean crust is later , the chlorite and serpentine are converted into new minerals ( garnet and pyroxene ) and the water that is released migrates into the overlying mantle , where it contributes to flux melting . Greenstone and metamorphism Convection of seawater Convection of seawater Figure Regional Metamorphism Crustal Rock on Either Side ofa Spreading Ridge . Image is used under a Creative Commons Attribution International License . At a subduction zone , oceanic crust is forced down into the hot mantle . But because the oceanic crust is now relatively cool , especially along its upper surface , it does not heat up quickly , and the rock remains several hundreds of degrees cooler than the surrounding mantle . A special type of metamorphism takes place under these very but relatively conditions , GEOGRAPHY

producing an amphibole mineral is known as glaucophane ( which is blue and is a major component of a rock known as . If you never seen or even heard of , it not surprising . What is surprising is that anyone has seen it ! Most forms in subduction zones , continue to be , turn into eclogite at about 35 depth , and then eventually sinks deep into the mantle never to be seen again . In only a few places in the world , where the subduction process has been interrupted by some tectonic process , has partially rock returned to the surface . One such place is the area around San Francisco the rock is known as the Franciscan Complex . Figure 59 Regional Metamorphism of Oceanic Crust at a Subduction Zone . Image by Steven Earl is used under a Creative Commons Attribution International License . Magma is produced at convergent boundaries and rises toward the surface , where it can form magma bodies in the upper part of the crust . Such magma bodies , at temperatures of around , heat up the surrounding rock , leading to contact metamorphism . Because this happens at relatively shallow depths , in the absence of directed pressure , the resulting rock does not normally develop foliation . The zone of contact metamorphism around an intrusion is very small ( typically meters to tens of meters ) compared with the extent of regional metamorphism in other settings ( tens of square kilometers ) GEOGRAPHY

Figure Contact Metamorphism around a Crustal Magma Chamber . Image by Steven Earl is used under a Creative Commons Attribution International License . Regional metamorphism also takes place within mountain ranges , and because of the extra heat associated with the volcanism , the geothermal gradient is typically a little steeper in these settings ( somewhere between and ) As a result , higher grades of metamorphism can take place closer to the surface than is the case in other areas . Another way to understand metamorphism is by using a diagram that shows temperature on one axis and depth ( which is equivalent to pressure ) on the other . The three heavy dotted lines on this diagram represent Earth geothermal gradients under different conditions . In most areas , the rate of increase in temperature with depth is . In other words , if you go down into a mine , the temperature will be roughly warmer than the average temperature at the surface . In most parts of southern Canada , the average surface temperature is about , so at depth , it will be about . That uncomfortably hot , so deep mines must have effective ventilation systems . This typical geothermal gradient is shown by the green dotted line in Figure . At 10 depth , the temperature is about and at 20 it about . In volcanic areas , the geothermal gradient is more like 40 ' to , so the temperature at 10 depth is in the to range . Along subduction zones , as described above , the cold oceanic crust keeps temperatures low , so the gradient is typically less than . GEOGRAPHY

UNIT 11 SUMMARY Metamorphism is controlled by five main factors the composition of the parent rock , the temperature to which the rock is heated , the amount and type of pressure , the volumes and compositions of aqueous fluids that are present , and the amount of time available for metamorphic reactions to take place . Metamorphic rocks are classified based on the texture and mineral composition . Foliation is a key feature of metamorphic rocks formed under directed pressure foliated metamorphic rocks include slate , phyllite , schist , and gneiss . Metamorphic rocks formed in environments without strong directed pressure include hornfels , marble , and quartzite . Almost all metamorphism can be explained by processes . Oceanic crustal rock can be metamorphosed near the spreading ridge where it was formed , but most other regional metamorphism takes place in areas where mountain ranges have formed , which are most common at convergent boundaries . Contact metamorphism takes place around magma bodies in the upper part of the crust , which are also most common above convergent boundaries . GEOGRAPHY