metamorphic rocks are formed when other rocks known as parent rocks are chemically Andor physically transformed by increases in temperature Andor pressure and possibly by the addition and interaction of chemically active fluids usually hot waterers rich in dissolved ions metamorphic rocks can form under a number of geologic settings anywhere in which temperatures and our pressures are raised usually a given parent rock will form very different metamorphic rocks based on the metamorphic conditions and hence we can look for clues in the Rock to help us identify the original geologic formation setting by the end of this learning module you should be able to list the most common metamorphic rocks recognize their textures and compositions and Link them to their formation environment and history for some parent rocks the same metamorphic rock will form regardless of environment in that case studying the metamorphic rock will not give us any information on metamorphic setting only the original parent rock material for example in this pile of metamorphic rocks there is one that is made almost entirely of calite or a magnesium Rich related mineral known as Dolomite when a calyer rock such as limestone is exposed to increased pressure or temperature the crystals grow larger because the parent rock is homogeneous in composition mostly calcium carbonate as long as no new chemicals are added during metamorphism the composition of the final rock is unchanged calcium carbon carbonate we call this metamorphic variety marble pure calite parent rocks produce pure calite metamorphic Marbles and the only indication of metamorphic intensity just how high the temperature and pressure got is the size of the crystals the same is true of pure quartz parent rocks like CHS and quartz sandstones the corresponding metamorphic rock is called quartzite and the only indication of metamorphic intensity or grade is the size of the crystals so if we can identify a marble or a quartzite what do we know we know some likely candidates for the parent rock but we have little to no knowledge of the metamorphic setting in addition to Crystal size another textural clue found in metamorphic rocks is something called foliation which is the alignment of crystals highly foliated rocks display High alignment of crystals within the Rock calide and quartz crystals tend to be uniformly shaped with no long axis more like rounded balls or Pebbles micas with their sheety structure can be aligned similarly long Prismatic crystals like cenite and actinite can also be aligned if we see alignment of these minerals in a rock we call the Rock foliated the extent of foliation combined with the types of minerals present can tell us a lot about metamorphic setting for example foliation can happen only when a rock is subjected to increasing pressure notice this pile of pencils and pens completely unaligned or unfoliated as I start to compress this system along one axis the pens and pencils that are free to move will start to align themselves with each other the greater the pressure the greater the alignment or foliation added to the foliation evidence is mineral evidence many minerals have a limited range of temperatures and pressures at which they are stable if we change those conditions so that a mineral is out of its stability range the unstable mineral will want to rearrange its atoms and bonds to create a new mineral that is more stable under the new conditions calide and quartz are stable under a very large range of pressures and temperatures in fact all of the temperatures and pressures found in typical metamorphic settings thus they do not if present alone recombine to form new minerals however if calite is present with quartz and with other minerals then as temperatures and pressure rise the stability of calide and quartz might end up being less stable than a new mineral with a new chemical composition and thus the atoms in the calite and quartz might rearrange themselves to form new minerals for example W asite ca3 might be more stable so the stability of a mineral depends on temperature pressure and chemical composition of the parent rock and any introduced chemicals from active fluids it's important to note here that when chemical constituents of a metamorphic rock rearrange themselves they do so without melting The Rock how increased temperatures will increase the motion of the atoms in the solid and if that vibrational energy is high enough the atoms can jump around in the solid and do things like clean up their crystal structure remove imperfections combine to form larger crystals and rearrange into more stable compounds so we can separate metamorphic rocks into three piles foliated nonfoliated and weakly foliated now let's go to the non-foliated rocks if these contain minerals that could foliate but haven't then what do we know the rock has not been subjected to high pressures the only metamorphic settings in which there is little to no increased pressure are contact metamorphism heating up rocks around a magma chamber and hydrothermal metamorphism hot Waters percolating through cracks or pores in the Rock when the non-foliated Rock contains a mixture of large crystals such as this one with some calite some cord some garnet and some epidote we call the final Rock scarn scarns are rocks that form in contact metamorphic environments with high temperatures and likely high amounts of chemically active fluids but little to no pressure we would find these environments underground at all volcanic settings such as hotspot volcanism divergent plate boundary volcanism and subduction zone volcanism a Sandstone that contains some mineral and rock fragments plus some shell fragment fragments would be a good parent rock for a scarn the mineral compositions and sizes that we find in a scarn can further tell us how much temperature and what kinds of fluids were available we would expect to see the minerals stable at the highest temperatures in a Zone closest to the magma chamber and those indicating lower temperature stabilities further away this other rock that is dark black with a sugary texture is called hornfels it is also non-foliated so also found in the same settings just described for scarn but it's what results when the parent rock was a mudstone or Basalt let's pause for a moment and review all our non-foliated rocks marble quartzite scarn and hornfels all of these can form in a contct metamorphism setting and each is transformed in that same setting to a different Rock product not because of different temperatures and pressures but because the original Parent Rock differed rocks with clear foliation show a clear alignment of minerals either invisible bands or microscopically as micas that are aligned and give the rock a Shey or slate cleavage last week we talked about how muds siiz Clays can collect at the bottom of lakes or the ocean floor and get buried in compressed or cemented to form mudstone continued compaction will align the clay minerals which are sheet silicates with sheety cleavage and that gives the rock a pler fabric a little bit of compression means we get a siment rock called shale a bit more compression moves us into the metamorphic realm the Clays are beginning to change chemically into other sheet silicates such as chlorite or musite or biotite these perfectly aligned microscopic sheet silicates give the rock slady cleavage and the rock is called slate you can see how dense it is in fact you can even hear a difference among these three rocks mudstone is less dense and compacted Shale is more dense and compacted slate is the most dense and compacted continued metamorphism leads to larger and larger crystals like biotite which as they grow begin to give the rock a satiny luster sometimes other minerals will form as large crystals surrounded by a sea of microcrystal and biotite those larger crystals are called porer blasts they form by the movement of atoms within a solid no melt just migration this process allows the crystals to grow quite large and with perfect edges pyrites and garnets are common porer blasts in metamorphic rocks this saty phase of foliated rock with or without porer blast is called fidic texture and the rock is called a filite as metamorphic intensity or grade increases the mic crystals become visible to the naked eye they can grow so large they give the rock a scaly appearance we call this texture schistosity and the rock is called a schist as metamorphic grade continues to increase the dark and light colored minerals begin to separate into bands known as NYSC texture and the rock is a nice nice form form through high grades of metamorphism of mudstone or granite in settings where pressure increases with temperature such as deep burial converging continents or subduction zones increasing temperatures at this point can cause the light colored bands to melt so that half or a fraction of the rock is molten but not all of it this situation leads to folding of the bands and a rock displaying contorted folding alternating colored bands when the molten bit solidify they create small layers of granite within the metamorphic rock we call this rock am migmatite any further increase of temperature will cause full Rock melting and lead to ous rocks now what about these rocks which can show some weak foliation but not the same as shown in the sequence of increasing grades of pressure and mudstone let's start with this familiar green rock made enely of the mineral Serpentine the rock is called serpentinite and it forms when Olivine is hydrothermally metamorphosed a typical geologic setting for this California State rock is deep under a mid ocean ridge where seawater penetrates cracks is heated up by magmas and then interacts with mantle Rock rich in Olivine where that rock is closest to the surface because of the thin crust after serpentinite forms in such a setting it takes advantage of any and all opportunities to rise upward along cracks in the Rocks because its density is so much lower than the surrounding rocks as such we typically find serpentinite migrating up subduction zones and accreting to the edge of the continent as terrains accrete we also see it migrating up transform faults and Fracture zones on the bottom of the seafloor creating serpentinite ridges these other two rocks represent the metamorphism of Basalt when Basalt is subjected to lowgrade metamorphism in a burial setting converging continents or subductions Zone small crystals of chlorite epidote and or actinolite begin to form this produces A fine grain dense green rock called greenstone usually it still retains much of the shape and texture of the original bassal if this rock under goes increasing pressure in temperature in a converging continence or deep burial setting the crystals will grow larger and eventually actinolite crystals will dominate The Rock we then call it a green shist if the green stone is in a sub uction Zone environment on the other hand where pressures quickly rise while temperatures lag behind and where water contents are high this rock blue shist will form followed by this rock eite at the highest grade of subduction blue shist is a fine grain bluish colored shist dominated by Blue amphiboles and other silicates IDE is distinguished by its green ground mass produced by small green piine crystals a different variety than the one we saw in the minerals lab and Scattered red garnets it also can contain some cenite crystals which only form at the highest pressures in a subduction zone environment so looking at these rocks again all together we see that with metamorphic rocks in particular the identification and naming process leads directly to the parent rock and metamorphic formation setting once you name a metamorphic rock you can learn quite a lot about its formation history for example which of these rocks has Basalt as a parent rock if Basalt is placed in a contact metamorphic setting it turns to hornfels if it's placed in a subduction zone it turns to greenstone then blue shist then eite if it's buried deeply it becomes greenstone then green shist then a rock we didn't include here called anilite and a mudstone it turns to hornfels in a high temperature low pressure environment and a slate filite shist nice or migmatite in increasing Gra in a high press environment serpentinite forms through hydrothermal metamorphism of partite at mid ocean ridges and contact metamorphism of a mixture of minerals like a greay wacky Sandstone leads to a scarn and remember what happens to a limestone or chirt Limestone turns to Marble and chirt turns to quartzite in any and all metamorphic settings in this review of some basic metamorphic rock types we have greatly simplified the very complicated field of metamorphic patrology our goal was to Simply give you a starter guide for identifying the most basic and common metamorphic rocks which you're likely to encounter as you travel the world's outcrops [Music]