Physical Geology 101 15 Metamorphic Rocks I p 223 233 New Rocks from Old What is the Greek origin of the word metamorphism meta means morphe means Definition metamorphism refers to all the changes that occur in the minerals chemistry and structure of solid rocks as a result of the physical and chemical conditions deep in the crust So metamorphism involves changing existing rocks to make new ones This is different to igneous and sedimentary rock processes Igneous rocks made from molten magma or lava Sedimentary rocks diagenesis is the lithification process that binds the particles of sediment together to form rock When rocks undergo metamorphism we say that they have been Does metamorphism involve any rock melting YES or NO Changes that occur during metamorphism existing minerals convert into new minerals minerals change their appearance because of the forces that are involved in metamorphism Depth of Metamorphism Metamorphic rocks form in the Earth s crust underneath where sedimentary rocks form near the surface i e where the process called occurs but above the depths where magma begins to form through the process of where present The depth range of metamorphism will thus depend on the local What is the lower temperature limit of metamorphism What is the deepest depth approximately that diagenesis occurs Pressures at this depth are more than times greater than atmospheric We have to have high enough pressures AND temperatures for metamorphism to occur by recrystallization the growth of new minerals What is the upper temperature limit of metamorphism 1 Physical Geology 101 What mineral starts to melt at this temperature if present in the rock The three grades of metamorphism are only occurs at shallower depths than Do low grade metamorphic rocks resemble the original rock YES or NO Do high grade metamorphic rocks resemble the original rock YES or NO Controlling Factors The type of metamorphic rock that forms is very dependent on what the original rock is but the exact changes that occur to the original rock will vary depending on what the metamorphic conditions are The controlling factors are 1 2 3 4 Pressure Pressure refers to the effect of the increasing weight of the overlying rocks as you go deeper and deeper into the crust This pressure is referred to as the This pressure is the same from all directions just like water pressure deep in the ocean If pressure is the same from all directions do objects decrease in volume because of the pressure YES or NO do objects change their shape because of the pressure YES or NO Under lithostatic pressure rocks decrease in volume the mineral grains get more and more tightly packed and they start to forming a metamorphic rock If forces are stronger in one direction than other directions unlike pressure we use the term to talk about the forces acting on the rock This may happen because of the forces of the tectonic plates pushing against each other In particular if there is a greater push from one direction compared to another direction results in a type of stress called 2 Physical Geology 101 Because of these unequal stresses rocks get squished in one direction more than others This causes platy minerals e g which grow in sheets to align with each other perpendicular to the direction of the largest stress This alignment of minerals in metamorphic rocks is called Because it is a physical characteristic of the rock we refer to this foliation as a type of metamorphic In low grade metamorphic rocks the grain sizes are usually very small and the foliation may be difficult to see A tell tale clue is that the rock breaks apart easily along the foliation just like with cleavage in minerals forming flat plate like fragments This characteristic is called In intermediate grade metamorphic rocks recrystallized mica and chlorite grain sizes are bigger and the foliation is easier to see and is often wavy in appearance This type of foliation is called In high grade metamorphic rocks recrystallized minerals segregate into light and dark bands forming a This type of foliation is also called Temperature Heat is a crucial part of metamorphism because chemical reactions occur faster as temperatures INCREASE or DECREASE What are the two sources of heat for metamorphism 1 or 2 Eventually the temperatures will get high enough that a certain rock type can no longer remain solid at the existing pressures and partial melting begins If partial melting doesn t melt all of the rock we can end up with rocks that are part metamorphic and part igneous usually granite These type of rocks are called Fluids What are 2 examples of holes or openings in rock that can contain fluids or volatiles 1 gaps between grains in sedimentary rocks 2 What types of minerals contain water in their chemical makeup e g clay 3 Physical Geology 101 What types of minerals contain OH in their chemical makeup e g mica and amphibole can be used to make H2O in chemical reactions What are 2 examples of fluids or volatiles that are used in metamorphic reactions and Fluids are important for metamorphic reactions because they allow existing minerals to dissolve or they allow new minerals to crystallize from the chemical ingredients present in the fluid Do fluids speed up or slow down chemical reactions during metamorphism SPEED UP or SLOW DOWN Time How long does it usually take for one silicate mineral to fully convert into a different silicate mineral even when the temperatures and pressures are very high Types of Metamorphic Rocks We can broadly divide metamorphic rocks up into two categories Those that have a foliation and those that don t In other words and Foliated In order from the lowest to the highest metamorphic grade forms when shale undergoes low grade metamorphism Develops a foliation called a What products are made from slate slightly higher grade metamorphism of shale and forms an obvious foliation intermediate grade metamorphism of shale with big mica crystals that form a pronounced foliation called a form from high grade metamorphism of clay rich sedimentary rocks and granite The metamorphic minerals segregate into light and dark bands producing a texture 4 Physical Geology 101 forms when undergoes low grade metamorphism The resultant rock is similar to a schist that develops from a shale but in the case of basalt contains mostly green chlorite minerals intermediate grade metamorphism of basalt Contains amphibole high grade metamorphism of basalt Contains pyroxene Non Foliated