Geology - Metamorphic Rock Types - Quick Guides | Geology

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Study GuideGeology–Metamorphic Rock Types1. Factors That Control MetamorphismMetamorphism happens when rocks are changed by heat, pressure, water, and stress deep insidethe Earth. Even though these conditions can strongly affect a rock’s texture and minerals, oneimportant thing usually stays the same:the overall chemical composition of the rock.1.1 Rock Chemistry: The StarƟng Point MaƩersIn most cases, a metamorphic rock keeps a chemistry very similar to its original, orparent, rock.•Aquartz sandstone, which is rich in silica, will turn into a metamorphic rock that is also richin silica.•Alimestone, which is rich in calcite (calcium carbonate), can only metamorphose into acalcium-rich metamorphic rock.Because of this, a quartz sandstonecannotchange into a calcium-rich rock. Metamorphismrearranges minerals—it does not completely change the rock’s chemistry.1.2 Temperature and Pressure: Driving Mineral ChangesTemperature and pressureare two of the most important controls on metamorphism.•Different minerals form under different combinations of heat and pressure.•Each mineral is stable only within a certain range, called itsstability field.•When temperature or pressure moves outside that range, the mineral breaks down and new,morestable minerals form.Higher temperatures also:•Cause minerals that are generallyless denseto form•Speed up chemical reactions, making metamorphism happen more efficiently

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Study Guide1.3 Water: A Powerful HelperWater plays a major role in metamorphic reactions.•Theamount of water availableaffects how quickly and how strongly metamorphism occurs.•Water helps minerals dissolve and recrystallize, making chemical reactions easier.Most metamorphic rocks form very slowly—often over10 to 20 million years or more—so time isjust as important as water.1.4 GeostaƟc (Confining) Pressure: Pressure from BurialGeostatic pressure, also calledconfining pressure, comes from the weight of overlying rock.•It pushesequally from all directionson a deeply buried rock mass.•The deeper a rock is buried, the greater the geostatic pressure it experiences.This type of pressure changes mineral structures without causing rocks to stretch or flatten in onedirection.1.5 DifferenƟal Stress: Unequal ForcesDifferential stressoccurs when pressure is applied unevenly, usually because of tectonic forces.•The rock is pushed more from one direction than another.•This causes the rock mass tostretch and become elongated.•The stronger the differential stress, the more stretched the rock becomes.Rock components such as mineral crystals, fragments, or pillow structures can be stretched so muchthat they are hard to recognize.

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Study Guide1.6 Compressive Stress: Squeezing from Both SidesCompressive stressis a special type of stress applied fromdirectly opposite directions.•Instead of stretching the rock, itcompresses and flattensit.•This type of stress is common in mountain-building regions.1.7 Shearing: Sliding Past Each OtherShearinghappens when parts of a rock mass slide past each other.•It is closely related to differential stress.•Minerals tend to align or grow along flat surfaces calledshear planes.These shear planes:

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Study Guide•Become zones of weakness•Allow mineral grains to becrushed or recrystallized•Let water enter the rock, which speeds up metamorphic reactions1.8 FoliaƟon: Layered Textures in Metamorphic RocksWhen rocks experience long-lasting compressive stress, differential stress, or shearing, mineralgrains can become aligned.This alignment formsparallel layers or bandscalledfoliation.•New metamorphic minerals often grow along these foliation planes.•The direction of foliation depends on the direction of stress.•Foliation may cut across the original sedimentary layers, sometimes completely hiding them.1.9 Types of FoliaƟon TexturesDifferent conditions produce different foliated textures:•Slaty cleavage○Rock splits easily along closely spaced, parallel foliation planes○Common in low-grade metamorphic rocks like slate•Schistose foliation○More massive texture○Coarser mineral grains grow along foliation planes○Foliation planes are more widely spaced than in slate•Gneissic texture○Found in intensely metamorphosed rocks○Extremely hightemperatures and pressures make the rock soft and plastic○Light-and dark-colored minerals separate into distinct bands

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Study Guide2. Types of MetamorphismGeologists group metamorphism intotwo main typesbased onwhereit happens andwhat causes it:regional metamorphismandcontact metamorphism.2.1 Regional Metamorphism: Changes Over Large AreasRegional metamorphism(also calleddynamothermal metamorphism) is themost commontypeof metamorphism.It happens when rocks are affected by:•Strong tectonic forces•High pressure•High temperatureThese conditions usually occur deep in the Earth duringmountain-building events, where largesections of the crust are squeezed, buried, and heated.Key features of regional metamorphism:•Affectslarge regions, not just small areas•Rocks are oftenfoliated(minerals line up in layers or bands)•Rocks are commonlydeformedby pressure•Many of these rocks are thought to beremnants of ancient mountain ranges2.2 Metamorphic Grades: HowIntense Was the Change?During regional metamorphism, different minerals form depending on thepressure and temperatureconditions. These mineral groups are calledmetamorphic facies(or grades).Each facies represents aspecific range of pressure and temperaturewhere certain minerals arestable.From low grade to high grade metamorphism:•Zeolite facies

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