1Earth Materials: Metamorphic Rocks• What is metamorphism• Agents of metamorphism• Types of metamorphism1• Foliation• Classification of metamorphic rocks• Metamorphic grade• The rock cycleMetamorphismMetamorphic rocks are formed from igneous rocks, sedimentary rocks, or other metamorphic rocks through by 2y, p gychanges in pressure, changes in temperature, and/or the action of fluids.Causes/Agents of MetamorphismPressureconfining pressure (lithostatic pressure)-equal pressure in all directions- increased confining pressure can cause minerals with more compact (denser) structures to formdirected pressure- pressure which is greater in a specific direction- may cause folding or cause mineral to grow with a specific orientation3Temperature- different minerals are stable at different temperatures- changing the temperature can cause the types of minerals present in a rock to changeHydrothermal fluids (mostly water and/or carbon dioxide plus dissolved material)- can change the overall chemical composition of a rock by adding or removing material- many important ore deposits have been formed by this metamorphic processMajor Types of MetamorphismRegional metamorphism- the most wide-spread- occurs over very large areas- changes in pressure and temperature are the most important factors - commonly results from:- mountain building - with directed pressure4- deep burial - lithostatic pressure onlyContact metamorphism- occurs near the edges of igneous intrusions- the intensity of metamorphism is greatest close to the intrusion and less farther away- increased temperatures and hydrothermal fluids are the most important factors- the zone of contact metamorphism is called a contact aureoleTwo of the major types of metamorphismRegional MetamorphismExample with directed pressure(foliated rocks)Contact Metamorphism(non-foliated rocks)5Fig 3.5 Understanding EarthMetamorphic texturesFoliated metamorphic rocks- foliation refers to a preferred orientation of mineral grains found in some metamorphic rocks- metamorphic rocks which have a preferred orientation f i l i ll d f li t d t hi k6of mineral grains are called foliated metamorphic rocks- the preferred orientation is the result of directed pressureNon-foliatedmetamorphic rocks- lack a preferred orientation of mineral grains27Foliation in metamorphic rocksNote how some minerals are oriented perpendicular to the direction of maximum pressureFig 8.5 Understanding EarthCommon types of foliated metamorphic rocksslate• has slaty cleavage• dull• crystals too small to seephyllite• similar to slate but is shiny8• crystals too small to seeschist• has visible crystals• often composed primarily of micasgneiss• has visible crystals• has bands of different colorsSlateFig 8.8 Understanding Earth9Geology - ChernicoffSchist composed of micas(mica schist)Schist with micas and large garnet crystals10Fig 8.10 Understanding EarthS. Kuehn 2005Geology - ChernicoffGneissNote the lighter and darker colored bands11Fig 8.8 Understanding EarthOther metamorphic rocks(composition of major importance for classification)Typically non-foliated or weakly foliated:quartzite - composed primarily of quartzmarblecomposed mostly of calcite and/or dolomite12marble-composed mostly of calcite and/or dolomiteTypically non-foliated:hornfels - variable compositionskarn- contains mostly calcite and/or dolomite with garnet3Quartzite, a type of non-foliated metamorphic rockThe parent rock is sandstoneFig 8.9 Understanding Earth13Marble, a type of non-foliated metamorphic rockThe parent rock is limestoneFig 8.9 Understanding EarthOther metamorphic rocksmylonite- formed in the deepest parts of fault zones- a type of gneiss (so always foliated)14migmatite- formed when a rock begins to partially melt- represents the transition between metamorphic and igneousrock- usually foliated15Mylonite Produced by shearing in the deepest parts of fault zones. Arrows indicate the shear direction in this sample Understanding Earth16MigmatiteThe lighter-colored part of this rock represents a granite-like liquid produced by partial melting Geology - ChernicoffMetamorphic grade- refers to how much a rock has been changed by metamorphism17- greater pressures and greater temperaturescorrespond to highergrades of metamorphismFig 8.2 Understanding EarthDifferent degrees of metamorphismLow grade(relatively little change)High grade (much change)Original rockModified from: Fig 8.7 Understanding Earth4In contact metamorphism, the intensity (grade) decreases with distance from the source of heat (intrusion).At different 19temperatures, different minerals form in the metamorphic auriole around the intrusion.A geologist studying the metamorphism could use the minerals like a thermometer.Fig 8.15 Understanding EarthRock Cycle exercise20The rock cycleFig 3.10 Understanding Earth21Rocks, Minerals, and the Rock CycleRock cycle; Formation of igneous, sedimentary, and metamorphic rockshttp://serc.carleton.edu/NAGTWorkshops/visualization/collections/rock_cycle.htmlRock cycle http://www.rocksandminerals.com/rockcycle.htmRocks, minerals, rock cycle, etc. http://www earth2class org/er/teachers/lessonplans/index php22http://www.earth2class.org/er/teachers/lessonplans/index.phpRocks & minerals at Volcano Worldhttp://volcano.oregonstate.edu/education/vwlessons/lessons/lesson4.htmlRock identification (ppt)http://www.gc.maricopa.edu/earthsci/imagearchive/rock_identification.htmResources for