Geology 101 Outline of Last Lecture Chemical Sedimentary Rocks Common Chemical Sedimentary Rocks Classification texture Turning Sediments into Rocks Diagenesis Sedimentary Environments Metamorphism Agents of Metamorphism Outline of Current Lecture Test corrections Metamorphic Textures Test Corrections o Earth is 4 6 billion years old o Correct order of earth s layers center to outer inner core outer core mantle crust o Uniformitarianism present key to past o The nebular theory propose the bodies of our solar system where formed at same time from a rotating cloud of dust and gases o Outer core liquid metallic region of earth s interior o True rock cycle any type of rock can be transformed to any type of rock at anytime o Strong evidence that the universe is expanding comes from the fact that the light emitted from distant galaxies appears to be red shifted o Heavy elements up to atomic 26 were formed from supernova o New oceanic lithosphere divergent o Red sea rift zone may eventually open into major ocean o Cooler older lithosphere sinks at mantle at subduction zones o Mt St Helen and cascade volcanoes young active volcanoes built on continental marking above sinking slab of ocean lithosphere o True oldest rock on sea floor much younger than oldest rocks on continents o Himalayan mts product of collision between India and Eurasia began about 40 mil years ago o Subduction zone down going of plate is always composed of oceanic lithosphere Density Oceanic is heavier o Every plate boundary can be recognized with presence of an earthquake belt o Apparent wandering continents drift while magnetic poles stay during geologic past o Alternating symmetrical bands of magnetic anomaly in oceanic crust are produced at mid ocean ridge explained by the iron rich minerals in magma align with existing magnetic field of earth as they crystallize Geology 101 o o o o o o o o o o o o o o o o o Most of the large plates have both oceanic and continental crust Largest plate pacific New oceanic crust forms at 5 cm per year at divergent Cascades and Andes are examples of continental volcanic arc Most important force in drive plate motion slab pull Distance smaller at Aleutian larger descending angle at Aleutian Isotope different number of neutrons Silicate minerals contain oxygen and silicon Element most abundant in earth s crust by weight oxygen Share atoms covalent True two distinct minerals may have same chemical formula Texture size shape and arrangement of mineral grains Slow cooling and crystallization Phaneritic Magma felsic viscosity will increase Major dissolved volatile constituent in both magmas and volcanic gases water Associated with deep mantle plumes Yellowstone national park High viscosity and dissolved gases tend to increase the explosive potential of magma body beneath a volcano o Kilauea and Mauna Lao basaltic shield volcanoes o Mt St Helen explosive stratovolcano o Volcanic activity at Mt St Helen explosive stratovolcano o Volcanic activity at Yellowstone park related to Intraplate hot spot volcanism o Volcanism at Cascade range related to plate subduction o A sill intrusive igneous rock body that is tabular and concordant o Formation of magma not caused by loss of volatiles o Caldera eruptions may cause the volcano to collapse producing broad depression o Magma erupts from mantle plumes in Hawaii has low silica content and low viscosity o Tectonic setting with most explosive volcanic eruptions subduction zones o Igneous rocks of felsic composition have greater silica content and lower melting temperature relative to mafic o Classification of igneous texture and composition NOT ALL QUESTIONS COVERED Metamorphic Textures o Texture Size shape and arraignment of mineral grains o Foliation Planar nearly flat arrangement of mineral grains or structural features within a rock Fundamental feature Driven primarily by differential stress Examples of foliation Parallel alignment of platy mineral Parallel alignment of flattened pebbles Compositional banding Rock cleavage where rocks can be easily split into thin tabular sheets Geology 101 Foliation formation Rotating of platy and or elongated minerals Recrystallization of mineral in preferred orientation Changing the shape of equidimensional grains into elongated shapes that re aligned o Slippage along crystal structures o Dissolving materials at high stress depositing at low stress Foliated textures Rock or slaty cleavage o Closely spaced planar surface along which rocks split Shale slate Mica flakes platy mineral concentrate along narrow parallel zones along which slates split Schistosity o Platy minerals are discernible with unaided eye due to minerals growth at high temperature and pressure o Exhibit a planar or layered structure o Rocks having this texture are referred to as schist Gneissic o During higher grades of metamorphism ion migration resulting in the segregation of minerals o Gneissic rocks exhibit a distinctive banded appearance with separated dark and light minerals o Rock gneiss Nonfoliated Lack foliation In environment where deformation is minimal Typically composed of minerals that exhibit equidimensional crystals Nonfoliated textures Porphyroblastic o Large grains surround fine grains matrix of other minerals