PYTS/ASTR 206 – Earth1z AnnouncementsLt HW3 dLate HW3s due now 50% late credit Thursday office hours? No one was concerned We’ll drop this slot – email me for a time to meet instead Drop day is todaypy y Calculate your expected grade… Add HW1 and HW2 percentages together, multiply by 0.15 Add the two in-class activity marks (out of 5) together, multiply by 2 Take your mid-term score (out of 45), multiply by 1.111 Add these three things together…. And compare to grade table This is a REALLY rough guess, you can easily move up/down a gradePYTS/ASTR 206 – Earth2EarthPTYS/ASTR 206 – The Golden Age of Planetary ExplorationShane Byrne – [email protected]/ASTR 206 – Earth3In this lecture…In this lecture…It d ti t th E thzIntroduction to the Earth Recap on Interiorz Two types of crustOceanic crustOceanic crust Continental crustz Plate Tectonics Current plates Evidence for plate tectonicsz Plate motion Spreading centers and subductionHistory of plate motionsHistory of plate motionsz Early Earth Building continents Early atmosphere Forming the oceansz Life When it formedRise of OxygenRise of OxygenPYTS/ASTR 206 – Earth4z What’s unique about the Earth? Impact craters? – Nop Volcanoes? – NoPYTS/ASTR 206 – Earth5z What’s unique about the Earth? Fluvial erosion? – No Rivers? – NoPYTS/ASTR 206 – Earth6z What’s unique about the Earth? Glaciers? – No Tectonics? – NoPYTS/ASTR 206 – Earth7z What’s unique about the Earth? Wind action? – No So what is unique about the Earth?PYTS/ASTR 206 – Earth8z What’s unique about the Earth?z Two main things to talk about in this lecture…Plate Tectonics LifePYTS/ASTR 206 – Earth9Recap interior…Recap interior…z Rocky planets have several parts Core – Iron/NickelMantleRockyMantle –Rocky Crust – Rocky (different composition)z Strong rocks near surface Colder rocks = stronger rocks Lithosphere Rocks are brittlez Weak rocks deeper Hotter rocks = weaker rocks Asthenospherep Rocks flowz CoreSolid in centerinner coreSolid in center –inner core Surrounded by liquid iron – outer corePYTS/ASTR 206 – Earth10z Compositional vs mechanical termsCt tlCrust, mantle, core are compositionally different Lithosphere, Asthenosphere, Mesosphere, Outer Core and Inner p,Core are mechanically different Lithosphere is divided into plates…PYTS/ASTR 206 – Earth11z Hotter interior temperatures cause convection in the mantleconvection in the mantleShijie Zhong, U. ColoradoPYTS/ASTR 206 – Earth12z Convection causes meltingMlti itd dMelting point depends on pressureMaterial rises to lower pressureMaterial rises to lower pressure zone Previously solid, but can now y,partly meltzPartly molten zonePartly molten zone Called ‘asthenosphere’ Able to flowPYTS/ASTR 206 – Earth13z LithosphereB ittl lBrittle upper layer Can move around on plastic asthenosphereDivided into many platesDivided into many plates 1000-10000 km across Most plates contain both continental and oceanic crustPYTS/ASTR 206 – Earth14Two types of crustTwo types of crustz Crust is a compositional distinctionz Most rock is in the mantle Mixture of two main minerals Olivine and Pyroxenez Earth has two types of crust Oceanic crust – low-lying Continental crust – high-standing Oceans cover oceanic crust and some of the continental crustz How do we manufacture this crust? Through volcanic activityPYTS/ASTR 206 – Earth15z Oceans cover oceanic crust and some of the continental crustNot always the caseNot always the case More continental crust exposed during the last ice age when sea levels droppedContinental crustOceaniccrustPYTS/ASTR 206 – Earth16z Oceanic and continental crust comparedOceanic ContinentalDensity 3000 kg m-32700 kg m-3Thi k5k20100kOceanOceanic cr stContinen-tal Thickness5km20-100kmComposition Basalt GraniteAge <0.1 billion years > 1 billion yearsMantleOceanic crustcrustPYTS/ASTR 206 – Earth17zThese large plates are constantly movingPlate TectonicsPlate TectonicszThese large plates are constantly moving Most plate move at centimeters/year – like growing fingernails… Measured with GPS and Very Long Baseline InterferometeryPYTS/ASTR 206 – Earth18z Plates move around from…Cti tConvection currents Pull of sinking slabs Still not totally understoodShijie Zhong, U. ColoradoPYTS/ASTR 206 – Earth19z Old evidence for plate tectonics… Similar fossils in locations that are widely separated today Continental shapes fit together like a jigsaw Matching rock types Plate tectonics was already a theory before modern instruments confirmed itPYTS/ASTR 206 – Earth20z More-recent evidence for plate tectonicstectonics… Seafloor surveys revealed mid-ocean spreading ridgesocean spreading ridges Magnetic reversals across ridges Locations of earthquakes and qvolcanoes Pacific ring of firePYTS/ASTR 206 – Earth21z When plates separate…NitNew oceanic crust Partial melting of mantle materialz Characteristic stratigraphic sequence: Gabbro (large grained basalt)Sheeted dikesSheeted dikes Each sheet was the wall of the inner ridge Pillow basalts Blobs of basalt that are quickly quenched Ocean sediments Fine-grained mudsPillow BasaltPYTS/ASTR 206 – Earth22z Magnetic reversals match across spreading ridgesridges Earth’s magnetic field switches north and south pole every so oftenNew rock gets magnetized before it coolsNew rock gets magnetized before it cools Magnetic record is preserved Same pattern on each side of the spreading ridgePress & Siever, 2ndeditionPYTS/ASTR 206 – Earth23z Crust moves away from spreading centersCrust near the spreading centers is still youngCrust near the spreading centers is still youngPYTS/ASTR 206 – Earth24z What happens when plates collide?AllddditOceanContinen-tal All depends on density Oceanic crust has a higher density than the continentsOceanOceanic crustcrustMantlez Three possible collisions Ocean-continent South America and the Nazca plate Ocean-ocean West pacific region Continent-continent India collides with EurasiaPress & Siever, 2ndeditionPYTS/ASTR 206 – Earth25zA continent-ocean plate collisionzA continent-ocean plate collisionz West of South-Americaz Ocean
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