Plate Tectonics/Continental Drift Scientific Method: Collect Data Form a hypothesis Develop a method to test your hypothesis Test hypothesis and collect data Evaluate data and revise hypothesis based on results Repeat with new hypothesis if necessary Share results through publications and presentations:: Peer Review Theory: a hypothesis that has been tested many times and has an established body of evidence for support Can be disproven but never proven Law: idea considered absolutely correct. If not true, the Universe as we know it would not exist Alfred Wegener: proposed Continental Drift Question: Is earth?s surface stationary? Idea was that there was a super continent called ?Pangaea? and over time, continents drifted apart over 800 billion years OBSERVATIONS: evidence to support theory Obvious fit of continents Paleoclimate data: glacial striations If all continents were connected, one large ice cap explains record Striations-direction till deposits-perimeter Similar fossils separated by oceans Rock type and PLATE TECTONICS:: Rejected the idea of continental drift and realized there was plate tectonics going on Composition change between crust and mantle doesn?t allow for movement, depends on behavioral layers Asthenosphere behaves in a ductile manner (a solid that can flow, hot, soft layer); tectonics plates can slide over this substance and lithosphere (relatively rigid, acts like rock) Boundary between lithosphere and astheonosphere is temperature (1280 degrees C) Lithosphere floats on astheonophere Continental crust: made of granite; high in silica; low density Oceanic crust; made of basalt/gabbro; moderate silica level; moderate density Mantle; make or periodotite; low silica; high density Core; made on iron; no silica; highest density Active margins ? Passive margins Active: the edge of the continent is the same as the edge of a plate Passive: nearest plate boundary is far from the continent edge WHERE OCEANIC AND CONTINENTAL CRUST MEET NJ and Morocco split 280 mill years ago Metamorphic rock formed by heat and pressure; formed by being covered under shallow ocean Plate Boundaries: Divergent: place where two plates are moving apart Place where new crust is formed (mid-ocean ridges); most of crust is made out of igneous rock (melted and cooled) As rock is pushed away from mid-ocean ridge, it is becoming thinner and denser ? colder temperature Ocean crust is made up of maphic igneous rock Rifting: the development of a divergent boundary (pulling apart of a continent); not yet creating ocean crust Drifting: when you?re creating oceanic crust; as you keep pulling apart, form igneous rock and form ocean crust in between a continent Age of the Ocean Floor:: Oldest ocean crust is closer to continents and far from new mid-oceanic crust Started being made about 180 mill years ago; older up north Continental crust is about 4 bill years old Pangaea began to split north to south, not all at the same time Convergent: plates come together Crust is destroyed Plates move towards each other (oceanic/oceanic plates, continental/oceanic plates. continental/continental plates.) Subduction: part of the crust goes down; the older oceanic crust (maphic, denser) ? continental crust NEVER subducts OCEANIC/OCEANIC That is why continental crust is so much older, doesn?t get destroyed ever The older oceanic plate is denser because it is cooler OCEANIC/CONTINENTAL Oceanic subducts CONTINENTAL/CONTINENTAL ? creates mountains Transform: places where plates are sliding past each other; not together or apart; no crust created or destroyed Very visible on Earth?s surface (displacement of steams, roads etc.) IMPROTANCE OF PLACE TECTONICS: Regulates CO2 on geologic timescales Released into atmosphere by volcanoes Determines where we get volcanoes and how much action they have Volcanoes formed at subduction zones, divergent boundaries Creates Mountains; CO2 removed from atmosphere by weathering mountains away Determines topography (relief of landscape) Determines locations of many hazards (earthquakes, volcanoes, udslies0 Responsible for creation of continents DIFFERENTIATION OF CRUST:: Continental crust (35-40 km think; silicic ? low density) Oceanic crust (7-10 km think; mafic-higher density First crust of mafic/oceanic ? how to get silicic rx? Rocks made of minerals Different minieral behave differently Silicic minerals melt at lower temperatures DEEP TIME:: HOW OLD IS OLD? The earth is 4.6 bill years old The universe is 14 bill years old Catastrophism: Led by Georges Curvier Proposed that Earth had been created in a series of rapid, catastrophic events Sedimentary rocks an fossils were deposited during these rare times of upheaval Thought earth was created Oct 4004 BC (6000 years ago) Uniformitarianism: processes we see today operated in the past 1785: James Hutton ?The present is the key to the past? Same process, different times? Same things that occur today happened over the course of the past Mud cracks now?and then Processes act slowly earth must be older Today, we believe in both; there were catastrophes and also slow processes that happened over long periods of time 3.9 bill first unicellular life 800 mill years ago first multicellular life 500 mill years ago diversity of life 200-100 mill years ago dinosaur age 60 mill years ago mammals DATING GEOLOGIC EVENTS Absolute dating Exact age of rock body Uses physics of radioactive decay Relative dating How old rock are in relation to one another Determine sequence of events First method used to date rocks (and now used if there is no radioactive decay) Absolute time and Radiometric dating Radioactive elements decay at a constant rate, measured in lab and specified in years We measure naturally occurring radiogenic isotopes to figure out numerical values (ages) Parent to daughter matter THEORY: Radioactive (unstable) isotopes decay to non radioactive (stable) isotopes The decay rate is constant and known If we know the decay rate and the amount of parent and daughter remaining, we can calculate when this reaction started Half life ? time it takes for ½ of the parent isotopes to decays into daughter isotope Radiometric clock starts when crystals cool enough for both parent and daughter isotopes to be locked into the crystal lattice (blocking temperature) Provides age of mineral Igneous rocks ? date when lava or magma cooled (if rock cooled quickly) Metamorphic rocks ? date when rock cooled from high metamorphic temps OTHER MARKERS: Count tree rings to provide a chronometer for geologic time Rings also preserve information on climate and environmental changes through time Lake varves River deposits Chemically precipitated sedimentary rocks Shells Glacial ice RELATIVE DATING:: Nicolas steno (1669) Said fossils were remains of ancient life Suggested first laws for relative dating of fossils Principle of Superposition: states that in undeformed sedimentary rocks, the youngest layer is on top Principle of original horizontality: layers of sediment, when originally deposited, are fairly horizontal Principles of Original Continuity: layers of sediment naturally forms a fairly continuous sheet across a basin Principle of Cross-Cutting Relations: a geologic feature that cuts across another feature is younger than what it cuts Principle of inclusions: if a rock contains fragments of another rock, the fragment are older than the rock they are in Unconformities ? gaps in the rock record Hiatus ? missing time Principle of Faunal Succession Curtain fossils are found in well defined time intervals (range) Relative ages of strata can be determined from fossil content Index fossils: Easy to identify Geographically widespread Limited to a short span of geologic time (evolves and goes extinct rapidly) Biostratigraphy ? using fossils to determine relative ages of rocks
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