Geo 100 notes ch 3-4 continued
Geo 100 notes ch 3-4 continued GEO 100
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This 5 page Class Notes was uploaded by samantha Flavell on Friday February 12, 2016. The Class Notes belongs to GEO 100 at State University of New York at Oswego taught by Rachel Lee (P) in Fall 2015. Since its upload, it has received 21 views. For similar materials see Physical Geology in Geology at State University of New York at Oswego.
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Date Created: 02/12/16
Mapping the Sea Floor *Bathymetry: Shape of the sea floor surface *Used to drop weight to measure, now we can use sound waves. Using the time it takes for the sound wave to deflect off the floor to determine the distance. SeaFloor Sediments *Sediment thickness is almost zero at the ridge. *Sediment age increases as you move away from the ridge. Heat Flow *Consistency in heat flow patter (hottest at ridge) something is happening here. Earthquake *Don’t occur randomly Occur at trenches SeaFloor Spreading *New melt comes up at ridge axis *Solidify to form new crust *New crust cracks and moves away from ridge axis *This hypothesis makes a number of testable predictions Evidence of SeaFloor Spreading *Magnetic Anomaly: difference between expected strength of Earth’s magnetic field and the actual measured strength of the magnetic field in a location. *Positive and negative anomalies *Positive=normal polarity *Negative=reversed polarity *The pattern of magnetic anomalies on the sea floor can be used to determine the rate of sea floor spreading. *Regions of the sea floor with positive magnetic anomalies were formed during times when Earth’s magnetic field had normal polarity. Wandering North Pole *Earth’s magnetic pole isn’t fixed in space and time *Coincides with geographic pole when averaged over time Magnetic Declination *Angle between longitude and direction compass needle points. Magnetic Field: Declination and Inclination *Rock magnetism has two components: direction of magnetic ‘pointing’ and inclination of this with the Earth’s surface. Magnetic inclination goes from nearly horizontal at equator to vertical at the magnetic pole. *Thus, magnetic records give an indication of where the rock was on the surface when it was magnetized. Paleomagnetism *We can measure declination and inclination magnetism in rocks. *Different declination and inclination indicate that the rock didn’t form in the present setting. Polar Wandering Paths *Earth’s north magnetic pole has moved through time *Calculated by triangulation using lavas of similar age from the same continent **Multiple magnetic poles aren’t possible *Polar wandering paths vary by continent; coincide only if continents move with respect to one another. Key Features of Plate Tectonics 1) Earth’s crust is constantly being created and destroyed (recycled) 2) Ocean crust, formed at divergent margins, is mafic and dense 3) As ocean crust ages and cools, its great density relative to the continents results in SUBDUCTION as plates CONVERGE.( As a results old ocean crust don’t persist, whereas old parts of the buoyant continents can survive for eons.) 4) The other kin of plate margins, transform are parallel to the current motion of the plates. *Plates= Crust and lithospheric mantle* *Active margins= plate boundaries (where the continent boundary coincides with plate boundary) *Passive margins= ARE NOT plate boundaries (Continent boundary does NOT coincide with the plate boundary) Testing Plate Tectonics *This model is consistent with a large number of detailed tests, including: 1) Sea floor spreading 2) Paleomagnetic ‘paths’ 3) Locations and focal depths of earthquakes 4) Seismic tomography 5) Age structure of Continents 6) Hot Spot Tracks 3 Types of Plate Boundaries 1) Divergent: Plates move away from each other 2) Transform: Plates move sideways past each other 3) Convergent: Plates move toward each other Plate Tectonics: Divergent Boundaries *Shallow Earthquakes *Mainly Basalt Volcanism *Normal faulting—thinning crust Oceanic: *Ridges volcanically and seismically active but hidden by oceans Continental: *Rift valleys (Great Basin of Western US, East Africa) *Closedbasin lakes (evaporates) Rapid clastic sedimentation Divergent Plate Margin *Slow ~1cm/yr *Fast ~10cm/yr Continental Extension *The north to south of the Basin and Range Province of the Western US come from crustscale extension. With continued extension this could develop into an ocean basin. Plate Tectonics: Transform Margins *Transform exist to accommodate movement of other plate margins on the globe of the Earth. Plate motion can never be entirely convergent or divergent: Somewhere there needs to be places where plates slide past one another. *Hence, the orientations of Transforms will parallel the direction of motion of the plates involved. *Most common in ocean crust *Abundant intense but shallow earthquakes *Essentially no volcanism *Strikeslip faulting Transform Plate Margins *Transforms were the last plate tectonic margin to become well understood. These boundaries where plates slide horizontally past one another, typically are not volcanically actie. TRANSFORMS FRACTURE ZONES: *Transform plate boundaries became obvious with the development of high resolution views of the sea floor. Transform Faults *Transform faults on ocean floor connect and offset ridge segments. Active faulting as indicated by Earthquakes only occurs in the part of the transform between ridge axes. Continental Transforms *Transforms are rarely seen on land. Unfortunately, for Californians, the best natural example is the San Andreas Fault. Divergent and Transform Margin Earthquakes *The similarity between earthquakes at these kinds of margins is that all of the foci will be shallow (in the upper <60Km) *The difference between earthquakes at these kinds of margins is that transform (strikeslip) margins will have much greater forces involved and so that earthquakes will be much stronger. Plate Tectonics: Convergent Margins *Deep focus earthquakes *Seismic tomographic evidence for subduction *positions of oceanic trenches *Island arcs *Nature and age of volcanic rocks *Orgenic belts (granite, batholiths, high pressure rocks, fold/thrust belts) *Ophiolites *microplate terranes
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