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Plate Tectonics 2 Making oceans and continents

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1 Plate Tectonics 2 Making oceans and continents

2 Pangea* seen at about 225 mya Sir Francis Bacon 1620 Benjamin Franklin 1782 The crust of the earth must be a shell floating on a fluid interior. Thus the surface of the globe would be broken … by … movements of the fluids…. Wegener 1912: evidence * Breakup begins about 200 mya, floods about 190 mya Collision of Laurasia and Gondwana

3 Continental drift hypothesis Continental drift hypothesis Continents "drifted" to present positions Continents "drifted" to present positions Evidence used in support of continental drift hypothesis Evidence used in support of continental drift hypothesis Fit of continents Fit of continents Fossil evidence Fossil evidence Rock type and mountain belts Rock type and mountain belts Paleoclimatic evidence Paleoclimatic evidence Alfred Wegener 1912

4 Evidence: Precise Matching of Continental Shelves of Circum-Atlantic Continents

5 Ranges of Triassic Reptiles

6 6 Similar Rocks on opposite shores Example, NJ and Morocco

7 Why wasn’t it popular? Why wasn’t it popular? Objections to drift hypothesis Objections to drift hypothesis Inability to provide a mechanism capable of moving continents across globe Inability to provide a mechanism capable of moving continents across globe Wegener suggested that continents broke through the ocean crust, much like ice breakers cut through ice Wegener suggested that continents broke through the ocean crust, much like ice breakers cut through ice

8 Attitudes change During the 1940s and 1950s technological strides permitted extensive mapping of the ocean floor Seafloor spreading hypothesis was proposed by Harry Hess in the early 1960s

9 Seafloor spreading in detail Harry Hess: Convection currents in mantle Seafloor spreading occurs along relatively narrow zones, called rift zones, located at the crests of ocean ridges As plates pulled apart, low pressure causes mantle mellting,magma moves into fractures and makes new oceanic lithosphere

10 Hess’s Seafloor spreading (cont) New lithosphere moves from the ridge crest in a conveyor- belt fashion Newly created crust at the ridge is elevated because it is hot and therefore occupies more volume Area also seems to be pushed up by mantle upwelling

11 Magnetic North and South exchange places at irregular intervals average = 100K years BUT Very high variance Dates when polarity of Earth’s magnetism changed were determined from radiometric dating of lava. Vine and Matthew’s Test

12 Geomagnetic reversals are recorded in the ocean crust Spreading predicts matching bands of lava polarity on either side of ridge. Vine and Matthews looked for symmetric magnetic stripes in the ocean crust Tow magnetometers record N or S NOT FALSE Vine and Matthew’s Test

13 Ideas:Earth's Convection Cells 1963 This is the part we have been discussing. But the hottest area is the core.

14 Ideas: Earth's Convection Cells Jason Morgan (1971) In this model, Plumes and convection cells are aspects of the same thing

15 Ideas: Earth's Convection Cells Combination Hawaii MORAndesitic Basaltic A combined model.

16 Hot Spots and Hawaii Hot Spots are caused by heat from deep in the mantle. As plates move over them, new volcanic Seamounts are formed. Any that stick up above the ocean’s surface as islands are eroded away, and as they move away from the Hot Spot, they cool, contract, and are submerged. They are then called Guyots Their lavas are datable

17 Hot Spot and Hawaii-Emperor Chain Speeds vary from 1 to 10 centimeters per year Hey look, the direction changed! Initial Flood Basalt was partly subducted Speed = distance/time

18 Hot Spots & Plate Motions Average 5 centimeters/year

19 LAGEOS and GPS satellites determine that plates move 1-10 cm per year, avg 5 Global Positioning Satellites

20 Active Rifting of A Continental Plate Note 3-D Triple Junction Discussion: Eggshells

21 Active Rifting of A Continental Plate Inactive Branch: Aulocogen;Subsided Passive Margins

22 East African Rift Zone Active: Red Sea and Gulf of Aden. Failed Arm: Great Rift Valley (aulocogen) Zagros Mts

23 Mid-Ocean Ridge System Motion Fracture Zones and Transform Faults Shallow weak earthquakes

24 Subduction-Zone Features Note sequence from land to trench If a continent converges from the left, what rocks will fold in the collision? Himalayas Note ocean plate rocks that don’t get subducted in a collision

25 Abyssal plains Can be sites of thick accumulations of sediment Sediments thickest away from MOR Abyssal plains found in all oceans Studded by old cold seamounts and MORs Trench Accretionary wedge Fore-Arc Basin Volcanic Arc Back-Arc Basin Continent

26 Mélange from California Coast Source: Betty Crowell/Faraway Places Sea-floor and land-derived sediments, + some volcanics. When stuffed down trench into Low Temperature- High Pressure zone, with plenty of ion-rich water, result is Blueschist Metamorphic Facies

27 More Terms Canadian Shield, North America’s crystalline core exposed by glaciers Continental Platform Terms: Shield + Platform = Craton, Coastal Plain

28 Exotic ( Displaced) Terrains Continental Crust is thick and silica- rich, so low density. Buoyant and hard to subduct. Also Erosion resistant. Collisions with Volcanic Island Arcs and microcontinents Made of volcanic island arcs, backarc basins and microcontinents Moved by transform faults, then accreted Sutures Anecdote Western California

29 Paleogeography Reconstructions Orientation of magnetic minerals gives latitude (north or south of equator) Radiometric dates of ocean floor basalts, plus distance from ridge, gives paleolatitude for last 200 million years Explains continental movements

30 Origin of Pangaea

31

32 100 mya Exotic terrane hits western N. Am. 50 mya Opening of the South Atlantic 40 mya Separation of Australia and Antarctica 35 mya India hits Asia forming Himalayas 3.5 mya Formation Isthmus of Panama isolates N. Atlantic Last 150 Million Years

33 Mapping the ocean floor Three major topographic units of the ocean floor Continental margins (few 100’s of meters below surface) Deep-ocean basins ……(Abyssal Plain 4 km below surface) Mid-ocean ridges ……( top is 1 km below surface) Trenches (may be 11 km below surface)

34 Ocean surface depths Trenches 11 km deep. Abyssal Plain 4 km deep, MOR’s average 3 km high so 1 km deep

35 Bathymetry of the Atlantic Ocean Abyssal Plain A few hundreds 4 km 1 km average Atlantic – Passive Margins – Note no trench – no subduction

36 A passive continental margin Found along coastal areas that surround large oceans. Coast is far from central MOR Not near active plate boundaries Little volcanism and few earthquakes Example: East Coast of US (turbidites, graded bedding) (Microfossil Ooze) Very thick sediments sand muds Carbonate and silica ooze High-angle Normal Faults Of Divergent Margin (Initial Rifting)_

37 An active continental margin Continental slope descends abruptly into trench Example: Pacific Ocean margin. Note Accretionary Wedge at trench

38 Seafloor sediment Abyssal Plain is mantled with sediment Sources Turbidity currents on continent margins Sediment that slowly settles to the bottom from above – fine mud and plankton Thickness varies Thickest in trenches – accumulations may exceed 9 kilometers. Thinnest at MOR – new ocean floor

39 Foraminifera (a.k.a. Forams) Form deepwater carbonate oozes, depths less than 4 km

40 Chert sample Diatoms (siliceous Ooze SiO 2 ) Only Silica below carbonate line So Depth of pure Chert >4 km Small Phytoplankton (tiny floating plants)

41 The structure of oceanic crust Black Smoker on cracks near magma MORs characterized by an elevated ridge Closely spaced normal faulting Mantle flow below pulls the crust apart Newly formed basalt ocean floor fills in cracks

42 Ophiolite Suite 2 km 3-6km Some Serpentine due to hot water circulation

43 Hydrothermal Metamorphism

44 Black Smokers Circulation of hot water in cracks at mid-ocean ridge dissolves metals which are reprecipitated as sulphide ores

45 Outcrop of pillow lava End Plate Tectonics 2


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