1 Continental Drift

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4 Evidence: Fossils

5 Matching Mountain Ranges

6 Glacier Evidence

7 “Uh-Oh!”: Problems with Continental Drift….

8 …an alternate explanation?1928: Arthur Holmes suggested magma rises toward the crust, spreads, and sinks again. (Convection current) 1930: Wegner dies, unable to adapt Holmes’ idea to his theory.

9 Mantle/Crust Structure

10 A new hope: Sea-Floor SpreadingMID-ATLANTIC RIDGE 50,000 km. Long mountain range on the ocean floor

11 Evidence for Plate TectonicsDuring WWII, the U.S. in an effort to find any advantage that would help out submarines in the war effort, scientists found that the iron in the rock at the mid-ocean ridge sometimes pointed North and sometimes pointed South Lava is a mixture of melted minerals of which Iron is a very common one. Iron is attracted to the Earth’s magnetic North Pole and prefers to line up with Earth’s magnetic field. While lava is liquid, the bits of iron will tend to line up with a magnetic orientation pointing at the North Pole. When the lava solidifies, the magnetic orientation is frozen into the rock- essentially; it will have a “north end” and a “south end.” Back in World War II, the United States, in an effort to find any advantage that would help out submarines in the war effort, collected any type of data possible about the ocean and ocean floor. When they dragged a magnetic detector along the ocean floor, they discovered that the rocks making up the ocean bottom either had a magnetic orientation pointing to the North Pole or to the South Pole. As the magnetic detectors were dragged along the ocean floor, the orientation of the rock flipped from North to South- but never any other direction. They discovered a very quirky behavior of our planet: for some still-unexplained reason, at random times in Earth’s past, the magnetic orientation of the planet reversed- several times. In other words, if you traveled back in time with a compass, sometimes it will point north and other times it will point South. The significance of these two points becomes clear when we have a map of the magnetic orientation of the entire Atlantic Ocean floor. Imagine that the North-oriented rocks (usually referred to as “normal polarity”) are colored white while the South-oriented (“reversed polarity”) rocks are colored black. What we will see is a striped pattern which runs parallel to the coasts of the Atlantic shores. In addition, the magnetic stripe pattern that emerges as we cross the Atlantic floor from the USA to the Mid-Atlantic Ridge is an exact mirror-image of the pattern from the Mid-Atlantic Ridge to Europe. This implies that the ocean floor is splitting at the Mid-Atlantic Ridge, new rock is being made at the ridge, and the new rock (along with its magnetic orientation) is split and each half is carried away from the ridge as the ocean floor spreads.

12 How could this be? Basalt is iron rich and contains magnetite. Magnetite acts like a little magnet and aligns itself with the orientation of earth’s magnetic field. When magma solidifies, the alignment is “locked in” recording Earth’s magnetic orientation at the time of cooling.

13 MAGNETIC STRIPE EVIDENCENorth Oriented Rocks are said to have NORMAL POLARITY South Oriented Rocks are said to have REVERSED POLARITY

14 The earth’s magnetic field has reversed hundreds of times

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16 POLARITY FLIPPING AGE INCREASES AWAY FROM OPENING - RIDGE

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18 Evidence for Plate Tectonics Ocean Drilling The data on the ages of seafloor sediment confirmed what the seafloor spreading hypothesis predicted. The youngest oceanic crust is at the ridge crest, and the oldest oceanic crust is at the continental margins.

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20 Which graph best represents the geologic age of the surface bedrock on the ocean bottom?

21 Sea-Floor Spreading is a Divergent Plate BoundaryCreates New Crust

22 SPLITS APART CONTINENTS TO CREATE NEW OCEANS

23 Spreading Center

24 New Ocean forming – linear Red SeaWhen it grows up, it will be like the Atlantic Ocean

25 EAST AFRICAN RIFT VALLEY – CONTINENTAL RIFT

26 In Afar, Ethiopia, a 40-mile magmatic rift that opened up 1 year ago

27 CONVERGENT BOUNDARIES – DESTROYS CRUSTIf new crust is created at divergent boundaries, is earth getting larger? CONVERGENT BOUNDARIES – DESTROYS CRUST Oceanic Crust converging with Continental Crust Oceanic crust converging with Oceanic Crust Continental Crust converging with Continental Crust

28 OCEAN CRUST - CONTINENTAL CRUST SUBDUCTION – TRENCH FORMS

29 Oceanic-Continental Convergent Boundary - SUBDUCTIONDENSER OCEANIC CRUST GOES DOWN INTO MANTLE

30 OCEAN CRUST –OCEAN CRUST CONVERGENCE - SUBDUCTION

31 Oceanic Crust – Oceanic CrustOlder, denser oceanic crust is SUBDUCTED (sinks below) VOLCANIC ISLAND ARC FORMED – ALEUTIAN ISLANDS TRENCH FORMS

32 STARTS OUT AS OCEAN-CONTINENT CONVERGENCE AND THEN AS OCEAN CRUST IS SUBDUCTED TURNS INTO CONTINENT TO CONTINENT WHY YOU FIND MARINE FOSSILS ON TOPS OF MOUNTAINS

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34 Continental Crust – Continental Crust Convergent Boundary NO SUBDUCTION - COLLISION ZONEWHY YOU FIND MARINE FOSSILS ON TOPS OF MOUNTAINS

35 Collision of India and Asia

36 TRANSFORM PLATE BOUNDARIESTransform Fault Boundaries TRANSFORM PLATE BOUNDARIES At a transform fault boundary, plates grind past each other without creating or destroying the lithosphere. SHALLOW EARTHQUAKES  Transform faults • Most join two segments of a mid-ocean ridge. • At the time of formation, they roughly parallel the direction of plate movement.

37 Boundaries: Transform

38 SAN ANDREAS TRANSFORM FAULT

39 Transform Fault Boundary AT MID-OCEAN RIDGES

40 More Evidence for Plate Tectonics Earthquake Patterns Scientists found a close link between deep-focus earthquakes and ocean trenches. The absence of deep-focus earthquakes along the oceanic ridge system was shown to be consistent with the new theory.

41 Convergent plate (subduction) boundaries have deeper earthquakes Divergent plate boundaries and Convergent Continent-Continent collision zones have shallow earthquakes

42 Convergent Indian-Australian Plate and the Pacific Plate (oceanic crust to oceanic crust) are converging forming the Tonga Trench. f07_58_pg196 Deep EQ’s f07_58_pg196.jpg f07_58_pg196

43 The greatest number of earthquakes in the cross section occurred at:Sea level Between sea level and a depth of l00 kms. 3. at a depth between 100 – 300 kms. 4. At a depth between 300 – 600 kms.

44 Which cross section has arrows that best represent the relative motion of the crustal plates along the Wadati-Benioff zone beneath the Tonga Trench?

45 SUMMARY 3 PLATE BOUNDARIESDIVERGING CONVERGING (SUBDUCTION ZONE) Oceanic Crust – Continental Crust Oceanic Crust - Oceanic Crust CONVERGING (COLLISION ZONE) Continental Crust – Continental Crust TRANSFORM

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47 So, what causes plates to move?Convection currents: drag and move the lithospheric plates above the asthenosphere (three sources of heat produce the convection currents): Leftover heat from earth’s formation Decay of Radioactive elements Plate friction

48 Convection Cells

49 Convection Currents in the mantle drag and pull the lithospheric plates above them

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51 3 types of plate boundaries caused by convection currentsCASCADES MID-ATLANTIC RIDGE TRENCH

52 ARROWS IN THE ASTHENOSPHERE SHOW PLATE MOVEMENT DUE TO CONVECTION

53 The arrows shown in the asthenosphere represent theinferred slow circulation of the plastic mantle by a process called (1) insolation (3) conduction (2) convection (4) radiation

54 Mid-ocean ridges (rifts) normally form where tectonic plates are(1) converging (3) stationary (2) diverging (4) sliding past each other The motion of the convection currents in the mantle beneath the Atlantic Ocean appears to be mainly making this ocean basin (1) deeper (3) wider (2) shallower (4) narrower

55 More Evidence for Plate Tectonics Hot Spots A hot spot is a concentration of heat in the mantle capable of producing magma, which rises to Earth’s surface; The Pacific plate moves over a hot spot, producing the Hawaiian Islands. Hot spot evidence supports that the plates move over the Earth’s surface.

56 HOT SPOTS Major regions of volcanic activity in the interior of plates away from plate boundaries. The cause of these hot spots is thought to be plumes of magma rising up from mantle producing sites of active volcanism. As the plate moves over a hot spot, a chain of volcanic mountains forms, like Hawaiian Islands

57 Hot Spot

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59 This diagram provides evidence that the Pacific TectonicPlate is moving toward the (1) south (3) southwest (2) east (4) northwest