Title: Theory of Plate Tectonics Brown 24-25

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Theory of Plate Tectonics What are the 5 Pieces of Evidence for existence and motion of plates:

1. Shape of the Ocean Floor (1940s-50s) 2. Locations of Volcanoes and Earthquakes (1950s) 3. Age of the Ocean Floor (1950s-80s) 4. Hotspots (1960s) 5. Paleomagnetism (1960s) 1. Shape of the Ocean Floor

Seafloor Mapping

Following WWII, exploration of the ocean floor was made a priority in U.S. Assisted by new technology called

so und navigation and ranging, a.k.a. sonar 1. Shape of the Ocean Floor Continued

Marie Tharp and Bruce Heezen

compiled all the sonar data and turned it into the first map of the North Atlantic in 1957 Discovery that the ocean floor was far from flat: a huge mountain range runs down middle of each ocean Contributed directly to development of the theory of plate tectonics Marie Tharp

Google Doodle of her here Mid-Atlantic Ridge, East Pacific Rise, Indian Ridge WHAT CAUSES THIS? Another mystery... Tharp and Heezens 1977 complete map of the ocean floor Sea Floor Spreading and Harry Hess

1. Harry Hess was a former Navy Admiral and Princeton geologist who studied the seafloor. Two questions had always interested him. a. If the oceans have existed for at least 4 billion years, why has so little sediment accumulated on the ocean floor? b. Why are fossils found in ocean sediments no more than 180 million years old? 2. Using Tharp and Heezens research and maps -published sea floor spreading hypothesis in 1962 3. Proposed that the mid-Atlantic Ridge was a spreading center (divergent boundary), a place where two plates move apart and it was driven by convection in mantle 4. Revived interest in Wegeners Pangea, had a mechanism for how the continents move. How does Seafloor Spreading work?

As the plates move away from each other, asthenosphere rises to surface and melts (lower pressure). Once it cools, new ocean crust is created. The continents grow apart as new crust forms between them The movement of the plates is powered by convection Formation of new ocean crust at Mid-Atlantic Ridge

Ocean crust is made of the dense igneous rock called basalt

When basaltic lava erupts underwater, this rock cools quickly to form rounded pillow basalt Pillow Basalts! Iceland is one of the few places on Earth where the Mid-Atlantic Ridge crosses land 2. Volcanoes and earthquakes are not located randomly

Sitting next to Tharp at Columbia University was a graduate student named Howard Foster Foster was hired by Heezen to plot earthquake data in the Atlantic while Tharp was compiling sonar data Tharp noticed a pattern between the mid-ocean ridges and earthquakes 3. Age of the Ocean Floor

Ocean Drilling 1968-1983 Deep Sea Drilling Project NSF (National Science Foundation) collected data about the age of the ocean floor Found supporting evidence: Whole ocean floor is much younger than continental crust Parallel to MOR pattern of youngest to oldest moving away from MOR 4. Hot Spots

Not all volcanoes are on plate boundaries. Hot Spots in mantle are pretty random, not always on boundaries. Plume of magma that rises towards the surface of the earth, usually in the center of a plate, and causes volcanism. Hot Spot Example: Hawaii Hawaii Continued

Lets look at the pattern of the age of the volcanic islands, along with the alignment of the islands when viewed on a map.

Hawaii forms a chain of volcanoes that help us track and prove plate motion

Hot spot island chains show the movement of plates across a stationary mantle plume

If there was no movement of plates, there would only be 1 active volcano Hawaiian Islands and Emperor Seamount Chain Newest Hawaiian Island: Loihi Earth is a giant magnet. Its magnetic field has a north and south pole This field is generated as liquid outer core spins around solid inner core Every so often, the magnetic poles of Earth reverse, so that the magnetic north pole is now located over the geographic south pole. Then, later on, they reverse back. Today we are at Normal Polarity. When the South pole is the magnetic pole, its called Reversed Polarity

5. Paleomagnetism and dating the rocks Paleomagnetism Continued When is the next polar reversal expected to occur?

On average over the last 20 million years, the polar reversal has taken place every 200,000-300,000 years Our last reversal took place 780,000 years ago Were overdue for a reversal! What will happen? 1. Compasses will tell you the north is in Antarctica (location of south pole) 2. Animals (birds, salmon, and sea turtles) that use Earths magnetic field for navigation will get lost during their routine journeys 3. Auroras will be visible at lower latitude locations Bands of Polarity

As igneous rock cools, magnetic minerals within it (like magnetite) cool and record the magnetic field of the moment they cool That mineral alignment becomes fixed as they cool below the Curie Temperature (580C), or temperature that magnetite solidifies Records positions of north and south pole relative to this rock when it was formed. As sea floor spreads, bands of equal width and polarity occur on opposite sides of the ridge Symmetrical pattern shows that material comes up at the spreading center and then moves away in both directions at the same speed. Smoking Gun for seafloor spreading

Seafloor Spreading and Paleomagnetism Interpreted the magnetic stripes as products of steady creation of new ocean crust over geologic time, supporting Hess sea floor spreading

Frederick Vine and Drummond Matthews (1968) 2nd Piece of Evidence due to the Cold War Paleomagnetism Practice Problems

1. How many episodes of normal polarity are represented? __________ 2. How many episodes of reverse polarity are represented? __________ 3. Which of the bands (A, B, C, D) are of the same age? __________ 4. Which band is oldest? __________ 5. Which band would be the densest? __________ 6. What do you notice about the pattern of the width and polarity of the band if you compare the east and west of the ridge sections? _______________ A

B

C DSo whats the difference between Theory of Plate Tectonics and Continental Drift Hypothesis?

Continental Drift Hypothesis Theory of Plate Tectonics

Continents move independent of oceanic crust, even plough through oceanic crust

Tectonic plates with continents and ocean lithosphere move as one slab across asthenosphere

No science based driving mechanism A science-based driving mechanism: convection including mantle plumes, slab pull, ridge push

4 pieces of evidence of Pangea supercontinent: Coastlines matchup, Fossils, Mountain ranges and rocks and glacial evidence

5 pieces of evidence of plates & motion: shape of ocean floor, age of ocean floor, locations of earthquakes and volcanoes, hotspots, paleomagnetism