Plate Tectonics Mrs. Griffin

Drifting Continents Average human lifetime- drift is slow On a geologic time scale- Whoa Nelly! South Africa is moving away from Africa- 2 to 3 cm/y Hawaii volcanic islands are migrating NW ~8 to 9 cm/y What’s the cause?

Early Observations Eduard Suess- single land mass know as Gondwanaland. 1 st hypothesis taken seriously was proposed by Alfred Wegener –Called his hypothesis continental drift –200 mya supercontinent Pangaea began to break apart

Evidence from Rock Formation If mountains were once formed together then fractured…… Wegener hypothesized similar rocks types must exist on opposite sides of the Atlantic Ocean. –Appalachian Mountains –Greenland and Europe

Evidence from Fossils Wegener also gathered evidence from fossils. Land dwellers could not have swam the great distances that exist today. Glossopteris fossils have been found on many continents

Ancient Climatic Evidence What types of rocks provide clues to the past? –Sedimentary Wegener also found evidence of vast climatic changes on some continents. –Coal deposits in Antarctica, forms from dead swamp plants…..??? –Was Antarctica close to the equator? Glacial deposits in Africa, India, Australia, and South America. –Continents once near the south pole?

A Rejected Hypothesis Wegener's theory was not accepted by scientists due to two major flaws, what were they? –What is the driving force behind the movement of the continents? –How were the continents moving without fracturing? Wegener died in 1930, and it wasn’t until the early 1960’s that new evidence was revealed that could explain how and why. Where do you think this evidence was found? –Seafloor

Alfred Wegener

Review The theory of continental drift was proposed by which individual? This supercontinent means “all the Earth.” What type of evidence did the above individual base his hypothesis on? What were the two “flaws” regarding the theory of continental drift? Where was the evidence found to explain the two missing flaws?

Seafloor Spreading Help from technology: –Development of echo-sounding methods. Sonar, uses sound waves to measure water depth. Magnetometer- a device that can detect small changes in magnetic fields, towed by ships, records the magnetic field strength in rocks on the ocean floor. –Used to make maps of the seafloor.

Sea-floor Topography

Ocean Floor Topography What did these maps reveal? –Underwater mountain chains called ocean ridges. Form the longest continuous mountain ranges on Earth; earthquakes and volcanoes are common among these areas. –Maps also revealed deep-sea trenches, counterparts to ocean ridges. Mariana Trench is over 11 km deep.

Ocean Rocks and Sediments Rock samples taken from areas near ocean ridges are younger than samples taken from areas near deep-sea trenches. Ocean floor rocks are much younger than continental rocks. Why is this so?

Seafloor Spreading Topographic, sedimentary, age, and magnetic data combined laid the foundation for Harry Hess to propose his theory: –Seafloor spreading: new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches.

Seafloor Spreading Seafloor spreading provides the answer to the how question. Continents are merely passengers that ride with ocean crust as it slowly moves away from ocean ridges.

The Theory of Plate Tectonics Why are some regions of Earth dotted with many active volcanoes while other regions have none? Why do earthquakes occur frequently in certain areas? –The theory of plate tectonics: Earth’s crust and rigid upper mantle are broken into enormous slabs called plates. They move in different directions and at different rates.

Earth’s Tectonic Plates

Plate Boundaries Tectonic plates interact at places called plate boundaries which include divergent, convergent, and transform boundaries.

Plate Boundaries Places where two tectonic plates are moving apart are called divergent boundaries. –Found mostly on the seafloor, form ocean ridges. –Some form on continents; when crust separates, the stretched crust forms a long narrow depression called a rift valley.

Convergent Boundaries Places where two tectonic plates are moving toward each other are convergent boundaries, 3 types. –Oceanic-oceanic –Oceanic-continental –Continental-continental In subduction zones- one of the two plates descends beneath the other ->this creates a deep-sea trench. –Recycling of the plates begins

Transform Boundaries Where two plates slide horizontally past one another. –Here crust is only deformed or fractured. –Characterized by long faults. –Rarely occurs on continents…..except for where? San Andreas Fault

Causes of Plate Motions Convection currents are thought to be the driving mechanism of plate movements. Convection: the transfer of thermal energy by the movement of heated matter. –Convection currents in the asthenosphere are set in motion by the transfer of energy b/w Earth’s hot interior and its cooler exterior.

Mantle Convection How is mantle convection related to movement of plates? Hypothesized: ridge push/slab pull. –Ridge push: formation of ocean ridge causes asthenosphere to rise; the weight of uplifted trench pushes plate. –Slab Pull: sinking region of a mantle convection could be pulled down at subduction zones.