Plate Tectonics Theory What is the Theory of Continental Drift? What is Plate Tectonics Theory? What causes the plates to move? What is the evidence for Plate Tectonics Theory? What was the missing piece to Alfred Wegener’s theory of Continental Drift?

Continental Drift Alfred Wegener: German meteorologist and explorer The Origin of Continents and Oceans, 1915 Introduced the Theory of Continental Drift Proposed the supercontinent Pangaea Pangaea: “all lands” Very controversial at the time Couldn’t explain the mechanism that moved the continents

Pangaea: the Supercontinent 250-300 mya: Paleozoic and Mesozoic Eras Included all the major continents 200 mya: Early Mesozoic (Triassic) Atlantic Ocean began forming

Pangaea: the Supercontinent 145 mya: Middle Mesozoic (Jurassic) India moves northward Atlantic expands 65 mya: Late Mesozoic (Cretaceous) Australia separates from Antarctica Present Day: Continents continue to move

Wegener's Evidence Shape of continents Better maps let people see the “puzzle” pieces People noticed the coastlines of South America and Africa

Wegener's Evidence Fossil Correlation Mesosaurus: freshwater reptile Found in South America and Africa Cynognathus: land reptile Lystrosaurus: land reptile Found in Africa, India, & Antarctica Glossopteris: fern Found in South America, Africa, India, Antarctica, & Australia

Fossil Correlation

Wegener's Evidence Rock Types and Structures Mountain Chains When the continents are put together, mountain ranges match up EX: Appalachian Mts., British Isles, Caledonian Mts.

Wegener's Evidence Paleoclimate Glacial Deposits and Formations Layers of glacial till Glacial grooves and scratches on bedrock Areas now near the equator, were once at the south pole and covered by ice

Wegener's Evidence Paleoclimate Coal Deposits Areas now in the Northern hemisphere were once at the equator Lush tropical swamps eventually formed the coal beds of the eastern U.S., Europe, and Siberia

Wegener's Evidence Paleoclimate Sedimentary Rocks Sedimentary rocks form in specific environments Shale: calm water; swamps, lakes, etc. Limestone: warm shallow seas Sandstone: running water; rivers, beaches or dry, windy environments like deserts Sedimentary rocks contain fossils that give clues about past climate

Where did Wegener go wrong? How did the continents move? Tidal force of the Moon? WRONG ☹ Centrifugal force of Earth’s rotation? WRONG ☹ Believed continents “plowed” through the oceanic crust like an icebreaker through ice Couldn’t explain the “how” so other scientists rejected it His theory was before its time!

Plate Tectonics Theory A New Theory New technologies and discoveries led to a reconsideration of Wegener’s theory Lithosphere The Earth’s crust and upper mantle are a rigid layer Plates: pieces of the lithosphere Continental and Oceanic Asthenosphere The weaker layer underlying the lithosphere Behaves like a fluid over a very long time

Plate Tectonics Theory: The Evidence Paleomagnetism Earth’s magnetic field switches polarity Recorded in igneous rocks

Plate Tectonics Theory: The Evidence Earthquake and Volcano Distribution Earthquakes and volcanoes coincide with plate boundaries

Plate Tectonics Theory: The Evidence Ocean Drilling Deep Sea Drilling Project 1968-1983 Age of ocean floor increases as you move away from mid-ocean ridges Ocean basins are much younger than continents Oceanic crust: none older than ~ 180 million years old Continents: 4.0 Billion years old

Plate Tectonics: The Plates 7 major plates and many smaller plates Plates move relative to one another About 5 cm/year on average

Driving Forces of Plate Movement Mantle Convection Uneven distribution of heat in the Earth’s interior causes thermal convection within the mantle. Warm mantle rises Mantle Plumes Cold mantle sinks Computer model of mantle convection

Driving Forces of Plate Movement Slab Pull and Ridge Push Mantle convection pulls oceanic crust down into the asthenosphere Rising convection currents at mid ocean ridges push plates apart