Presentation on theme: "Theory of Continental Drift Alfred Wegener, German meteorologist Proposed in 1912 in articles Single large continent (Pangaea – “all earth”) started."— Presentation transcript:
Weakness of Theory What kind of forces could be strong enough to move such large masses of solid rock over such great distances? Wegener suggested that the continents simply plowed through the ocean floor, but Harold Jeffreys, a noted English geophysicist, argued correctly that it was physically impossible for a large mass of solid rock to plow through the ocean floor without breaking up.
Earth’s “Inner Furnace” To scale Crust 0-100 km thick Crust Mantle Outer core Inner core Not to scale 2,900 km 5,100 km 6,378 km Lithosphere (crust & uppermost solid mantle) Mantle Core
After World War II, new evidence from ocean floor exploration and other studies rekindled interest in Wegener's theory, ultimately leading to the development of the theory of plate tectonics.
Evidence for Plate Tectonics Ruggedness & youth of ocean floor Confirmation of repeated reversals of Earth’s magnetic field in geologic past Emergence of the seafloor-spreading hypothesis and associated recycling of oceanic crust Documentation that the world's earthquake and volcanic activity is concentrated along oceanic trenches and submarine mountain ranges.
What is the nature of the forces propelling the plates? Conceptual drawing of assumed convection cells in the mantle. Below a depth of about 700 km, the descending slab begins to soften and flow, losing its form.
Faults Fault Faults Fractures in Earth’s crust along which motion may occur Mark plate boundaries
Types of Crust Continental crust Makes up landmasses Granitic (felsic) Lighter Less dense Thicker Average age 2.3 billion years, oldest is 3.96 billion years Oceanic Crust Lies under oceans & seas Basaltic (mafic) Heavier Denser Thinner Average age 55 million years, totally recycled every 150 million years
Three Types of Plate Boundaries Divergent: new crust forms as plates pull away from each other Convergent: crust destroyed as plates collide, one dives under the other Transform: crust neither created nor destroyed, plates slide past one another
Ocean: Sea Floor Spreading Ocean plates are being pushed apart by convection currents and new ocean floor is made Crustal Features: Rift Valleys on land and Mid-Ocean Ridge in ocean DivergentBoundaries
ConvergentBoundaries Continental-Continental: huge mountains Continental-Oceanic: Subduction! The more dense oceantic crust sinks under the continental. Trenches, mountains, and volcanoes occur. Oceanic-Oceanic: Subduction! Very deep trenches and volcanoes
Find the Nazca plate on the map. What direction is it moving? The Nazca plate is moving east
Find the South American plate and describe its movement. The South American plate is moving west
What do you think will happen as these plates continue to move? As the plates move, they will collide (convergent boundary) Since the Nazca plate is oceanic crust and the South American plate is continental crust, the Nazca will plunge beneath the South American plate