Theory of Continental Drift Alfred Wegener,

German meteorologist

Proposed in 1912 in articles

Single large continent (Pangaea – “all earth”) started breaking apart about 225-200 MYA

Permian 225 MYA Triassic 200 MYA

Jurassic 135 MYA Cretaceous 65 MYA

Present day

Evidence for Continental Drift

Fit of African and South American continents (1858 drawing by geographer Antonio Snider-Pellegrini)

Additional Evidence

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

CrustMantle

Outer core

Inner coreNot 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.

Magnetic Striping & Polar Reversal

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

Convergent Plate Boundaries

Continent-ocean collisions (denser oceanic crust dives under)

Ocean-ocean collisions (older denser crust dives under younger crust, form island arcs)

Continent-continent collisions (form mountain ranges – Himalayas)

Main Types of Plate Boundaries

Earthquake Concentration

Mid-Ocean Ridges

Earth’s tectonic plates

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

Divergent Boundaries

mid ocean ridge

Sea Floor Spreading

Rift Valley on Land

Convergent Boundaries

•Continental-Continental: huge mountains

Continental-Continental

Convergent Boundaries

•Continental-Continental: huge mountains

•Continental-Oceanic: Subduction! The more dense oceantic crust sinks under

the continental. Trenches, mountains, and volcanoes occur.

Oceanic-Continental

Convergent Boundaries

•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

Oceanic-Oceanic

Transform

A plate boundary where two plates move past each other in opposite directions

Makes faults and causes earthquakes

Boundaries

Transform Boundaries

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