Plate TectonicsPlate TectonicsEvolution of the Earth

and Our Restless Planet: Chapter 33

Review: InteriorReview: Interior

33.1 The Theory of Continental Drift

33.1 The Theory of Continental Drift

• Scientists in the early 20th century believed– Oceans and continents were fixed in place– The surface is a skin spread over a cooling

interior– Cooling caused contraction of the skin=

wrinkles and bumps aka mountains and valleys

• Scientists in the early 20th century believed– Oceans and continents were fixed in place– The surface is a skin spread over a cooling

interior– Cooling caused contraction of the skin=

wrinkles and bumps aka mountains and valleys

Alfred Wagner Thought Differently!

• He believed:– The continents are slowly moving– All the continents has once been attached in a

supercontinent and this was called Pangaea “all land”

– Pangaea split and the continents moved apart– The boundary of each continent is not at the

shoreline but the continental shelf

Alfred Wegener cont.

No one believed Wagner…..at least not yet!

Scientific Revolution

• Discovery of the Earth’s Magnetic Field gave a window into the past

• Paleomagnetism (magnetism from the geologic past…shows 3 major things)– The position of the magnetic north versus the

geographic north pole– Direction to the magnetic pole at the time the rock was

formed– Magnetic latitude of the rock’s location at the time it was

formed– The movement of the magnetic pole over time(500

million years) suggests that either the pole moves around or the continents move

• The Earths North and South pole have flipped many times

• These leaves magnetic ‘stripes’ in rock containing iron minerals

Seafloor Spreading: H. H. Hess

• 1950’s had detailed mapping of the ocean floor

• Figured out – Some really deep parts are next to the

continents and some shallower parts are out in the middle due to the mountains under the water

– Why?

Finally Wagner's’ Ideas became the: Theory of Plate Tectonics

Finally Wagner's’ Ideas became the: Theory of Plate Tectonics

The earth’s outer shell, the lithosphere, is divided into 8 large plates and a number of smaller ones

Most earthquakes, volcanoes, and mountains occur along the borders

All Based on MotionAll Based on Motion

• It turns out that the upper section of the mantle is stuck to the underside side of the crust to form what we call tectonic plates

Evidence of Tectonic Plates

Evidence of Tectonic Plates

Our first evidence of tectonic motion is based on similar fossils and rock types on opposing sides of the ocean

Tectonic PlatesTectonic Plates

Today plate boundaries are determined by examining the location of volcanoes and earthquakes.

Volcanoes result from the friction (heat) of the platesmotion.

Earthquakes occur where plate rub against one another

Tectonic PlatesTectonic PlatesVolcanoes

Tectonic PlatesTectonic Plates

Why do the Plates Move?Why do the Plates Move?• No single idea explains everything but we can

identify several forces that contribute to the movement of the plates.– Slab pull

• The sinking of the cooled dense oceanic plates pulls on the rest of the plate

– Ridge rises• The material deposited on the top of the ridge slides downs

from the rise pushing on the plate

– Convection• Movement within the mantle could be part of the driving force

behind the motion of the plates.

Why do the Plates Move?Why do the Plates Move?

Why are they spreading?

• Why is the Atlantic still getting wider

• The plates are pulled apart by convection currents in the mantle below

• Caused by heat released from natural radioactive processes

• At the mid Atlantic ridge molten rock from below rises up to fill the gap with new basaltic rock

Plate BoundariesPlate Boundaries• Convergent – plates move toward

one another

• Divergent – plates move away from each other

• Transform-Fault – plate moves sideways from each other

Plate BoundariesPlate Boundaries

Convergent PlatesConvergent Plates

Convergent PlatesConvergent Plates

Ocean-Ocean Convergence

Ocean-Continental Convergence

Continental-Continental Convergence

Divergent PlatesDivergent Plates

Spreading centers on the ocean floor and on land (create rifts)These create the beginning of a new ocean basin

Transform PlatesTransform Plates

Transform PlatesTransform Plates

San Andreas Fault

Mid-Plate HotspotsMid-Plate Hotspots

Hawaii

PangaeaPangaea

PangaeaPangaea

PangaeaPangaea

The break up of Pangaea

Where are we going?Where are we going?

We appear to be headed for another super continent as North America,

South America, Asia and Australia converge in the ever shrinking Pacific Ocean