Plate Tectonics

Chapter 8

What Is Plate Tectonics?

• The Earth’s crust and upper mantle are broken into sections called plates

• Plates move around on top of the asthenosphere like rafts and support both continents and oceans

• Plate tectonics is a theory that describes the formation, movements, and interactions of plates

Plate Tectonics

The Theory of Continental Drift

Continental Drift Theory

• Proposed by Alfred Wegener (German scientist) in 1912

• 250 million years ago, all of the continents were combined into one super-continent called “Pangaea”

• The continents gradually drifted apart to where they are today

Evidence to Support the Theory

“Puzzle Pieces”

• Continents look like they could be part of a giant jigsaw puzzle

Distribution of Fossils

• Plant and animal fossils found on the coastlines of different continents

Sequence of Rocks

• Same rock patterns found in South America, India, Africa, Antarctica and Australia

Ancient Climates

• Tropical plant remains (coal deposits) found in Antarctica

• Glaciation in Africa, South America, India, and Australia during the same time

Problems With The Theory

• Wegener couldn’t explain how continents moved

• Wind and currents could possibly move fossils• Theory was not accepted by scientists

The Theory of Plate Tectonics

• Recall: continents are embedded in lithospheric plates. As the plates move they carry continents and oceans with them

• Discoveries in the 1950s & 1960s added some support to Wegener’s ideas:– Earthquakes– Magnetism– Age of rocks on the ocean floor

Location of Earthquakes and Volcanoes

• Earthquakes and volcanoes do not occur randomly – instead in concentrated belts– Belts mark plate boundaries• Boundaries are where 2 plates are pushing toward,

pulling away, or sliding past each other• Strain builds up along plate boundaries and can cause

fractures to occur leading to an earthquake• Boundaries are also areas of high heat flow molten

rock moves upward to the surface causing volcanic activity

Magnetism and the Age of the Ocean Floor

• Some igneous rocks contain minerals that are magnetic provide a record of the direction of Earth’s magnetic field at the time when molten matter that formed the rock cooled– One important discovery made by scientists studying

these rocks:• Some rocks recorded reversals in the direction of Earth’s

magnetic field (north become south and south became north)• A number of magnetic reversals have taken place over

millions of years

Magnetism and Age of the Ocean Floor

• Igneous rocks on the sides of mid-ocean ridges• Mid-ocean ridge: long chain of volcanic mountains

on the ocean floor with a deep central valley• Discovery:– Bands of rocks on both sides of a mid-ocean ridge

record periods of magnetic reversals– Rocks at the center of the mid-ocean ridge are the

youngest and the rocks are older as you move farther from the ridge

Magnetism and Age of the Ocean Floor

• Conclusion:– Mid-ocean ridges represent boundaries where

plates are moving APART– Why?• The newer rock along the ridge are formed by hot,

molten rock rising between spreading plates• As the new rock forms, older rock moves away from the

ridge on either side

Types of Plate Boundaries

• Divergent• Convergent• Transform

Divergent Boundary

• Plates are moving apart• Creates rift valleys• Deep valleys at the center of a mid-ocean ridge

Features of Divergent Boundaries

• Mid-ocean ridges• rift valleys• Volcanic activity• Earthquakes at mid-ocean ridges

Convergent Boundary

• Two plates are moving toward each other

• 2 Types:– Subduction boundaries

– Collision boundaries

Subduction Boundary

• Oceanic plate plunges beneath another plate: the oceanic plate is said to be subducting beneath the overriding plate

• Important feature: deep-sea trench (deepest part of the ocean floor)

• Subduction boundaries can be:– 2 oceanic plates colliding OR– Oceanic plate colliding with a continental plate

Two Oceanic Plates Colliding

• Creates an island arc and a trench• Example: Japan

Aleutian Islands, Alaska

Oceanic Plate Colliding with a Continental Plate

• Oceanic plates = denser than continental plate and subduct under the continental plate

• Forms a volcanic mountain range and trench• Example: Cascade and Andes Mountains

Andes Mountains, South America

Collision Boundaries

• Two continents collide and a re welded into a single, larger continent

• Forms a folded mountain range• Example: Himalayas, Alps and Appalachian Mountains

Transform Boundaries

• Boundary where two plates slide past each other• Forms fault lines and causes earthquake activity• Example: San Andreas Fault Line (California)

San Andreas Fault, CA

Causes of Plate Movement

• There are 3 hypotheses:– Mantle Convection

– Ridge Push

– Slab Pull

Mantle Convection

• Mantle may be moving plates along with it as it convects

How it Happens…

• Hot magma in the Earth moves toward the surface, cools, then sinks again.• Creates convection currents

beneath the plates that cause the plates to move.

The Problem With It…

• Model does not account for the enormous force needed to move the plates

Ridge Pull

• Recall what happens at a mid-ocean ridge– Molten magma rises at a mid-ocean ridge– The magma cools and forms rock

• New rock closest to the ridge is less dense and more buoyant because rock is very hot but as it cools and ages it becomes more dense

• Gravity causes the older, denser rock to slide away from the ridge • As the rock slides away, new molten magma wells up at the mid-

ocean ridge

• The cooling, subsiding rock exerts a force on spreading plates that could help drive their movement

Slab Pull

• At a subduction boundary, one plate is denser and heavier than the other plate

• The edge of the subducting plate is much colder and heavier than the mantle, so it continues to sink, pulling the rest of the plate with it

• The force produced is called slab pull• Currently: scientists find this explanation to be

the strongest in driving plate movement