Plate Tectonics

Tectonic Plates of the Earth

How can we explain the rock cycle?

• Need to identify mechanisms by which rocks are–Heated and melted–Buried and metamorphosed–Pushed up into mountains to be

eroded

Earth as a jigsaw puzzleMatching continental edges: recognized for centuries

Pangaea: supercontinent proposed by Wegener (1912)

Wegener’s contributioncontinental drift

= pattern of moving continents

Theory of fixed continents

Traditional assumption: continents & ocean basins fixed in position

Problems

1. Matching continent edges

2. Matching fossils among continents

3. Matching geology among continents

Evidence against fixed continents

Matching edges

Matching geology

Bathymetry of the ocean floor

continentalshelf

abyssalplain

trenchm

id o

cean

ridg

e

Thickness of the Sediment

x 550 m

x 102 mx 18 m

x 112 mx 430 m

35 m x

The Driving Mechanism

Paleomagnetism

Radiometric dating

Red = younger, green = older

Plate tectonics

= process for continental drift

Thermal convection system within Earth1. Making crust

2. Destroying crust

Explains the rock cycle!

Making crust

• magma rises up from mantle, spreads out, & cools

• occurs at spreading boundaries (called ridges)

• process of sea-floor spreading

Magma

RidgeNew crust

Sea-floorspreading

Asthenosphere

Lithosphere

Sea-floorspreading

• crust descends into mantle & is recycled

• occurs at converging boundaries (called trenches)

• process called subduction

Destroying crust

Asthenosphere

Subducting plate

Trenc

h

Making crust + destroying crust = conveyor belt system =

rock cycle (sort of)

Convectionor Buoyancy

Ridge push

Slab pull

Convection cell in mantle

Where are ridges & trenches?

ridges trenches

Geologic Excitement at Tectonic Plate Boundaries

Global Seismicity

Active Volcanoes

3 Major plate boundaries

1. Spreading

- plates pulling apart

2. Converging

- plates pushing together

3. Transform

- plates sliding past each other

1. Spreading boundaries

• also called divergent boundaries• includes ridges and rift valleyse.g. Mid-Atlantic ridge, Great Rift Valley

2. Converging3 types

A. Oceanic-Continental volcanic rangese.g. Andes

B. Oceanic-Oceanic volcanic island arcse.g. Japan, Aleutian islands

C. Continental-Continental mountain rangese.g. Alps, Himalayas

Convergent boundaries

Oceanic-Continental

Oceanic-Oceanic

Continental-Continental

3. Transform boundaries

• associated w/ major seismic activitye.g. San Andreas system

Continental Drift Animationbased on paleomaps from

University of Texas Arlington

http://www.ucmp.berkeley.edu/geology/anim1.html

Plate tectonics: predicting the future

Plate tectonics in the future: 50 my

• Australia will straddle the equator

• E. Africa will form new continent

• Mediterranean will close off

• Atlantic Ocean will grow

• Pacific Ocean will shrink