Continental Drift

Plate Tectonics

Break-up of Pangaea

Plate Tectonics • Plate tectonics = mechanism of continental drift (1950s – 1960s) • Lithosphere is broken into at least 27 plates, which are in motion due to convection in mantle

• E. coast of S. America and W. coast of Africa fit together (fossils similar) • Alfred Wegener proposed “continental drift” hypothesis in 1912 • Said all continents joined into a supercontinent called Pangaea, which began breaking 180 mya

Break-up of Pangaea

Evidence? In the 1960’s the continents were fitted together again, this time by the computer best fit to the edges of the continental shelves. The matches are astonishing!

1

Today’s Puzzle of Plates

RIGID PLASTIC

RIGID

Mantle Convection • Convection in the mantle drives the movement of the plates (causing seafloor spreading) • Convection currents are due to alternate heating and cooling of mantle material • The plates ride atop the fluid portion of the mantle, as if they were on a giant conveyor belt

Heat, composition and fluidity of materials change with depth

USA to China: 13.3 hr flight

2

Hot fluid magma, density differences, plus convection produce continental drift

Mid-Ocean Ridges • New oceanic crust is formed at mid-ocean ridges • Most famous is mid-Atlantic ridge • New crust is formed at the ridge, then carried away from the ridge by convection in the mantle • Movement of crust away from ridge = seafloor spreading

Mid-Ocean Ridges

Evidence for Seafloor Spreading • Earth’s magnetic field reverses polarity 2-3 times every million years • Magnetite orients itself towards the magnetic pole • The orientation of magnetite flip-flops back and forth as you move away from the ridge, indicating seafloor spreading

3

SEAFLOOR SPREADING

• PREDICTIONS – OLDER ROCKS FARTHEST FROM RIFT – SEDIMENTS THICKER AWAY FROM RIFT – RIFT SHOULD BE HOT

newest

Other Evidence

oldest

Types of Plate Boundaries • 3 types of plate boundaries: – Divergent = plates move apart – Convergent = plates move together – Transform fault = plates grind past each other

Divergent Boundaries • Divergent boundaries result in upwelling of material from the mantle, creating new sea floor (new crust made) • Found at mid-ocean ridges

Convergent Boundaries • Convergent boundaries involve plates moving into one another • 3 types: – Oceanic/Continent – Continent/Continent – Oceanic/Oceanic

4

Deep rocks are denser than surface rocks

Basalt: fine-grained and heavy

Granite: coarse-grained and light

Oceanic/Continent Convergence • Oceanic plate moves under continental plate (subduction) • Trench = deep ocean area formed at this type of boundary – crust destroyed here • Earthquakes and volcanoes abundant

Continent/Continent Convergence • Continents are of near equal density, so collision causes crumpling of crust • Crumpling = mountain-building • Himalayas are an example

Oceanic/Oceanic Convergence • One oceanic plate normally dips below the other • This normally causes volcanoes to form, which eventually form an island arc • Aleutian Islands are an example

5

Transform Fault Boundary • 2 plates slide past one another • Place where they slide past one another = a fault • Lots of friction builds stressed – released as earthquakes • San Andreas fault is an example (N. Am. And Pacific plates)

Distribution of earth quakes delineates the edges of the plates

A bad day in LA

Depth and distribution of earthquakes indicate the angle of subduction of the Pacific Plate beneath Japan

Island arcs

Hawaii

6

Effects on Biogeography

MARSUPIALS AND MONOTREMES EVOLVE

Primitive Placentals Evolve and Radiate

ISOLATION OF MONOTREMES AND MARSUPIALS

65 MYA EXTINCTION OF

Advanced MARSUPIALS & MONOTREMES Placentals

ISOLATION OF EXTINCTION MARSUPIALS OF MAMMALS & PRIMITIVE PLACENTALS EXTINCTION OF MONOTREMES

CONTINUED ISOLATION OF MARSUPIALS AND MONOTREMES

Migration of Advanced Placentals into South America

FAUNAL EXCHANGE

EXTINCTION OF MANY MARSUPIALS

AN EVOLUTIONARY EXPERIMENT

STILL ISOLATED, ONLY CONTINENT WITH MARSUPIALS AND MONOTREMES AS DOMINANT MAMMALS

7

Most lava reaches the surface of the earth at spreading centers but, except for Iceland, most rifts are in the deep sea. Mid-Atlantic ridge runs through the center of Iceland.

Iceland is growing rapidly.

Perhaps too rapidly.

8