Plate Tectonics

A Unifying Theory

Early Theory• Contraction Theory–Young earth was made up of molten rock –Contracted (got smaller) as it cooled

forming mountain ranges.

Early Theory

• Realized that the earth was heated by the breakdown of radioactive elements– Earth might be getting

warmer not cooler and expanding

Alfred Wegener

• Wasn’t buying it!• In 1910, Started

lecturing and writing about his new hypothesis

The Continental Drift Hypothesis

• Continents were all one supercontinent called Pangaea.–Pangaea fragmented

and pieces drifted to the current location of the continents

Problems with Continental Drift• Scientist could not explain what caused

continents to move• Didn’t believe the evidence was conclusive

Evidence for Continental Drift

• Matching Coastlines– South America and

Africa• Matching Geology– Similar age of rocks

on the coast– Similar geologic

features

Evidence for Continental Drift• Fossil Evidence–Glossopteris fossils in South Africa, South

America, Antarctica and India

Evidence for Continental Drift• Glacial Deposits–Grooves from glacial sheet movement

match up

Evidence for Continental Drift• Paleomagnetism- The

magnetic poles of the earth have changed over time– Igneous rocks magnetize

based the location of the poles while they are cooling. (mineral grains will point to magnetic north)

The Missing Piece• Paleomagnetism in

the sea floor– Sea floor was made

up of bands of igneous basalt with alternating polarities

– Bands were symmetric across the mid-Atlantic ridge

Sea Floor Spreading• Rocks on either side of

the ridge were moving away from each other

• New molten rock was filling the gap as the floor spread.

• Older rocks are farther from the ridge and new rocks are closer

How fast do the plates move?