Wegener’s Continental Drift Hypothesis

• States that the continents once formed a single landmass, broke up, and drifted to their present locations.

• His evidence includes:– Shape of the continents– Fossil evidence– Similar rocks

The shape of some of the continents look like they could fit together like …

…Puzzle Pieces

Fossil Evidence

Fossils of the same species are found on two different continents.

Rock type and structure correlation

Coal Deposits

Glacial Deposit

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Wegener had a lot of evidence…

• He could not satisfactorily explain the forces moving the continents.

• Scientists could not explain how the continents were moving.

• Flaws in Wegener’s data had the continents moving much to far and too fast.

…so why was his hypothesis almost universally rejected?

The large land mass was called Pangaea.

Pangaea

Continental Drift - animation

Mid-Ocean Ridges and Sea-Floor Spreading

Mid-ocean ridges are places where

sea-floor spreading takes place.

Mid-Ocean Ridges and Sea-Floor Spreading

As the tectonic plates move away from each other, the sea floor spreads apart and

magma fills the gap.

As the new crust forms, the older crust gets pushed away from the mid-ocean ridge.

This means that the older crust is farther away from the mid-ocean ridge than the

younger crust is.

Evidence for Sea-Floor Spreading: Magnetic Reversals

Throughout Earth’s history, the north and south magnetic poles have changed places many

times.

Magnetic Reversal of Earth’s Poles

Magnetic Reversals and Sea-Floor Spreading

• The molten rock at the mid-ocean ridges contains tiny grains of magnetic minerals like iron.

• These minerals align with the magnetic field of the Earth while liquid, then when they cool and harden, they make a record of the magnetic poles.

• This record is then carried slowly away from the spreading center of the ridge as sea-floor spreading occurs.

Magnetic Reversals and Sea-Floor Spreading