Evidence for Plate Tectonics

As it turns out, Alfred was right.

1. Alfred Wegener’s Evidence for Continental Drift

• The puzzle-like fit of some of the continents (such as those along the Atlantic Ocean)

1. Alfred Wegener’s Evidence for Continental Drift

• The puzzle-like fit of some of the continents (such as those along the Atlantic Ocean)

• Mountain ranges that match up when continents are fit together

1. Alfred Wegener’s Evidence for Continental Drift

• The puzzle-like fit of some of the continents (such as those along the Atlantic Ocean)

• Mountain ranges that match up when continents are fit together

• Evidence of glaciers in areas that can’t have glaciers today (glaciers in Australia?)

More evidence for Continental Drift:

• Fossils of identical animals (ones that can’t swim or fly) in far away places

• Coal, deserts and salt deposits in places they couldn’t form today

Problems with the idea?

• How do you get the solid rock of the continents to “drift” through the solid rock of the ocean floors?

• What mechanism can possibly move so much rock?

N. America Europe

Ocean

2. Harry Hess and the Sea Floor

• Mid-Ocean Ridges—People expected the ocean floor to be shaped like a bowl.

Instead, Hess found tall mountain ranges in the middle of the ocean.

North America

Europe

• Hess also noticed flat-topped mountains below the surface (“guyots”)

Hess reasoned that the flat-topped mountains could be explained if they had been flattened by wave erosion at the surface, then dropped below sea level at a later time.

Topography of the Atlantic Ocean FloorSeamounts and Guyots

Mid-Atlantic Ridge

Sea Floor Spreading• Hess speculated that the features seen on the

ocean floor could be explained if the sea floor was spreading away from mid-ocean ridges, with new ocean floor being made at the ridge.

Hot, upwelling mantle material raises the surface and fills in the gaps as spreading occurs

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3. Evidence from Paleomagnetism (“fossil magnetism”)

• Magnetic minerals pointing in weird directions in continental rocks—they should be magnetized to the north, but some pointed in other directions. “Polar Wandering”?

• Magnetism within rocks of different ages on the same continent were not consistent.

The “Last Nail”: Mirror-image magnetic stripes along mid-ocean ridges!

From Continent to Rift to Ocean: Animations 2.3, 2.5

4. The oldest sea floor is next to the continents; the youngest is along the mid-ocean ridges.

The oldest sea floor rocks found are only about 200 MY old.

But if new ocean floor is being made at the ridges, and the Earth isn’t getting any bigger, where is

the old ocean floor going?

The Tonga Trench

5. Benioff Zones: Hugo Benioff noticed that earthquake foci near ocean trenches plot along a descending line. Earthquakes can only happen in brittle rocks.

• Mantle rocks at these depths are “plastic”, not brittle, so earthquakes cannot occur in them.

• The earthquakes are due to oceanic lithosphere diving down (subducting) at the trench, thus solving the ocean crust problem.

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6. Worldwide distribution of earthquake epicenters

• Earthquake epicenters “frame” the plates. Earthquakes are much less common inside the plate boundaries.

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7. Hot Spots (Mantle Plumes)• Certain places on Earth

stood out as exceptions to the rule

• ex. Hawaii—constant volcanic and seismic activity, but not at a plate boundary

• But why? Instead of ruining the new theory, The answer provided great confirmation of plate movement

• The Hawaiian hot spot records the movement of the Pacific plate as a linear series of islands and seamounts.

Development of Island Chains above Mantle Plumes--Animation