Who is Harry Hess?

• Harry Hess was a geologist and Navy submarine commander during World War II.

• Part of his mission had been to study the deepest parts of the ocean floor.

• Scientists began to use magnometers to study the ocean floor in the 1950s.

• The Great Global Rift (mid-ocean ridge) was discovered.

• What are mid-ocean ridges?

What are mid-ocean ridges?• Mid-ocean ridges are underwater

mountain ranges that extend into all of Earth’s oceans.

• They curve like the seam of a baseball along the sea floor.

What did Harry Hess propose?

• The discoveries inspired Hess to look back at his data from years before.

• In 1960, Harry Hess, proposed that the movement of the continents was a result of sea-floor spreading.

• What is sea-floor spreading?

What Is Sea-Floor Spreading?• In sea-floor spreading, the sea floor

spreads apart along both sides of a mid-ocean ridge as new crust is added. As a result, the ocean floors move like conveyor belts, carrying the continents along with them.

Convection Currents and Sea-Floor Spreading

What is the evidence for Sea-Floor Spreading?

• Several types of evidence supported Hess’s theory of sea-floor spreading: eruptions of molten material, magnetic strips in the rock of the ocean floor, and the ages of the rocks themselves.

• What are magnetic strips?

• The Earth acts like a giant magnet.

What causes the magnetic field?

• Scientists think that the movements in the liquid core create Earth’s magnetic field.

The magnetic poles are constantly changing and this is recorded in the Earth’s rocks.

http://openlearn.open.ac.uk/file.php/2717/S279_1_006i.jpg

This is a reading of the magnetic strips along the Mid-

Atlantic Ridge.

Magnetic minerals within igneous rocks align themselves with the magnetic

poles.

Do we track plate movement?

• Scientists use satellites to track the movements of the Earth’s plates.

Interesting Fact!• Because of sea-floor spreading, the

distance between Europe and North America is increasing by a few centimeters per year.

Rock samples• Scientists have also collected core samples.