The document summarizes evidence that supports the theory of seafloor spreading and plate tectonics. It describes how Harry Hess first proposed in 1962 that new ocean crust is formed at mid-ocean ridges through volcanic activity and magma cooling. As more magma is added, it pushes older ocean crust further from the ridges, providing evidence that the plates are moving. Studies of magnetic reversals in ocean floor basalts also indicate the age of the crust increases further from ridges, confirming seafloor is continually being created and destroyed over time.

1. THE PROOF-----SEAFLOOR SPREADING
•An American scientist named Harry Hess
proposed the seafloor spreading theory in
1962.
•The Sea floor spreading theory states that
new ocean crust is being created at mid-
ocean ridges (which are large mountain
chains underwater) and destroyed at deep-
sea trenches.
•This is proof that the plates are moving
along on a “conveyor belt” so Wegner’s
idea on continent drift was correct.

2. The force responsible for driving or moving the plates is
_____________________.
Convection Currents occur within the mantle of the earth
when hot magma rises and cool magma sinks
convection currents

3. •Magma, because it is hotter and less
dense, is forced up towards the ocean
floor cooling off when it touches
water.
•When the magma hardens, a small
amount of new ocean floor is added
to the Earth’s crust. As more magma
rises and cools it pushes the new sea
floor away from the ridge. This process
is called sea floor spreading.

4. PROOF THAT THE SEA FLOOR IS SPREADING
1. The oldest ocean floor rocks ever found are 180
million years old. Remember the Earth is 4.6 billion
years old. This proves that ocean floor is being
destroyed therefore all ocean floor rocks are young
compared to the age of Earth.
2. The rocks closest to the ocean ridge were younger
than the rocks found further from the ridge. This means
that new rocks are formed at the ridges and push the
older rocks away from the ridge.

5. Animation of Sea Floor Spreading and Ages…2.5

6. Animation of Magnetic Reversals…2.3 on Previous Animation
3. Geologists also discovered that the rocks found at
oceanic ridges showed the magnetic reversals that the
Earth has undergone.

7. Magnetic Reversals occur
• When magma cools, the iron cools into the mineral
magnetite. It lines up parallel to the Earth’s present
magnetic field. This iron is like compass needles, pointing
north. So when the rock hardens, a record of the Earth’s
magnetic field at that time is locked in stone----so to
speak!!

8. 2. Studies of these magma flows have showed a
magnetic reversal over geologic time.
3. A magnetic reversal (called a FLIP) is a change in
Earth’s magnetic field. North pole becomes south pole
and south becomes north.

9. 4. A magnetic field that has the same orientation as Earth’s
present field is said to have a normal polarity
5. A magnetic field that is opposite to the present field
has reversed polarity

10. YOU NEED TO LOOK AT THE ARROWS
AND LEARN HOW
TO READ THEM FOR PLATE
MOVEMENT
→← CONVERGING
←→ DIVERGING
↑↓SLIDING PAST