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.

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

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.

PROOF THAT THE SEA FLOOR IS SPREADING 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.

Animation of Sea Floor Spreading and Ages…2.5

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. Animation of Magnetic Reversals…2.3 on Previous Animation

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.

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!!

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

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

AMAZING EARTH SCIENCE FACTS 1. ____________________is created at mid oceanic ridges. 2. Age of rock ____________ as distance from ridge increases (____________ rock is at the ridge). New sea floor increases youngest ____________________________________drive plate movement. Convection currents