In the mid-twentieth century American geologist Harry Hess mapped and viewed the sea floor while on naval patrol. He saw volcanoes erupting from the sea floor, making long mountain ridges. He realised that new crust was being created here and he called this process sea-floor spreading. This is the mechanism by which plates could move.
Proof that sea-floor spreading occurs
1. The different magnetisms locked into the rocks either side of the mid-ocean ridge match up.
2. The age of the sea floor is youngest at the mid-ocean ridges and older further away from it.
3. Young volcanic islands are found near the ridge; older volcanic islands are found further away from it.
Sea
Magma rises and forms new sea floor
Sea floor Rift valley as crust pulled apart Younger Oceanic crust Older
Fig. 10 Magnetism locked into rocks as they cool is a proof of sea-floor spreading. Lava contains grains of iron that point toward the magnetic pole as the lava solidifies. The grains are a permanent record of the earth’s magnetic field. The earth’s magnetic poles have reversed many times in the past. Scientists have matched the magnetic bands in the rock either side of the Mid-Atlantic Ridge and used them as proof that sea- floor spreading occurs.
NN N R
Molten magma
Mantle
convection currents
Upper mantle N = normal magnetism R = reversed magnetism
The Atlantic Ocean is getting wider at the same speed that your fingernails grow.
Passive margin USA
North American plate
Atlantic Ocean Mid-ocean ridge Europe
Eurasian plate
Fig. 11 Exam Diagram: How sea-floor spreading is making the Atlantic Ocean wider. Note the passive margin. This is where continental crust and oceanic crust share the same plate. The junction of the two types of crust is not an active plate margin.