Earth science for schools by Moorland School

Are the oceans spreading at the mid-ocean ridges?

Before the 1950's we knew very little about the ocean floor. It was quite easy to find out the shape of the ocean floor because a ship could use sonar to scan the seabed  directly below the ship. The problem was that such a vessel would have to sail over just about every spot in the worlds oceans to produce an accurate map.

After World War II the Russians and Americans were locked into the 'Cold-war'. Submarine warfare and control of the oceans was strategically important to both sides. The US Navy badly needed accurate maps of the worlds ocean floor and the only way to do this was for a ship to sail backwards and forwards taking soundings. The results produced some astonishing pictures.

Magic magnetism

The yellow dot shows a massive underwater chain of mountains that stretch right down the Atlantic. It is called the Mid-Atlantic Ridge and it is an area where new plate is constantly being created. Oceanic surveys found that such mountain chains extend all over the world. A more detailed picture can be found here.

The ocean floor rocks are made from magma that has been erupted into the water at the mid-ocean ridge. The two plates are moving away from each other at the ridge and new plate is created. The rock is called Basalt (What size crystals would they have??   answer )  These rocks contain minerals that are magnetic. One such mineral is called magnetite. If magnetite is heated over 500c (called its Curie temperature) it loses its magnetism. So the magma pouring out at the ridge will not be magnetic until it cools to form new Basalt. The key point is that when magnetite is cooled it once again becomes magnetic (a phenomena called remanent magnetism).


The next bit is definitely tricky..... NORTH is SOUTH. To explain this we need the think about what gives the Earth its magnetic field. The answer is that the core is made up of iron and nickel which are both magnetic, however at 5000c it's not quite like a bar magnet that we're used to in the science lab. The outer core is molten and the convection currents are thought to act like a giant dynamo which produces electricity and induces the Earth's magnetic field. What is interesting is that the poles keep flipping every couple of million years, so north does indeed become south. These reversals give us the next big clue, here is the sequence:-

  • Magma rises and erupts onto the ocean floor forming new basalt rock at the mid-ocean ridge.

  • The magnetic polarity will be locked into the rock as it cools on the sea-floor

  • These eruptions continue  and the plates grow and spread apart.

  • After a couple of million years the Earth's polarity flips.

  • New magma rises and erupts onto the ocean floor. The NEW magnetic polarity will be locked into the rock as it cools.

  • These eruptions continue  and the plates grow and spread apart.

If this were to continue you would expect rocks to be magnetised either one way or the other:-


Detailed study of the sea floor with magnometers (devices which measure magnetism) gave some amazing results. The zebra like pattern below is what a segment of ocean crust actually looks like (in magnetism terms that is). The pattern was centered along, and symmetrical to, the mid-ocean ridge. :-

Diverging ridge magnetism.gif (32515 bytes)

Lava erupting today would preserve a positive magnetic anomaly because the Earth's north magnetic pole is in the northern hemisphere.

VIPThe key point to all this is that the magnetic characteristics of the rocks helped geologists to prove the theory of sea floor spreading.


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