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.
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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 500°c (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).
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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 5000°c 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:-
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Magma rises and erupts onto the ocean floor forming new basalt
rock at the mid-ocean ridge.
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The magnetic polarity will be locked into the rock as it cools
on the sea-floor
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These eruptions continue and the plates grow and spread
apart.
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After a couple of million years the Earth's polarity flips.
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. :-
Lava erupting today would preserve a positive magnetic anomaly because the
Earth's north magnetic pole is in the northern hemisphere.
 The
key point to all this is that the magnetic characteristics of the rocks helped
geologists to prove the theory of sea floor spreading.
Go back to the tectonics page
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