Download Are the oceans spreading at the mid

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 world’s 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.
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 ocean floor material 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? – Basalts have small crystals because the magma cools rapidly when it
touches the sea water.) These rocks contain minerals that are
magnetic. One such mineral is called magnetite. If magnetite is
heated over 5000°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 remnant magnetism) and aligns itself up with the Earth’s
current magnetic field.
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 of the Earth 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 flowing molten material 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 magnetized either one way or the other:
Detailed study of the sea floor with magnetometers (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 of this is that the magnetic characteristics of the rocks
helped geologists to prove the idea of sea floor spreading.
Paleomagnetics – the study of the magnetic properties of rocks.
When magma cools and solidifies the magnetic orientation of certain iron-bearing
minerals becomes permanently fixed in the rock. It lines itself up with the
magnetosphere (the magnetic lines of force surrounding the Earth).
The Earth’s magnetic field has reversed itself 9 times (that we know of) during Earth’s history.