By the early 1960s it was established that the earth was making new
crust, on the ocean floor, all the time. However, since the earth is not
expanding, where is this new land going?
The answers began to appear in 1964, following a major
earthquake in Alaska. Geologists visiting the area immediately after the quake
were, at first, confused by reports about changes in sea level. Some areas were
claiming that the sea level had fallen, and others that it had risen. Despite
searching, the geologists could find no evidence of a crack in the earth's
surface. A quake with a magnitude as great as this had to have an associated
fault line (crack), but where was it?
A particularly observant geologist noticed a strange
phenomenon. Along the entire coastline, there was a line of barnacles, still
living, way above the highest level that the sea reached. In some places it was
as much as 20 feet above the present levels of water. The fact that the
barnacles were still living meant that they had been in seawater recently. From
this, scientists were able to state that the sea level at the coast had
definitely fallen, or more accurately, the land had risen.
Further inland though, the story was the complete opposite.
Areas, which had been dry for years were suddenly sitting under water. There was
no doubt that these areas had dropped during the earthquake.
The Fault Line
By drawing diagrams, and making models, the scientists came
to the conclusion that the fault line must lie under the earth's surface. The
areas of uplifting and sinking indicated that the line ran from an area just off
the coast, diagonally downwards, under the land. They also noticed that just
where their theoretical fault line was, there was a deep trench in the ocean
floor. Trenches like it can be found all the way around the Pacific Ocean rim.
Putting it Together
Now the scientists had the following information:
- The earth makes new crust on the ocean floor
- The crust moves outwards as newer crust is formed
- The earth is not getting larger
- There is a trench all the way around the Pacific Ocean
- There is a fault line running from the trench, under the land
It didn't take them very long to decide exactly what was
happening to the extra land. It was disappearing down, under the
continents, at the trenches. The trenches are like a huge crack in the ocean
floor, where the newer rock slides under the older rock. As it slides, the
weight of the older rock pushes it downwards. Eventually it reaches the outer
core of the earth, which is extremely hot and molten. Here it heats up and
becomes molten, joining all the rest of the molten rock, to become magma again.
In a way it is like a natural recycling plant. At the mid
ocean ridge, hot molten rock, or magma is forced upwards. As it comes into
contact with the cool ocean it solidifies, becoming new solid rock. The rock is
gradually pushed outwards my more rock as it forms. The entire ocean floor is
being pushed outwards towards the continents. At the edges of the continents,
the new rock is pushed downwards, under the continents. The solid rock heats up
and becomes magma again. Then the cycle begins all over again
Is That the End of the Story?
It might seem that the story is finished now. We have
successfully explained how the continents were pushed apart. We have proved that
the ocean floor is constantly being renewed. We have seen where the extra ocean
floor goes. However, it isn't quite the end of the story. The theory of
continental drift once it got this far, became known as the theory of Plate
Tectonics. So now we need to look at Plate Tectonics, and also check those facts
about disappearing earth. Not all of it goes down under the continents, some of
it goes up!