>>In previous videos we have discussed the
basics of plate tectonics. Now we're going
to go into some more detail and present some
of the evidence for the theory. The overall
theory of plate tectonics started with Alfred
Wegener in 1912, 1915, approximately that
time. He proposed the hypothesis of continental
drift. And he suggested the continents were
all together at one time in a supercontinent
called Pangea, and then they separated to
produce the current configuration. What was
his evidence for this? Well, it was the fit
of the continents. You can put all the continents
together, and they sort of fit together as
a jigsaw puzzle. When you put the continents
back together, mountain ranges would line
up across different continents, and rock types
would be found in logical places. For example,
limestones, which form in warm, shallow water,
would be found near the equator. He also found
that fossil distributions lined up across
the continents. After he proposed his hypothesis,
it was debated, but it was not accepted. By
the 1930s, it wasn't even in textbooks. In
the 1950s, however, there was a revival of
interest because of new scientific evidence,
and there were two lines of evidence that
provided the impetus for this. The first was
fossil magnetism or paleomagnetism.
Before I explain how this is related to plate
tectonics, some basics. The earth has a magnetic
field. The field has intensity. It has a declination.
The declination is that needle points to the
north, it points toward magnetic north. But
the magnetic field also has an inclination.
And if you had a magnetic needle, the inclination
would be what the needle would do in the vertical.
It turns out at the North Pole, the inclination
is very steep down, 90 degrees. At the South
Pole, it's steep up at 90 degrees. And at
the equator it's 0 degrees. So it changes
from very steep at the poles to shallow or
zero at the equator. So why is this important?
It's important because it changes with latitude.
So let's think about this in terms of a rock.
Let's say we have a lava, and it's cooling.
As it cools, magnetic minerals within that
rock acquire the earth's magnetic field for
wherever they are on the earth. For example,
if you have a lava forming at the equator,
what will the inclination be? It will be very
shallow. On the other hand, if it forms at
very steep latitudes near the North or South
Pole, it's going to have a very steep inclination.
One of the first continents scientists looked
at was India. At about 180 million years ago,
rocks that formed on India had a steep up
inclination of about 65 degrees. By 65 million
years, the inclination had changed to 25 degrees
up. By 20 million years, the inclination was
down. So what actually happened to India?
It moved from the south, crossed the equator,
and then what happened to it? It hit Asia
and formed the Himalaya Mountains.
Fossil magnetism or these changes in inclination
provided evidence that Wegener was right and
the continents did move, but we still did
not have a mechanism to explain how they moved.
That mechanism comes from studies of the oceans,
and it's the second line of evidence that
was developed by scientists. There are magnetic
anomalies in the oceans that are parallel
to the mid-ocean ridges. We also know the
rocks at the ridge are younger. As you move
away from the ridge, they get older. What
happens is magma comes up, it becomes rock,
and then it moves off to the side. This is
called seafloor spreading, and you should
read about this in your online text. Seafloor
spreading provides the mechanism for moving
the plates.
The theory of plate tectonics is based on
seafloor spreading, paleomagnetism or fossil
magnetism, as well as Wegener's evidence for
the fit of the continents. The theory is also
based on the distribution of earthquakes,
which define the subducting plate at subduction
zones.
