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This is a high-resolution
physiographic map of the South Atlantic. Notice
South America on the left, Africa on the right, and the Mid-Atlantic
Ridge (MAR; a Divergent Boundary and
Mid-Ocean Ridge [MOR]) in the center.
As you can see, the coastlines of South America and Africa
are nearly parallel, and match just about perfectly to the
MAR. The reason for that is that the MAR is the
location of where Africa and South America before they
split into two continents. [If you want to see the whole map of the world, click here] [This map is reduced in size. To see it full size, right-click on the map and click on "View Image"] |
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This is a
simplified drawing of the above map, and you can see
that the east coast of South America and the west coast
of Africa have neatly split away from the MAR. The
shape of the split is dependent on many factors such as
pre-existing geology and the (still not completely
understood) processes that causes a continent to split
into two, but for our purposes we can think of that
shape as fairly random. |
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We run
into a problem because of the curved nature of the MAR
(or any MOR). In every case MOR that has been looked at, the direction of plate motion next to the ridge (the spreading direction) is ALWAYS at a right angle (90o) to the ridge. |
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This
means that the red arrows are the directions of
spreading. There are 2 problems with this. 1. If spreading is at 90o to the ridge, then at A, the crust would be getting compressed together and at B the crust would be stretched out - and there's no evidence for either. 2. We can see from the shape of the coastlines and the MAR that the direction of spreading MUST be as shown by the blue arrows. So how do we reconcile these two seemingly contradictory statements? |
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What
happens in a real MOR is that after the original break
between continents, the MOR breaks into a series of
short segments, each of which is perpendicular to the
direction of spreading*. In the drawing to the
left, the short segments of MOR are shown in brown and the direction of
spreading is in red. Let's look closer at the 2 segments within the oval. *I'll add that it's not entirely clear about how this happens, but it seems to happen automatically, probably due to a lessening of the forces necessary to cause the spreading. |
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Notice
that everything to the left of the 2 segments is moving
to the left. Therefore everything to the left of
the MOR is part of the same plate. Everything to the right of the 2 segments is moving to the right. Therefore everything to the right is part of the same plate. And next to the green line connecting the two MOR segments, the directions of motion show that the the two plates are sliding side to side. This tells us that we are looking at a TRANSFORM margin. Most transform boundaries around the world are within MORs. |
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The top left image here is
part of the image at the top of this page, blown
up. On the bottom left image here, the MORs are shown in brown,
the direction of spreading in red, the transform
boundaries in dark green, and the light green represents
"Fracture Zones", the scar left behind from the
Transform boundary. Within the Fracture Zone, no
relative movement between the sides occurs, so it is no
longer a plate boundary. The Fracture Zones are
parallel to the direction of plate motion, and can be
used to determine plate directions as a result.
Take a look at the map at the top of the page. If
you look at it full-sized, you can see the fracture
zones actually trace out movement, and you can see what
point on the east coast of South America corresponds to
its equivalent on Africa's west coast. Most Transform Boundaries occur like this. Every Divergent Boundary is formed from many short segments of Mid-Ocean Ridge separated by Transform Boundaries. There are only 2 Transform boundaries that I know of on land. The first is the Alpine Fault on the South Island of New Zealand, which is wrenching apart the island. The other is much better known in the US: the San Andreas Fault, running from the north end of the Gulf of California to Point Reyes National Seashore just outside of San Francisco. The Gulf of California is a new ocean, but is opening up mostly North-South rather than East-West. If you look at the global image I have for the entire Mid-Ocean Ridge system, you can see this. |