16. Divergent Plate Boundaries
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Contact Don Reed
Dept. of Geology
San José State University
Divergent plate boundaries have the following
characteristics:
1) marked by mid-ocean ridges,
2) plate motion is away from boundary,
3) lithosphere is created,
4) regions of high flow from the crust and mantle due to volcanism,
5) exhibit seafloor spreadingFrom the information in the diagram below, let's calculate the rate of seafloor spreading, in other words, how fast the two plates are moving apart at a divergent plate boundary.....
We previously learned that we could use the pattern of marine magnetic anomalies as evidence for seafloor spreading. We can also use the magnetic patterns to determine the age of specific regions of the oceanic lithosphere or when that region formed by seafloor spreading ...... since we have a time-scale for the reversals of the Earth's magnetic field over the past 200 million years and can match this scale to the magnetic patterns mapped by oceanographers.
Above the pattern of marine magnetic anomalies, you will find a small portion of the magnetic reversal time-scale -- back to 4.5 million years ago.
Let's determine the average rate of seafloor spreading since the lithosphere at point A was formed. Let's also assume for this problem that seafloor spreading is symmetric, i.e. the rate of plate formation is the same on the left side for Plate NA as it is for the right side for Plate EU. Often in science we need to make simplifying assumptions.
So now we need a distance and an age of A.
The oceanic lithosphere at A is 50 kilometers (km) from the divergent plate boundary marked by the mid-ocean ridge. So the plate on the left side of the plate boundary has moved 50 km since it was formed. By matching the marine magnetic anomalies to the time-scale we find that the oceanic lithosphere at A is 3 million years old. So the plate on the left has moved 50 km in 3 million years (my) or.....
50km/3my = 16.7 km/my (kilometers per million years), which is equivalent to 1.67 centimeters per year or about 0.7 inches per year....but we are not done yet. We need to consider the right side of the ridge....remember we assumed symmetric seafloor spreading.
We therefore need to multiply 16.7 km/my by 2 or
16.7 km/my x 2 = 33.4 km/my
Now this is our answer, which is the full rate of seafloor spreading or rate of motion of Plate NA away from Plate EU.
©Copyright 1999
Last Updated on June 21, 1999
