Geology Reference
In-Depth Information
They used their record of the seafloor rock ages and the distance of the rocks
from the Mid-Atlantic Ridge to calculate the distance each plate moved per year.
The answer is approximately 18 millimeters per year for the last 3 million years.
As they continued to map the magnetism and ages of the seafloor rocks, scientists no-
ticed that the rate of plate movement varies quite a bit. Along the Mid-Atlantic Ridge,
the crust appears to be spreading more slowly near Iceland than it is near more south-
ern parts of the ridge.
To refine calculations of plate velocities, modern scientists have begun using the Global
Positioning System (GPS) of satellites in orbit around the earth. You probably have a
GPS system in your car or phone that is very similar to (though much less precise than)
the one scientists use to measure plate movements. The changes in distance and the dir-
ection of plate movement have been recorded by the GPS satellites. These findings con-
firm previous calculations based on magnetic reversals in the seafloor basalts. Now geo-
logists rely on GPS tracking of the plates to help answer questions about current plate
movements and tectonic processes, which I describe in Chapter 9.
Unifying the theory
What began with ideas about continental drift has been developed into a foundational
explanatory theory of how earth's crustal plate system functions now and functioned in
the past. The confirmation of seafloor spreading in the Atlantic Ocean was just the first
step.
If the seafloor is spreading along the Mid-Atlantic Ridge, what is happening along other
edges of the crustal plate? This is where plate tectonics theory extends its explanation
to include mountain building, volcanoes, and earthquakes, which I explain in Chapter 9.
