February 1, 2008
During a recent visit to Phoenix, my grandkids, Donovan and Delaney, and I watched some ants wandering around on a great big leaf that was slowly drifting across a pond. I was tempted to point out how the ants and the leaf were analogous to people walking around on a continental plate that is slowly floating around the surface of the earth. I resisted the temptation because they are just kids who haven’t even started school yet. I figured they would like plates of cookies better than plate tectonics.
But, like it or not, plates of the earth’s crust are constantly on the move. Plates on either side of the East Pacific Rise, one of the fastest moving mid-ocean ridges, spread apart about 6 inches each year. North America and Eurasia spread apart along the Mid-Atlantic Ridge at a rate of about 1 inch per year. Now, this isn’t enough to explain the increase in airfares from New York to London, but throughout millions of years, things add up.
Last month, this column introduced Rocky, a grey and pink gneiss (layered metamorphic rock), which was rescued from a quarry near Morrison, Colo. His current location on earth can be recorded as 39º38’10” N latitude, 105º12’24” W longitude. But Rocky hasn’t always been at that location. The forces of plate tectonics have pushed Rocky’s home around the surface of globe for billions of years.
The three maps in Figure 1 show how the earth looks today and how it might have looked 300 million years ago (Ma) and 600 Ma. The grey dashed line shows the path of Rocky’s journey across the earth starting about 600 Ma. The arrowheads, spaced at 300-million-year intervals, point to what, today, would be considered north. You can see how, during the past 600 million years, North America has rotated about 90 degrees counterclockwise and moved all the way from the southern hemisphere to its current location in the northern hemisphere.
So how do scientists know that the continents have wandered over the earth? The answer lies in the fact that some rocks possess what is referred to as “remnant magnetism.” When crystalline rocks form, the magnetic minerals they contain are “frozen” according to the surrounding field at the time they were created. By carefully measuring the orientation of the remnant magnetism, it is possible to determine the latitude where the magnetic rocks were formed.
I figure that by the time North America and Eurasia drift another half a foot further apart, Donovan and Delaney will want to hear this story. Until then, you’ll have to do. Thanks for listening!