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Solution to Exercise 2
Earth is about 25,000 miles around the equator. The continents were all packed together as "Pangaea" about 250 million years ago. So I estimate that a typical continent has moved halfway around the world (12,500 miles) since the time of Pangaea. So that's a speed of v = 12,500 mi/ 250,000,000 yr = 0.00005 miles per year. At that slow rate, it's not really sensible to compute the speed in miles per hour - it'd be such a small number, there's no good reference. So let's try inches per year: v = 0.00005 mi/yr * 5280 ft/mi * 12 in/ft = 3.168 inches per year Again, since I've been using "numerical common sense" to estimate my numbers, keeping more than one or two decimal places would be adding false precision. So I estimate that the continents are moving 3 inches per year. (This means, by the way, that trans-Atlantic phone cables need to be lengthened every few years!) This answer, and the way I arrived at it, may seem way too simplistic. Am I really close to the right answer? Well, according to a page at the U. S. Geological Service website: "Pangaea broke up and gave birth to the Atlantic Ocean, which has been widening a few inches a year ever since, crowding our side of North America toward the Pacific." In other words, this simplistic calculation is right on target! |