June 1, 2008
Author’s note: The beautiful illustrations in this article are by Northern Arizona University’s Ron Blakey. For more illustrations of how the Earth evolved throughout time, go to his Web site at http://jan.ucc.nau.edu/~rcb7/globaltext2.html
Recent Carved In Stone articles have described the life of Rocky, a 1.8-billion-year-old rock from Morrison, Colo. Last month, this column described the childish antics of two paleontologists fighting about dinosaur bones in Rocky’s backyard. One of the dinosaurs discovered in Morrison was Apatosaurus, also popularly known as Brontosaurus (which, as any school kid will tell you, is incorrect).
Luckily for it and its friends, the climate in Colorado during Apatosaurus’s reign here was much different than the cool, dry, continental climate we have today. This is, in part, because the movement of continents over the Earth’s surface (referred to as plate tectonics) has a pronounced affect on its climate. As demonstrated in the series of photos accompanying this article, climatic changes due to plate tectonics take place throughout tens of millions of years. These long-term climate changes are not the same as the more rapid climate changes being debated by scientists in the present.
Let’s begin by looking at the Earth about 240 million years ago (during the Triassic Period). All the continents clumped together to form a supercontinent called Pangea. The large land mass extended nearly from pole to pole, severely disrupting the ocean currents. That fostered hot, arid, continental climates and the melting of the Polar ice caps. If you are wondering how ocean currents affect climate, just think of El Niño.
About 150 million years ago (during the Jurassic Period) when the Apatosaurus was here, North America was drifting away from Europe, and South America was just beginning to drift away from Africa. This opened a deep-water gateway across the equator (the Atlantic Ocean), which dramatically transformed oceanic currents and the climate. Morrison was wet, swampy, and warm.
The continents continued to drift apart, ocean currents modified their course and intensity, and the climate changed accordingly. About 90 million years ago (during the Cretaceous Period), global surface temperatures were about 18o F (10o C) warmer than those we have today. That is the hottest climatic period recorded in geologic history. Morrison, on the other hand, was at the bottom of a great interior seaway.
These illustrations demonstrate how the Earth’s climate has changed in response to the forces of plate tectonics. Climate has changed as the result of other natural forces such as asteroid impacts, changes in composition of the atmosphere, and ash and carbon dioxide from volcanoes. But that’s another story.