Studies of sedimentary rock in Antarctica have produced conclusive evidence of palm trees. How is that possible when nothing other than primitive vegetation grows there today? Satellite measurements confirm temperatures as cold as -135F and can reach well below zero even in the middle of “summer.” Yet the rocks provide clear evidence: Palm trees once grew there.
Sharks teeth are routinely found in sedimentary rock in Minnesota, yet there are no oceans here today for them to swim around in. Where did they come from?
These are just two examples that prove the planet was quite a bit different during prehistoric times. Antarctica was once a lot warmer and an ocean must have covered much of Minnesota. In northeastern Minnesota there is even more compelling evidence for a drastically different landscape: The iron ore deposits.
About two billion years ago, when an ocean and actual mountains occupied Northern Minnesota, vast amounts of iron washed into the sea. At this distant time, the atmosphere contained little free oxygen, but primitive plants in the oceans were using photosynthesis to release oxygen into the water from carbon dioxide. The released oxygen combined with iron producing iron oxide (what we call rust). Vast amounts of this substance settled to the ocean floor. Today, we mine these deposits as either magnetite or hematite.
The mountains of prehistoric Minnesota are long gone, having been worn away by wind, water and multiple glacial advances and retreats. In southern Minnesota, the seas that supported sharks have also long ago drained away. But road cuts through limestone deposits the ocean left behind are packed with a wide array of fossils of plants and animals from the distant past.
What happened to the ocean that once covered much of central and eastern North America? What happened to Antarctica to turn it from a tropical paradise to a perpetual deep freeze? Well, it’s complicated, but also extremely interesting.
Antarctica’s story begins on a place on Earth’s surface where the temperatures were subtropical. Carbon dioxide levels in the atmosphere were a lot higher than today, which contributed to greater warming. Ocean currents were not constrained by land masses as they are today. The result was a planet that did not have polar ice caps or winter anywhere. But Antarctica, through a process of plate tectonics, moved south towards the South Pole. Mountain chains rose, interrupting warming ocean currents. The Earth cooled, and the epochs of worldwide warmth came to an end. It is an amazing transformation and seems almost mythical if not based on solid evidence.
The ocean that once covered much of the U.S. drained away as the land slowly rose. Even though this process was nearly complete millions of years ago, there exists today clear evidence of this once great inland sea. There are the vast limestone deposits. Great Salt Lake in Utah is an existing remnant. The Sand Hill country of Nebraska is evidence of the gradually receding ocean coastline. In the eastern U.S., the vast sand and pine country that extend 100 miles into the Carolinas are the beaches of a receding ocean. How do we know? Because of evidence preserved in those deposits of the life that once lived there.
The Earth has drastically changed its face several times in its geologic history. By volcanism, by plate tectonics, by mountain building, and by the life that arose here. The Earth would be a very different place without the presence of life, from the earliest forms that changed the composition of the atmosphere and oceans and that literally created land in the form of limestone and iron deposits. And the Earth is not done changing. The formative processes will continue. Because in nature there is no end game, only continual change.