The massive tectonic collisions in the tropics may have caused the last three great ice ages on Earth.
Before each of these ice ages, new research discoveries, collisions between continents and island arches built long mountain ranges in tropical latitudes. These mountains could have prepared the ground for a cold climate: as they eroded into the seas, they would alter the chemistry of the ocean so that it could absorb more carbon from the atmosphere. Since atmospheric carbon traps heat, less carbon in the skies translates to cooler temperatures, allowing the formation of ice caps and glaciers
"This could provide a simple tectonic process that explains how the Earth enters and exits glacial periods" said study co-author Oliver Jagoutz, professor of geology at the Massachusetts Institute of Technology. [Earth̵
Brief history of ice
During the Phanerozoic Era, which covers the last 540 million years, the Earth has been free of ice 75% of the time, even to the north and south Pali But the planet has also seen three glacial periods, or glaciations, when at least some permanent ice sheets existed. The first was in the late Ordovician period, from 455 million to 440 million years ago, when the first gaping fish were evolving. The second was in the Permo-Carboniferous, from 335 million to 280 million years ago, the era of amphibians and bizarre reptiles similar to mammals such as the dimetrodon. The last ice age is underway. It began about 35 million years ago, when the Antarctic ice sheets of modern times were formed.
Small glacial advances, such as the ice age that ended about 11,700 years ago, are not the subject of this study. Brief progress and retreats of glaciers have occurred due to variations in the earth's orbit that redistribute the heat of the sun, Jagoutz told Live Science. The embarrassing question is why the Earth has periods of ice, period.
"It seems that the state of the climate in which the Earth loves to be warmer than today, and these periods of glaciation are unusual," said Jagoutz. "And if they are unusual, something specific must happen."
Collision and erosion
Jagoutz and his colleagues believe that "something specific" is the formation of mountain ranges in the tropics.
At first glance it might seem strange that tropical mountains can create an ice age. But the atmosphere, the oceans and the land are all connected. The continental crust is rich in silicate minerals. When these heavy silicate rocks corrode and dissolve in the oceans, they make seawater more alkaline or basic. The carbon dioxide from the atmosphere dissolves easily in this alkaline sea water. The more alkaline it is, the more carbon it can hold the ocean.
Currently, the human use of fossil fuels is exceeding the capacity of the ocean to retain carbon. Over the past 200 years, seawater has become 30 percent more acidic. Millions of years ago, huge mountain-building events could have done just the opposite, making the ocean more alkaline. The tropical mountains, in particular, would have done the job efficiently. The tropics are wet, so erosion occurs rapidly and the rocks raised by tropical tectonics are rich in easily soluble magnesium and calcium.
The idea that tropical erosion could have affected the climate was not new, but Jagoutz and his group were the first to collect a database of all the geological records of these important tectonic collisions and compare them with the beginning of the ice ages. They found that beyond the Phanerozoic, the length of the active collision areas between oceanic and continental plates – called "sutures" – ranged from zero to 18,640 miles (30,000 kilometers). Each of the main ice ages was preceded by a peak in the length of these collisions active in the tropics, when the sutures ranged between 6,214 miles and 18,640 miles (10,000 and 30,000 km).
"Whenever you had an ice age, you had an increase in the length of the suture area in the tropics," said Jagoutz.
The geological traces left by these ancient collisions are called ophiolites, which are oceanic and volcanic rocks pushed onto the continental crust. The researchers did not see any of these extreme ophiolites at times when the Earth was not frozen. And it was the ophiolites in the tropics, or regions less than 20 degrees latitude, that seemed important to cool the planet.
There are other theories as to why the Earth has ice periods, Jagoutz said, namely that volcanic activity varies, pumping more or less carbon into the atmosphere. But data on the history of volcanism do not always coincide with glacial periods, he said, and the volcano theory does not offer a good explanation of why ice ages should stop and begin. The tectonic explanation does a good job: once the mountain ranges rich in calcium and magnesium erode completely or move from the tropics through the drift of the continents, their effect on the climate fades and the Earth returns to its typical mild state.
Jagoutz and his colleagues requested a grant from the National Science Foundation to further their theory. Whether right or wrong, tropical mountains will not be able to save humanity from climate change caused by man at any time. This mountain-building process occurs over millions of years, Jagoutz said, and has little to do with the kinds of variations that determine whether, for example, Miami is habitable or flooded by rising sea levels. Some researchers, however, pondered geoengineering schemes that would demolish calcium or magnesium-rich rocks and spread them in tropical oceans, he said, or injecting carbon dioxide into similar rocks.
"People want to use this natural process to help with climate change caused by man, [but[therearemanyproblemswithmenandwomeninobtainingthisprocessinaclassoftimethatisrelevanttothoseoftheages?"Jagoutzhadetto"Thisishardtodo"
Originally published in Live Science .