Massive tectonic collisions cause ice ages, study finds

The Earth’s climate has remained remarkably stable for hundreds of millions of years, but there are some noteworthy exceptions—like the ice ages of 80 and 50 million years ago, during which the planet’s temperature and carbon dioxide levels plummeted dramatically.

There have been plenty of theories as to why these events happened, but now geologists from MIT have made a discovery that puts a fascinating new spin on how the ice ages got their start—and it involves continental collisions.

According to their research, which can be found in Proceedings of the National Academy of Sciences, massive tectonic collisions shook the Earth right before each ice age near the equator—a tropical area where rocks undergo heavy weathering thanks to environmental conditions like heavy rain. When this weathering happens in these rocks, large amounts of carbon dioxide are absorbed from the atmosphere, setting the planet up for rapid cooling.

“Everybody agrees that on geological timescales over hundreds of millions of years, tectonics control the climate, but we didn’t know how to connect this,” said Oliver Jagoutz, associate professor of Earth, Atmospheric and Planetary Sciences (EAPS) at MIT, in a statement. “I think we’re the first ones to really link large-scale tectonic events to climate change.”

What happened, exactly? How did this cause an ice age?

According to the researchers, the actions that led to the first ice age in question began around 90 million years ago, when the northeastern edge of the African tectonic plate collided with an oceanic tectonic plate in the Neo-Tethys Ocean, creating a chain of volcanoes. 10 million years later, as the continent of Africa continued to move northward, the oceanic plate was thrust farther up and over the African plate—which both exposed a large amount of ocean rock to the atmosphere while simultaneously terminating the volcanoes.

The second ice age, meanwhile, was preceded by the collision of India with Eurasia about 50 million years ago—an event which also pushed an oceanic plate up and over a continent (Eurasia in this case) and exposed rock to the atmosphere.

Both of these collisions happened in the Intertropical Convergence Zone, a region around the Earth’s equator where trade winds come together to create high temperatures and heavy rainfall. Moreover, the kinds of oceanic rock exposed—like basalts and ultramafic rocks—fall into the category of rocks that undergo silicate weathering upon exposure to intense heat and rainfall. Silicate weathering is a process by which these rocks undergo chemical reactions in such climates, causing the rock to absorb carbon dioxide from the atmosphere.

So naturally, the team hypothesized that the sudden exposure of massive amounts of ocean rock around 80 and 50 million years ago led to equally massive amounts of carbon dioxide being removed from the atmosphere—thereby plunging the Earth into ice ages. They also believed that the collisions buried volcanoes that had been adding carbon dioxide to the atmosphere, thereby turning off major contributors to the atmospheric greenhouse gas.

To test this idea, the researchers examined the weathering rates of different kinds of rock, including granites and basalts. By applying these rates to models that estimated how much oceanic rock had been exposed to the air in the two separate collisions, and by calculating how much carbon dioxide these rocks could absorb, they were able to calculate the change in atmospheric carbon at the time periods.

As it turned out, the levels of carbon dioxide plunged at the exact time the collisions occurred, and as did the temperature of the oceans—an effect that was drawn out over time as the tectonic plates continued to crash together, exposing more and more oceanic rock.

Modern analogue

Perhaps most interestingly, this same event is happening today—only on a much smaller scale. The Australian plate is colliding with and lifting an oceanic plate near Java, which has become a “a huge carbon sink,” according to Jagoutz.

“What nature shows us is, if you put a lot of these rocks in the tropics, where it’s hot, muggy, wet, and rains every day, and you also have the effect of removing the soil constantly by tectonics and thus exposing fresh rocks, then you have an excellent trigger for ice ages,” Jagoutz said. “But the question is whether that is a mechanism that works on the timescale that is relevant for us.”

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Image credit: Thinkstock

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