In somewhat regular cycles, Earth enters an ice age, and glaciers spread across the globe. This is normally driven by changes in Earth’s orbit around the Sun, with predictable fluctuations and “wobbles” causing the planet to receive significantly less sunlight than normal.
Every now and then, however, plate tectonics play a major role in altering the global climate. As new research published in the Proceedings of the National Academy of Sciences reveals, the creation and destruction of these massive continental and oceanic plates may have been responsible for two ice ages that occurred between 80 and 50 million years ago.
“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,” Oliver Jagoutz, an associate professor of Earth, Atmospheric and Planetary Sciences (EAPS) at the Massachusetts Institute of Technology (MIT) and lead author of the study, said in a statement. “I think we're the first ones to really link large-scale tectonic events to climate change.”
Plate tectonics are thought to influence the climate in a variety of complex ways. If more land is generated at higher latitudes, atmospheric and oceanic currents transporting heat around the planet are redirected. Sunlight-reflecting glaciers will form at higher latitudes, which will cool the world.
Rock exposed to the air also provides a sink for carbon dioxide. The two react in a process known as “chemical weathering;” carbon dioxide is taken out of the atmosphere as a result, which leads to global cooling. However, an increase in volcanic activity, generated by certain plate tectonic movements, will counter this by unleashing carbon dioxide back into the atmosphere.
The two major ocean rock exposure events (gray shaded) occur just before the region registers as negatively contributing (absorbing more than emitting) to the world’s carbon dioxide budget. Ocean temperatures fall as a result. Jagoutz et al./PNAS
Taking all this into account, Jagoutz’s team used a model, one developed to recreate the movement of plates for a different study, to simulate the effect of the destruction of the supercontinent of Gondwana on the world’s climate. The model was based on ancient rocks excavated from beneath the Himalayas that have geologically preserved the tectonic history of the entire region.
About 180 million years ago, Gondwana began to fragment, and the continental plates we see today began to take shape. Then, 80 million years ago, as Africa continued to move northwards, an overlying oceanic plate was suddenly forced upwards, exposing it to the atmosphere, while cutting off the magmatic supply to a vast range of volcanoes. 30 million years later, India collided with Eurasia and another part of this oceanic plate was pushed skywards.
Vast quantities of carbon dioxide began to weather away at these exhumed oceanic rocks – chemically speaking, the best kind to absorb this commonplace greenhouse gas – just as the carbon dioxide-producing volcanoes were smothered. Additionally, these exposures occurred in the wet, humid tropics, providing optimal weathering conditions.
Overall, there was a net reduction in carbon dioxide in the atmosphere on both occasions, which cooled the climate – just enough, the researchers think, to cause an ice age without the aid of Earth’s axial wobbling.
Plate tectonics have been implicated in this way before: The break-up of the ancient supercontinent Rodinia around 800 million years ago kick-started the Cryogenian period, where temperatures plummeted to such a degree that the entire world was essentially a “snowball.”
