Earth's steady state is warm and balmy, but half a dozen times over the past billion years, the planet developed ice caps and glaciers. Researchers have now amassed evidence that these cold snaps occurred when tectonic activity propelled continents headlong into volcanic island arcs in the tropics, uplifting ophiolites that rapidly absorbed carbon dioxide, cooling Earth. Once collisions stopped, CO2 again built up from volcanic eruptions and a runaway greenhouse effect warmed the planet.