When a plate located at one of the poles is isolated, it receives little to no sunlight in winter. Over millions of years, often in conjunction with other natural climate forcings such as the Milankovich Cycle, the land may eventually be covered in an ice sheets. Ocean currents also play a major role, as they can transport heat from the tropics to the poles, or block that heat when a plate is isolated. 

This is what happened to Antarctica. When the tectonic plates that made up the supercontinent Gondwana began breaking apart 118 million years ago, Antarctica headed south along with Australia and South America (Fig. 1). Around 30-35 million years ago both the South American and Australian plates separated from the Antarctic plate and headed north. This changed where the ocean currents moved. A new current, the Antarctic Circumpolar Current (ACC) also formed, preventing most of the warmer tropical currents from reaching Antarctica. As the continent grew colder, ice caps formed, which led to more cooling in a negative feedback loop. 

If two plates are separating at a region where the rocks contain significant quantities of carbon, large quantities of carbon dioxide (CO₂₎ can be emitted into the atmosphere. For example the East Africa rift releases about 20 megatonnes of CO₂ every year from magma below the crust (top image).