The climate of modern Antarctica is extreme. Located over the South Pole and in total darkness for six months of the year, the continent is covered by glacial ice to depths in excess of 3 km in places. Yet this has not always been the case. 50 Ma ago, even though Antarctica was in more or less the same position over the pole, the climate was much more temperate – there were no glaciers and the continent was covered with lush vegetation and forests. So how did this extreme change come about?

The modern climate of Antarctica depends upon its complete isolation from the rest of the planet as a consequence of the Antarctic Circumpolar Current that completely encircles Antarctica and gives rise to the stormy region of the Southern Ocean known as the roaring forties. The onset of this current is related to the opening of seaways between obstructing continents. Antarctica and South America were once joined together as part of Gondwana and were the last parts of this original supercontinent to separate. By reconstructing continental positions from magnetic and other features of the sea floor in this region, geologists have shown that the Drake Passage opened in three phases between 50 Ma and 20 Ma, as illustrated in Figure 32. At 50 Ma there was possibly a shallow seaway between Antarctica and South America, but both continents were moving together. At 34 Ma the seaway was still narrow, but differential movement between the Antarctic and South American Plates created a deeper channel between the two continents that began to allow deep ocean water to circulate around the continent. Finally, at 20 Ma there was a major shift in local plate boundaries that allowed the rapid development of a deep-water channel between the two continental masses.

The coincidence of the change in motion of the Pacific Plate with changes in plate motions between S. America and Antarctica shows how the motions of all the plates are interconnected – a change in the true motion of one plate leads to changes in the true motions of many others.

While these plate motions were taking place the effect on Antarctica was profound. By 34 Ma the climate cooled from the temperate conditions that previously existed. This was sufficient for glaciers to begin their advance, and was followed by a period of continued cooling until at about 20 Ma, glaciation was complete. Even though the Drake Passage first opened at 50 Ma it was not until it opened to deep water at 34 Ma that glaciation really took hold

Today, the Antarctic Circumpolar Current is the strongest deep ocean current and its strength is responsible for the ‘icehouse’ climate that grips the planet. The opening of the Drake Passage had both a local and a global effect, initially cooling the climate of Antarctica from temperate to cold and ultimately playing an important role in the change from global ‘greenhouse’ conditions 50 Ma ago to the global ‘icehouse’ of today.