5.6.2 Plate tectonics and climate change
This course began by considering the evidence in the Earth's past for the existence of supercontinents and how evidence of past climates recorded in continental rocks can be used to reassemble ancient continental configurations. The evidence was interpreted in such a way that the continents were considered as passive recorders of the surface conditions that they have experienced on their inexorable passage across the Earth's surface. While such an assumption is broadly correct, it does not take more than a momentary glance at a map of the world today to realise that the disposition of the continents has a marked effect on both local and global climate. Not the least of these effects results from the difference in the thermal properties of land versus ocean - a continental region will be colder in winter and warmer in summer than an oceanic region at any given latitude. Moreover mountain belts formed as a consequence of plate tectonic activity dramatically modify rainfall through the effects of orography - the development of a rain shadow on the leeward side of mountain belts.
Global climate is also strongly controlled by ocean currents. For example, northwestern Europe is significantly warmer than other regions at similar latitudes because of the warming effects of the Gulf Stream and North Atlantic Drift. The reversal of oceanic currents in the equatorial Pacific - a phenomenon known as El Niño - has a far-reaching effect on climate around the Pacific. Ocean currents depend on the geometry of the oceans and this is controlled by plate tectonics. Hence, over geological timescales the movement of plates and continents has a profound effect on the distribution of land masses, mountain ranges and the connectivity of the oceans. As a consequence, plate tectonics has a very direct and fundamental influence on global climate.
To illustrate this effect, the next page briefly describes the opening of a seaway between the southern tip of South America and Antarctica, and how that affected global climate.
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.
Question 24
What other major change in global plate motions occurred between 43 Ma and 50 Ma?
Answer
The change of orientation of the Hawaiian hot-spot trace shows that at this time the Pacific Plate changed from a northward velocity direction to a northwestward direction.
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.
This example shows how plate tectonics, continental drift and the opening and closing of seaways can have a profound influence on both local and global climate. Throughout the Phanerozoic there were long periods when the Earth was much warmer than today - often called a 'greenhouse' climate - and other times when it was cold - called an 'icehouse' climate. These cycles, like the Wilson cycle, occur over periods of 100 Ma, reflecting the timescale of plate movements and the growth and destruction of oceans. Given the clear link between ocean circulation and climate, and the similar timescales of global climate change and plate motions, it is inescapable that one of the chief controls on long-term changes in the global climate must be plate tectonics.