Earth: The Essentials

20 Jul 2005 - Special Reports, Land & Geology, Arctic, Antarctic

maps of Arctic and Antarctic

The polar regions provide unique opportunities for understanding the earth's geological history. Continents have shifted significantly over the eons and Antarctica was once a part of an enormous landmass containing all contemporary continents. Exploring the geology of the Antarctic continent and the Arctic Ocean seabed presents great challenges for polar scientists due to their inaccessibility. In overcoming these difficulties, polar research findings have added significantly to our understanding of the earth's history.

To begin at the beginning. Beneath their frozen appearances, the Arctic and the Antarctic could not be more different: the Arctic is, of course, an ocean surrounded by continents, while the Antarctic, a continent surrounded by oceans.

This fundamental difference in topography is linked directly to the Earth's geological history, notably plate tectonics and continental drift, going back billions of years. The different geological histories of the two polar regions are also major and determining factors in their distinctive glacial and oceanographic dynamics and consequently for their interactions with the earth's climate system as a whole.

Find out more in this feature about the essential issues surrounding the geology of Antarctica and the Arctic.

Mounts Belgica in Antarctica

Antarctica

With 98% of its landmass covered by ice sheets, ice caps and glaciers, studying Antarctica's geology is a particularly difficult proposition. Nevertheless, using a mix of traditional dating methods to study exposed rock and fossils, as well as more recent technologies such as radio echo-sounding, to study the topography of the continent beneath the ice, geologists have been able to piece together the history of the continent going back many hundreds of million of years.

Antarctica as we know it today originally formed the central block of Gondwanaland, an ancient super-continent which also incorporated Africa, South America, India, Australia and New Zealand. 180 million years ago, for reasons that are still being debated, Gondwanaland began to break up and Antarctica started to drift south toward its present location over the South Pole. According to fossil records, Antarctica benefited from a mild climate and was rich in both vegetation and animal life until about 35 million years ago when the Antarctic Peninsula eventually broke off from the southern tip of South America to create what is now called the Drake Passage.

Following the creation of the Drake Passage, Antarctica was isolated physically and climatically from the rest of the Earth. At this final disintegration of Gondwanaland, a cold circumpolar current formed that insulated Antarctica from warm waters further north and, along with atmospheric currents, super-cooled the continent so that the vast ice sheets we know today started to form.

These ice sheets became so thick (up to 4800 metres) that they covered entire mountain ranges and their sheer weight depressed the landmass by hundreds of meters. Were the ice sheets now to melt of Antarctica, the "isostatic" rebound, as it is known, would raise Antarctica's subglacial topography in places by as much as 950 meters.

Geologists are able to distinguish today between two main regions of Antarctica that differ sharply both geologically and physically. The ancient continental core of East Antarctica, dating back some 3000 million years, is comprised primarily of metamorphic "basement" rock (the "shield"), intermingled with granite intrusions and overlain with flat sedimentary rocks. Interestingly, because it was formed prior to Gondwanaland's break-up, the East Antarctic shield is similar to shields in Australia, South Africa, India, Madagascar and Sri Lanka. These also share many matching fossils and rocks.

In sharp contrast to the ancient and monolithic landmass of East Antarctica, West Antarctica is composed of four distinct continental blocks with a much more recent and complex history of formation and interaction involving micro plates, metamorphic mountains and volcanic activity. These continental blocks moved into their present position after the break-up of Gondwanaland and include the Marie Byrd Land, the Ellsworth-Whitman Mountains, the Antarctic Peninsula, and the Thurston Island block. However, because of the ice coverage of the region, the exact boundaries and relative movement of these blocks is still not totally understood. Indeed, the West Antarctic Ice Sheet, because it overlaps these blocks and also lies over marine regions and basins, is inherently much less stable and thus a much greater threat to global sea level rise than the completely continental-based East Antarctic Ice Sheet.

East and West Antarctica are clearly separated by the Transantarctic mountains that nearly bisect the continent and stretch some 3500 kilometres from the Weddell Sea to the Ross Sea. The Transantarctics themselves were formed by a rapid uplift thought to have occurred during the Paleozoic era, between 540 and 250 million years ago.

Many valuable minerals are known to occur in Antarctica, including gold, silver, beryl and graphite. Coal has also been found in some parts of the Transantarctic Mountains and oil is also believed to exist in relative abundance, both offshore in the Southern Ocean and onshore beneath the ice sheets. All mining is, however, banned in accordance with the Antarctic Treaty, although the logistics and costs of mining would likely be prohibitive.

Arctic Sea Ice

The Arctic

Because of the great logistical difficulty and cost of collecting adequate geological samples and data such as cores and geophysical measurements from the floor of a frozen ocean, the Arctic remains little understood whether in respect to marine geology or geophysics.

Of the world's five oceans, however, we know the Arctic Ocean is unique in being almost permanently ice covered and almost entirely enclosed by continental landmasses. Moreover, through the often very narrow seas and straits that link it to other oceans (the Bering Strait to the Pacific and the fairly constrained Nares Strait, Fram Strait and Barents Sea to the Atlantic) the Arctic Ocean is crucial in driving the ocean (thermohaline) currents that circle the globe and are part of maintaining the Earth's delicate heat and climate balance.

The Arctic Ocean Basin is separated into two distinct Eastern and Western regions lying on either side of the 1800 kilometre long and relatively narrow Lomonosov Ridge. On the Eastern side, the Eurasian Basins (Amundsen and Nansen) border the European continent, whilst on the Western side, the Amerasia Basins (Makarov and Canada Basins) are bounded by a more complex continental scale fault system that also affects the Russian Arctic Shelf.

The Arctic Ocean basins also contain deep sedimentary deposits, partly as a result of very substantial inflows of fresh water from a number of huge rivers from the surrounding continents, including the Yenisei, the Ob, the Lena and the MacKenzie.

These sedimentary deposits serve as vast archives of past climatic change. These sediment deposits help tell us that variations in the extent of Arctic sea ice over the last twenty thousand years have had significant and often rapid impacts on global thermohaline circulation patterns with shifts sometimes having occurred over just a few decades.

What's more, findings from the recent and groundbreaking Arctic Coring Expedition (ACEX) in 2004 suggest massive fluctuations in the water temperature and general environmental conditions of the Arctic over the past 55 million years. A sedimentary core extracted from the Lomonosov Ridge is the first to show evidence of ice-free, subtropical, shallow seas with water temperatures of 20°C compared to today's average temperatures of -1.5°C. This truly massive cooling of the Arctic Ocean, which is thought to be responsible for a mass extinction of certain sea-bottom-living organisms, suggests that the Earth's climatic conditions are much more variable than previously thought.

By: Jean de Pomereu