Winter’s settling in in the northern hemisphere and with it come warnings of snow and ice. This set me thinking about the polar regions, where snow and ice are not seasonal but permanent and where ice, kilometres thick, has persisted, at varying extents, throughout human history.
As a result of this, quite apart from the remoteness and hostility of the terrain, we know relatively little about what goes on beneath the ice. We can map the shape of the underlying terrain using a variety of techniques, but compared with what we know of the rest of the planet’s land surface, our understanding is sketchy and there’s much to be delivered.
With that in mind, this week I’m going to look at three pieces of research which add to our understanding of the geological history of the land underlying the Earth’s two major ice sheets, Greenland and Antarctica.
Under Greenland: A Newly-Discovered Impact Crater
Anyone who’s flown over Greenland in daylight will have the impression of an immense, featureless sheet of white. There are few clues to the topography of the rocky terrain that lies beneath. Using radar technology, however, scientists have recently discovered what seems to be the imprint of a major meteorite impact beneath the ice.
Meteorite impacts are nothing new — anything but. Any ten-year old with an interest in dinosaurs could probably tell you about the Chicxulub impact that’s held responsible for wiping out the dinosaurs, and many such impacts are much older than that. The formation of the solar system rests on the coalescence of planetary material, and the Moon bears the visible scars of craters formed from the early part of its history.
On Earth, most of these meteor craters have been erased, either by weathering or by the recycling of continental material through the process of plate tectonics. (Neither of these occur on the Moon.) It’s still possible, however, to see either the clear, more recent, examples such as Meteor Crater in Arizona (50,000 years old) or the ancient remains of those much as older, such as that at Sudbury in Canada (a venerable 1.8 billion years or so).
And others remain undiscovered. A report in the Earth science journal Eos describes how a review of existing topographical data revealed what’s described as: “a circular depression roughly 31 kilometers in diameter with an elevated rim and an uplifted area near its center” — in other words, the classic physical shape of an impact (rather than a volcanic) crater.
To verify this discovery, sediment studies uncovered further telltale signs of an impact in the shape of high stressed (shocked) mineral grains and the presence of indicator minerals such as gold, cobalt and nickel. The crater remains to be precisely dated but is between 11,000 and a few million years old, implying that it would have occurred during a period of ice cover and might have had significant environmental impacts.
At 31km across the crater would rank among the largest on Earth — and the fact that it has only recently been identified indicates how much more we have to learn about our planet.
Antarctica: The Continental Graveyard
Some continents, such as Australia and Africa are largely stable tectonically speaking; they contain the oldest rocks on the planet and the cycling and recycling of continents has taken place around them. Others, such as Eurasia, have broken apart and reassembled, been subjected and rebuilt and consist of a complex rearrangement of different ages of rock.
So what of Antartica? The bedrock reaches the surface only at the continental fringes and where mountains break the ice. In places the ice is almost 5km thick, so we know very little about what lies at the bottom of it and, although the basic layout and structure of the continent has been established, to date is has been impossible to say whether the continent has an old, stable tectonic interior or whether it’s the result of continental cycling.
Researchers reviewing data from a now-discontinued satellite have been able to use the information to piece together a jigsaw of the land beneath the ice.
The New York Times, in a follow-up interview, outlines more details of the discoveries, which indicate that Antarctica is essentially a composite continent, though it does have three core parts: “One … has geological similarities with some of Australia’s bedrock, while another resembles part of India’s. The third is an amalgamation of pieces of old seafloors.”
It’s worth noting that the data exercise wasn’t confined to the Antarctic, but it’s in the southern continent that the data reveals the ‘newest’ information.
Deception Island: An Antarctic Volcano
One thing we do know about Antarctica is that it’s volcanically active. According to the Global Volcanism Program website, the continent has 120 volcanoes, most of them in the South Sandwich Island arc and one of them, Erebus, currently erupting.
Because of its remoteness, however, the volcanoes of Antarctica are less well studied than those elsewhere, although they are monitored and fieldwork identifies past eruptions (and gives clues to the influence they may have had on climate etc).
This week my eye was caught by an article about Deception Island, a volcano in the South Sandwich Islands. Deception Island is a horseshoe-shape, the remnants of a crater blown away by a massive eruption, and the GVP lists numerous possible eruption over the past 10,000 years, though the dates of most are very poorly constrained.
Now there’s new research to date, and define, the caldera-forming eruption, and it’s reported in an article in Forbes magazine. Frustratingly, although this article links to the new research, the link in fact goes to a much older paper on basic geology going back to 2002, so I’m limited to reporting the Forbes article. That notes that: “Deception Island’s caldera-forming event ejected 30 to 60 cubic kilometres (roughly 7 to 14 cubic miles) of fresh volcanic debris, the chemical fingerprints of which made it as far as 4,600 kilometres (2,860 miles) from the vent”.
Date wise, the eruption is placed at around 4,000 years ago, which means it’s relatively recent in geologic terms. And it’s likely to have had significant impacts upon regional climate.