Binding Continents from Float

Too often in the field, you'll be looking at a hillside and there are no outcrops of bedrock. A poor second choice is relying on float—isolated stones in the soil that you must assume came from the bedrock. Float is no good for stratigraphy, because its original position is lost; nor is it any good for bedding features or orientation or any information that comes from the context of the rock. But if you're careful with float, it's better than nothing.

A paper in the 11 July Science ties two ancient continents together on the basis of a single chunk of stone found sitting on top of a glacier in the Trans-Antarctic Mountains. The press release from the National Science Foundation is very coy about the stone in a way typical of these documents: it was "a very specific form of granite." The paper's lead author, John Goodge, is quote saying it has "a particular type of coarse-grained texture." Thus most of the stories on the Web today are equally mushmouthed. It takes a clue from the paper's abstract, "a glacial clast of A-type granite," to deduce that they meant rapakivi granite. Why not just say so?

Rapakivi granites are very distinctive: they have big balls of alkali feldspar with shells of plagioclase. (Ole Nielsen's blog has a photo with his brief explanation of the Science paper.) A long series of them is strewn across North America in a wide belt of Proterozoic crust running from the Canadian Maritimes at one end to an abrupt cutoff in the Southwest. Where the belt continues is an important question because if you find it on another continent, that ties the continent to North America at a specific time in the deep past when both were united in a supercontinent.

Finding a chunk of rapakivi granite in the Trans-Antarctic Mountains, even just as float, is a key piece of evidence that the ancient supercontinent of Rodinia held Antarctica next to North America. You'll find a deep well of information about the background setting for this discovery in a 2006 paper in EPSL by Goodge and Jeffrey Vervoort. It shows how much preparation it takes to make the most of serendipity in the form of a lucky piece of float.