The face of the earth is ever changing. Plate tectonics is slowly but surely rearranging the locations and inter-connections of continents. However knowing this in the abstract doesn’t prepare you for the awed surprise of discovering that a section of crust formed in Scandinavia is now found in Alaska.
The evidence for this comes from a massive accretion of data from geologists across the world. Build up a geological history, underpinned by accurate dating, for enough parts of the world and you can start correlating ancient events. Other tools area palaeomagnetic studies that can tell the you the latitude of a piece of crust at a given time and suites of fossils that trace small areas where organisms with local characteristics were found.
The latest research (Beranek et al. 2013) uses most of these techniques to nail down the link between a portion of Alaska and rocks now in Northern Scandinavia. It’s long been known that Alaska and Pacific Canada is made up of portions of crust (terranes) that formed elsewhere. The portion in question, the Alexander terrane is not small, covering 100000 km2 from British Columbia up into Alaska. Here’s where they reckon this crust was half a billion years ago:
The area labelled Laurentia corresponds to North America today, Baltica to Scandinavia. Arctic Alaska and Farewell are other terranes now found a long way from home. If you’ve read my post about rotating continents, you’ll have spotted the way Baltica dramatically swings round.
How did these terranes get round to the other side of Laurentia/North America?
This figure from Colpron & Nelson (2009) shows how. We are 50 million years further on from above. Baltica and Laurentia are now fused together, the ‘Caledonides’ – a strip of fascinatingly deformed rocks – mark where they collided. The narrow subduction zone to the north of this area will over time pull itself forward, dragging the continental fragments over the top of Laurentia. A similar process is going on today in the Caribbean, where the Caribbean Arc is moving east, pulling ‘Pacific’ rocks further towards the Atlantic Ocean.
Ultimately these fragments end up plastered onto western Northern America in the area known as the Cordillera, waiting for cunning geologists to spot their true origins. I don’t know about you, but my flabber is well and truly gasted by this, by both the fact that it happened and that we can work out that it did so.
Colpron, M., & Nelson, J. (2009). A Palaeozoic Northwest Passage: incursion of Caledonian, Baltican and Siberian terranes into eastern Panthalassa, and the early evolution of the North American Cordillera Geological Society, London, Special Publications, 318 (1), 273-307 DOI: 10.1144/SP318.10
Beranek, L., van Staal, C., McClelland, W., Israel, S., & Mihalynuk, M. (2013). Baltican crustal provenance for Cambrian-Ordovician sandstones of the Alexander terrane, North American Cordillera: evidence from detrital zircon U-Pb geochronology and Hf isotope geochemistry Journal of the Geological Society, 170 (1), 7-18 DOI: 10.1144/jgs2012-028