Laurentia

Laurentia, ancient continent comprising much of what is present-day North America, including Greenland, Arctic Canada, and the islands of Svalbard (Spitsbergen). Laurentia had a history independent of South America prior to the assembly of Pangaea in the late Palaeozoic Era (about 250 million years ago). Laurentia came into existence following the break-up of a late Proterozoic supercontinent—somewhat analagous to Pangaea—which has been termed Rodinia. The basement rocks of Laurentia are accordingly Precambrian, the core of which comprises the Canadian Shield, predominantly composed of Archaean metasediments and other metamorphic rocks, with some Proterozoic “cover”. The oldest rocks known, from Isua, Greenland, were part of this ancient nucleus. The breakup of Rodinia happened about 560 million years ago. Laurentia was an independent entity for more than 100 million years until it collided with Baltica and part of Asia. The palaeomagnetic evidence for this interval of independence is well known and shows that for the bulk of the period Laurentia was close to the ancient equator. This is reflected in the kinds of rocks that accumulated upon it in the earlier Palaeozoic. Limestones are abundant in the Cambrian, Ordovician, Silurian and Devonian Periods. For the greater part of a period of 150 million years, shallow seas flooded over the peneplained surface (flat and featureless through erosion) of Laurentia. The details of the geography varied greatly, and at times the eastern part of Laurentia differed from the western, but the shelf seas were almost always replete with life, leaving beautifully preserved fossils as a legacy of independent Laurentian history. In the Cambrian, the celebrated Burgess Shale of British Columbia, Canada, accumulated off the edge of a carbonate “ramp”. Ordovician and Silurian strata include some of the first coral reefs, early fossil fish, and a wealth of trilobites. Starting in the Ordovician, subduction on the eastern seaboard led to the evolution of the Appalachian mountains and the accretion of terranes (including Avalonia), and eventually collision with the Baltica continent. The subsequent 'Old Red Sandstone' deltas and river systems were cradle areas for the evolution of terrestrial plants and animals, the earliest of the latter having been discovered in Greenland. The eastern margins of Laurentia do not coincide exactly with the present-day Atlantic Ocean, because of the way that ocean opened up after the break-up of Pangaea. Thus, Scotland was part of Laurentia, as was western Ireland; similarly, the western part of Newfoundland, including the Northern Peninsula, was part of Laurentia, but the eastern part belonged with Avalonia.