A Dynamic Earth Event supporting Homecoming Scotland 2009

SUPERCONTINENTS, SCOTLAND AND THE SCOTS

A public lecture to be given by Prof Ian Dalziel (FGS, FRSE)

on Monday 7 September 2009

		at Our Dynamic Earth Holyrood Rd, Edinburgh EH8 8AS (0131 550 7800) 1900hrs, Entrance £5 www.hubtickets.co.uk Remaining tickets will be on sale at the door

Satellite measurements demonstrate that all the continents on planet Earth are moving with respect to one another at rates of a few millimeters per year. Eighteenth Century Edinburgh physician and natural scientist James Hutton led the way to appreciation of the great antiquity of our planet. This eventually  let us understand that such tiny movements could, over tens, hundreds, and indeed thousands of millions of years, move over very large distances the apparently stable continents on which almost all humans make their homes.

Some of the earliest evidence for the existence of ‘supercontinents,’ comprising most or all of the low-density rocks on Earth’s surface, came from the North West Highlands of Scotland. Geologists working there for the Geological Survey of Great Britain realized as early as the mid-1800s that fossils in the Cambrian and Ordovician strata (540-460 million years old) capping several well known peaks of the region such as Beinn Arkle, Canisp and Quinag were more closely related to forms found in the equivalent age strata of North America than to those in the Welsh Border country of western England. Thanks to geophysical studies of the intervening ocean floor, we now know that the Highlands of Scotland are indeed a fragment of Laurentia, the ancestral core of North America and Greenland, and that until the opening of the Atlantic and Indian Ocean basins by seafloor spreading over the past 180 million years, all the continents of the present-day Earth were joined together in the supercontinent ‘Pangaea.’ This was surrounded by ‘Panthalassa,’ ancestor of today’s Pacific Ocean.

Pangaea, however, was itself a comparatively ephemeral entity in Earth history. The Caledonian and Appalachian mountain chains together with the Armorican mountains of Europe and the Urals of Russia represent ‘sutures’ along which more ancient ocean basins closed between 450 and 250 million years ago in earlier continental movements that resulted in the amalgamation of Pangaea from pre-existing continents. Knowledge of pre-Pangaea supercontinents is hard won, but Laurentia, ancestral North America and parent of the Scottish Highlands, appears to have broken out of a supercontinent towards the end of Precambrian times and appears to have left behind several older geological ‘calling cards’ like the Highlands that provide clues to past paleogeography. These include: the Argentine Precordillera that contains strata and fossils identical to those of Laurentia and the NW Highland strata; a remote part of Antarctica, discovered by the 1904 Scottish National Antarctic expedition, where the geochemistry and magnetism suggest a Laurentian origin; and the Transantarctic Mountains where rocks and minerals identical to those of Laurentia have recently been found. From these we can reconstruct the wanderings of Laurentia over the past 1,000 million years, and several supercontinents of which it appears to have been a part.

The processes of supercontinent amalgamation and fragmentation are the grandest surface manifestations of the constant tectonic activity of “Our Dynamic Earth.” They were major factors determining the environments in which life evolved, and in controlling the present distribution of plants and animals on the planet. The presentation will cover principally the ‘supercontinental cycle’ from Rodinia to Pangaea, main supercontints of the most recent 1,000 million years of Earth history, in the course of which life evolved from single-celled organisms to primates and humankind. It will also take a peek into possible future supercontinents.