Iapetus

Iapetus Iapetus is the name given to a Pacific-style ocean that existed from the Late Precambrian to the mid-Silurian period (around 600–420 million years ago). It separated several continental masses by a distance of up to 5000 km. To the north was the continent of Laurentia (present-day North America, Greenland, and northern Britain), while to the south lay Gondwanaland (now South America, Africa, most of Asia, Australia, Antarctica and most of Europe) and to the east was Baltica (Scandinavia). The configuration of these former continents is shown in Fig. 1.

Earth scientists are now confident of the existence of Iapetus, although the exact timing of its stages of development is more controversial. Several lines of evidence are used to infer the presence and position of the ocean, such as data from fossils, geophysical studies, and field geology.

Fossil evidence is provided by a phenomenon known as provinciality. Large oceans can act as barriers to the interaction of different species of animals living on the continental shelf. For instance, it is difficult for a shallow marine species on one side of an ocean to mix with any similar organisms on the other side, and hence they evolve separately. Thus, fossils found in Cambrian and Ordovician rocks on the northern side of the Iapetus Ocean, as in north-west Newfoundland, are different from those found on the southern side, such as in south-east Newfoundland. These fossils include graptolites, trilobites, and various types of microfossils visible only under the microscope.

Other evidence for Iapetus comes from rocks of similar age that occur in close association with each other but have very different physical and magnetic characters, indicating that they were not originally formed in the same place. For example, in Ballantrae in Scotland there are unusual rocks interpreted as representing oceanic crust in an area where there is no longer an ocean.

Present estimates place the formation of the Iapetus Ocean in the Late Precambrian (between 600 and 550 million years ago). During this time plate-tectonic processes caused the rifting of an old supercontinent, the pieces of which (Laurentia, Gondwanaland, and Baltica) drifted apart as oceanic crust was created at mid-oceanic spreading centres. The ocean continued to expand until mid-Ordovician times.

No ocean lasts for ever, and the continuous processes of plate tectonics that originally formed Iapetus were also responsible for its eventual destruction. During the Ordovician, oceanic crust at the edges of Iapetus began to slide, or subduct, under Laurentia, Gondwanaland, and Baltica, causing the ocean to shrink slowly and the continents to drift back together (Fig. 2). One of the initial consequences of the shrinking of Iapetus was a gradual decline in the provinciality of fossils. Organisms that earlier had been separated by thousands of kilometres of water were able to cross the ever-decreasing distance between the margins of the ocean. A gradual mixing of fossil species is thus seen in progressively younger rocks.

The climactic result of the closure of the Iapetus was the eventual collision of the opposing continents about 420 million years ago in the events known as the Caledonian and Appalachian orogenies. As Laurentia, Baltica, and the northern part of Gondwanaland approached each other, marine sediments and volcanic islands at the margins of these continents began to be pushed together, causing them to be deformed by folding and faulting and to become altered (metamorphosed) by the effects of pressure and heat. Thus, the continents slowly ploughed into each other to form mountain belts of deformed, metamorphosed Lower Palaeozoic rocks, within which is a record of the Iapetus Ocean. These mountain belts are known as the Caledonides and despite their great age, are still expressed today as, for example, the Appalachians in North America and the Highlands of Scotland. The mountain belt was later broken up by the rifting events that led to the formation of the North Sea and the North Atlantic ocean.

The join, or suture, between the old continents can today be located with the aid of geophysical techniques. Seismic reflection profiles, which provide an image of the structure of the crust by deep echo soundings, indicate that the suture line between Laurentia and the northern part of Gondwanaland stretches from the border between England and Scotland, through the centre of Ireland, across Newfoundland and down the east coast of the USA. If today you travel from the south-eastern to the north-western coast of Ireland, it is worth remembering that to have made this journey 500 million years ago would have required a 5000 km sea crossing across the Iapetus Ocean.

Dave Burnett and and Dave Quirk