Caledonian Orogeny: The Closing of the Iapetus Ocean

The Caledonian orogeny was a series of mountain-building events, spread over roughly 100 million years from about 490 to 390 million years ago, that closed the ancient Iapetus Ocean. As the ocean narrowed, the continents Laurentia and Baltica and the microcontinent Avalonia collided and welded together. The result was a great mountain belt whose worn-down roots now form the Scottish Highlands, the mountains of Norway, parts of Ireland and Greenland, and the northern Appalachians.

Background: an ocean before the Atlantic

Late in the Precambrian, around 600 million years ago, as the landmasses left over from the breakup of the supercontinent Rodinia continued to rift apart, a new ocean opened between the landmass of Laurentia (the core of North America, plus Scotland and Greenland) and Baltica (Scandinavia and the Baltic region). This was the Iapetus Ocean, named after the Titan Iapetus, father of Atlas.

A third piece, Avalonia, was a strip of crust that broke off the edge of Gondwana and drifted north. It included what are now England, Wales, southern Ireland, Belgium, parts of Atlantic Canada such as Newfoundland’s Avalon Peninsula, and parts of New England.

In 1966 the Canadian geophysicist J. Tuzo Wilson asked in the journal Nature whether the Atlantic had closed and then reopened. He noted that early Palaeozoic fossils from northwest Scotland resembled those of North America, while those from Wales resembled European ones. His answer, a “proto-Atlantic” ocean that later closed, became one of the founding ideas of plate tectonics.

What happened

The closure of Iapetus was not a single collision but a sequence of events, usually grouped into phases.

Grampian and Taconic phases (c. 480-450 million years ago). In the Ordovician, chains of volcanic islands collided with the margin of Laurentia. In Scotland and Ireland this deformed and metamorphosed rocks of the Grampian Highlands; in North America the equivalent event is called the Taconic orogeny.

Avalonia meets Baltica (c. 450-440 million years ago). The smaller Tornquist Sea between Avalonia and Baltica closed, joining the two.

Scandian phase (c. 430-400 million years ago). In the Silurian, Baltica collided head-on with Laurentia. Great slabs of rock were pushed many tens of kilometres eastward onto Baltica, forming the stacked thrust sheets of the Scandinavian mountains, while comparable structures formed in East Greenland. In northwest Scotland the Moine Thrust records this compression.

Final closure (c. 420-390 million years ago). Avalonia docked against Laurentia, closing the last of Iapetus. In North America the later stages are often called the Acadian orogeny. The merged landmass is known as Laurussia, or Euramerica.

Some geologists compare the Caledonian mountains at their height to the modern Himalayas, but their original elevation cannot be measured directly and remains an estimate.

Traces in the landscape

The Iapetus Suture, where the two sides finally met, runs beneath northern England and southern Scotland, roughly from the Solway Firth to the Shannon estuary in Ireland. Large faults that formed or moved during the orogeny, such as the Great Glen Fault and the Highland Boundary Fault, still shape the geography of Scotland.

As the mountains eroded, huge quantities of red sand and gravel were carried into basins on their flanks. These became the Old Red Sandstone of the Devonian, the setting in which early fish, and later the first tetrapod relatives such as Tiktaalik, lived on the same continent.

At Siccar Point on the Berwickshire coast, nearly vertical Silurian rocks, tilted during the orogeny, are overlain by gently sloping Old Red Sandstone. James Hutton visited the site in 1788 and used it to argue that Earth’s history involved immense spans of time.

Evidence from fossils and rocks

Several independent lines of evidence reveal the former ocean. Fossils are among the clearest. In the Cambrian and Early Ordovician, trilobites and brachiopods on the Laurentian side, in northwest Scotland and western Newfoundland, belonged to different faunas from those of Avalonia and Baltica, in Wales, England and Scandinavia. As the ocean narrowed during the later Ordovician and Silurian, the faunas became increasingly similar, because larvae and adult animals could cross the shrinking gap.

The rocks tell the same story. Slices of old ocean floor, called ophiolites, were pushed onto the continents during the collisions; examples include the Ballantrae complex in southwest Scotland and the Bay of Islands complex in Newfoundland. Thick piles of Ordovician and Silurian deep-water sediments in the Southern Uplands of Scotland are widely interpreted as material scraped off the descending ocean plate. Large granite bodies, formed as crust melted during and after the collisions, are found across Scotland, Ireland and Norway.

Timeline

• c. 600 million years ago: The Iapetus Ocean begins to open.
• c. 480-450 million years ago: Grampian and Taconic phases deform the Laurentian margin.
• c. 450-440 million years ago: Avalonia collides with Baltica.
• c. 430-400 million years ago: Scandian phase; Baltica collides with Laurentia.
• c. 420-390 million years ago: Final closure of Iapetus; Laurussia formed.
• 1788: James Hutton studies the unconformity at Siccar Point.
• 1880s: Eduard Suess names the Caledonian mountain belt.
• 1966: J. Tuzo Wilson proposes that a proto-Atlantic ocean closed and reopened.

Why it matters

The Caledonian orogeny assembled much of the crust of northern Europe and northeastern North America and created landscapes that still define Scotland and Norway. When the Atlantic began to open in the Mesozoic, during the breakup of Pangaea, it split roughly along the old Caledonian belt, leaving matching pieces of the same mountains on both sides of the ocean. Recognising this was a key step towards the theory of plate tectonics.