A history of the plate tectonics of Britain (Part 3): Britain breaks apart – rifting, volcanoes and the birth of the Atlantic
Jon Trevelyan (UK)
By the end of the Carboniferous, Britain had already passed through more than one major phase of mountain building, each followed by long periods of erosion and structural collapse. The Caledonian mountains were long eroded, and the Variscan belt of southern Britain was entering its final stages of decay. Yet tectonic influence did not fade away. Instead, the dominant regime changed. Britain was no longer being compressed – it was being stretched.
This shift from compression to extension marks one of the most important transitions in Britain’s tectonic history. It led not to new mountain belts, but to rifting, crustal thinning and, ultimately, widespread volcanism associated with continental breakup. The landscapes that record this phase – particularly along Britain’s western margins – are among the most dramatic in the British Isles, from the volcanic mountains and lava plateaux of Isle of Skye (Fig. 3.1) and Isle of Mull, to the columnar basalts of the Giant’s Causeway (Fig. 3.7) and Fingal’s Cave (Fig. 3.8).
Stretching the crust: early rifting and failed break-ups
The first signs of extension appeared during the Permian and Triassic, as the supercontinent Pangaea began to fragment. Across much of Britain, the crust was stretched and broken into fault-bounded blocks. Subsiding basins accumulated thick sequences of sediment, often under arid, desert-like conditions, producing the red sandstones and evaporites that underlie many lowland regions today (see, for example, A pebble across Deep Time: From Staffordshire to Budleigh Salterton and back again).
These rift basins rarely preserve abundant fossils, but where they do, they contain assemblages dominated by terrestrial and marginal environments, reinforcing the interpretation of continental rather than marine deposition. The absence of widespread marine fossils contrasts sharply with earlier Carboniferous successions and reflects the tectonic isolation of these basins.
These rift basins are not only evident in the fossil record, but are also expressed clearly in the modern landscape. Many form elongated lowlands bounded by fault-controlled escarpments, a geometry that reflects their origin as down-faulted blocks within an extending crust.
The Midland Valley of Scotland is a classic example, its broad, linear form reflecting long-lived extensional structures inherited from late Palaeozoic tectonics (Fig. 3.2). In northern England, fault-bounded Carboniferous basins flanking the Pennines remain visible as contrasting belts of lowland and upland terrain, while smaller rift basins beneath southern Britain are revealed indirectly through subsurface geology and subtle surface expression. The predominance of red beds, coal measures and terrestrial sediments within these basins, coupled with the scarcity of marine fossils, reflects their isolation from open seas and reinforces their interpretation as continental rift environments rather than shallow marine basins.
Crucially, most of these early rifts failed. Continental breakup stalled, leaving behind deep-seated faults and zones of weakness that remained embedded within the crust (Fig. 3.3).
Although major rifting episodes waned after the Triassic, the tectonic legacy of earlier extension continued to shape Britain throughout the Jurassic and Cretaceous. During this long interval, the region occupied a relatively stable, slowly subsiding setting on the northern margin of the Eurasian plate, far removed from the active plate boundaries associated with the closing Tethys Ocean to the south. When Atlantic rifting resumed in the Paleogene, it preferentially exploited this inherited extensional framework.
Earlier fault systems and basin geometries controlled patterns of sediment accumulation, giving rise to the thick marine successions that now form much of southern and eastern England, including the Jurassic Coast and the Chalk landscapes of the Cretaceous. While tectonic activity was subdued compared with earlier and later phases, this period was crucial in establishing the stratigraphic and structural framework that would later be gently inverted or uplifted during Alpine compression and Atlantic rifting (see A history of the plate tectonics of Britain (part 4): A quiet crust with a long memory – tectonic inheritance in the modern British landscape).
From rifting to rupture: opening the North Atlantic
True continental breakup did not occur until much later, during the early Paleogene. As Greenland and Europe began to separate, rifting intensified along the future North Atlantic margin. The crust thinned to the point of rupture, allowing magma to rise from the mantle in enormous volumes.
This process produced the North Atlantic Igneous Province, one of the largest volcanic events in Earth’s history, active mainly during the early Paleogene (Fig. 3.5). Unlike subduction-related volcanism, these eruptions were driven by decompression melting beneath a stretching lithosphere. Lava poured across the landscape and, as continental breakup progressed, onto the newly forming seafloor. Fissures opened repeatedly, and large central volcanoes developed where magma supply became focused, marking the transition from continental rifting to true oceanic spreading around the Paleocene-Eocene boundary.
Volcanic landscapes as tectonic evidence
The clearest expressions of this phase occur along Britain’s western seaboard. On the Isle of Skye, stacked basaltic lava flows form extensive plateaux, while erosion has cut deeply into former central volcanoes, exposing gabbros (Fig. 3.6), granites and dense networks of dykes and sills. Similar features occur on the Isle of Mull (For example, the Monro, Ben More), where the roots of Paleogene volcanoes are revealed in rugged uplands.
Across the Irish Sea, the Antrim Plateau preserves thick sheets of flood basalt, forming a stepped landscape that reflects successive lava eruptions. Along its northern margin, erosion has produced the columnar jointing of the Giant’s Causeway (Fig. 3.7), while offshore, wave action has carved Fingal’s Cave (Fig. 3.8) into similar basalt columns.
| Why Basalt forms hexagonal columns |
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| The striking hexagonal columns seen at Fingal’s Cave and the Giant’s Causeway are a natural consequence of how basaltic lava cools. When thick lava flows or shallow magma bodies cool slowly, they contract. Because contraction occurs evenly in all directions, the cooling rock fractures in a regular pattern that minimises internal stress. This process produces a network of cracks that propagate downward from the cooling surface. Although the fractures may begin irregularly, they quickly organise into polygonal shapes as cooling continues. A hexagon is the most efficient way to divide a surface into equal areas with minimal cracking, which is why six-sided columns are so common, although columns with five or seven sides also occur. The columns grow perpendicular to the cooling surface, not vertically by default. At the Giant’s Causeway, this surface was roughly horizontal, producing upright columns. At Fingal’s Cave, later erosion has exposed the columns sideways, revealing their internal structure in dramatic cross-section. Together, these sites show that the columnar jointing reflects cooling geometry rather than any inherent “hexagonal” property of the lava itself. |
These landscapes are particularly striking because of their youth. They formed around 60 million years ago – long after the dinosaur extinction – demonstrating that Britain’s geological history extends well beyond the Palaeozoic.
Volcanism, inheritance and erosion
The distribution of volcanic centres was strongly influenced by inherited structure. Faults formed during earlier phases of collapse and rifting provided pathways for magma ascent, while variations in crustal thickness controlled where volcanism was most intense. Once again, earlier tectonic events shaped the outcome of later ones.
Volcanic activity was intense, but short-lived. As seafloor spreading became established further west, for example, in Iceland in the present day, Britain moved onto the passive margin of the newly formed Atlantic. Erosion then became the dominant process, selectively removing softer rocks and leaving resistant basalts and intrusions to define much of the modern relief.
From fire to framework
By the end of the Paleogene, Britain occupied a tectonically quieter setting on the margin of the North Atlantic. Yet the faults, rifts and volcanic structures created during breakup would continue to influence landscape evolution.
The final article turns to that quieter phase, examining how ancient tectonic structures continue to shape Britain’s modern scenery – often in subtle but revealing ways.
| Other articles in this series |
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| A history of the plate tectonics of Britain (Part 1): Britain assembled – oceans, collisions and the making of a geological patchwork |
| A history of the plate tectonics of Britain (Part 2): When mountains fall – collapse, basins and the foundations of Britain’s lowlands |
| A history of the plate tectonics of Britain (Part 3): Britain breaks apart – rifting, volcanoes and the birth of the Atlantic |
| A history of the plate tectonics of Britain (Part 4): A quiet crust with a long memory – tectonic inheritance in the modern British landscape |