What Happens When a Continent Erupts? Lava on a Scale Nobody Has Seen
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Several times in earth history, eruptions have covered areas the size of countries in lava over intervals of a million years or less. Their timing lines up with mass extinctions closely enough to demand explanation.
The scale involved
These are not volcanoes in the familiar sense. Lava emerges from long fissures rather than from a cone, flows are extremely fluid and travel hundreds of kilometres, and successive flows stack into piles kilometres thick covering areas of hundreds of thousands to millions of square kilometres. The total volume of a single province can exceed a million cubic kilometres, which is thousands of times what any historical eruption has produced. Individual flows within a province can each exceed the volume of any eruption in recorded history. The eruptions occur in pulses over intervals ranging from tens of thousands to a few million years, which is geologically brief and long enough that nobody would experience it as a single event.
The known examples
A handful of provinces are well studied and several coincide with major extinctions:
- •The Siberian Traps, at the end of the Permian, coinciding with the largest extinction known
- •The Central Atlantic province, at the end of the Triassic
- •The Deccan Traps in India, at the end of the Cretaceous
- •The Karoo and Ferrar provinces in the Jurassic
- •The Columbia River basalts, more recent and much smaller, with no extinction attached
- •The North Atlantic province, associated with a sharp warming event around fifty six million years ago
How they kill
The mechanisms proposed are atmospheric rather than a matter of being covered in lava. Carbon dioxide released in enormous quantities warms the climate over the duration of the eruptions, and dissolving in the ocean acidifies it, which attacks organisms building carbonate shells. Sulphur dioxide produces aerosols causing short-term cooling and acid rain, alternating with the warming on different timescales. Halogens released damage the ozone layer. And where magma intrudes into sediments containing coal, salt or organic matter, heating those rocks releases further gases, which several researchers argue was decisive at the end of the Permian, since the Siberian eruptions passed through exactly such deposits. The combination produces rapid environmental change on several fronts at once, which is what makes extinction plausible.
What they left behind
The deposits are substantial landscape features and useful rocks. Stepped terrain forms where successive flows weather at different rates, producing the staircase profile that gives several provinces their name, since the word for such steps in Swedish supplies the term. Columnar jointing forms where a thick flow cools slowly and contracts, cracking into polygonal columns, which produces the celebrated formations at several sites in Ireland, Scotland and elsewhere. The rock itself is dense, hard and widely quarried for aggregate and for building. Weathered basalt produces exceptionally fertile soil, which is why the Deccan region supports intensive agriculture and why volcanic soils elsewhere are valuable. And the flows preserve records of the earth's magnetic field at the moment each cooled, which has been central to establishing the history of magnetic reversals.
The argument about the Cretaceous
The end of the Cretaceous is the case where flood basalt and another mechanism compete. The impact of a large asteroid at that boundary is established beyond reasonable doubt, with a crater, a global layer of impact debris and a precise date. The Deccan eruptions were also under way across the boundary, and the argument concerns how much each contributed, with some researchers arguing the eruptions had already stressed ecosystems and the impact finished them, others arguing the impact was sufficient alone, and a third proposal that the impact itself accelerated the eruptions. Improved dating has narrowed the timings considerably without settling the question. The debate matters beyond this one extinction because it bears on whether flood basalts are generally sufficient to cause one.
The takeaway
Lava emerges from fissures and stacks into piles kilometres thick over areas the size of countries, with single provinces exceeding a million cubic kilometres. The killing mechanisms are atmospheric, with carbon dioxide warming and acidifying, sulphur cooling, and gases released from sediments the magma passes through. At the end of the Cretaceous eruptions and an impact coincide and their relative contributions remain argued.