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The Flood Basalts

Flood basalts occur when a wound opens up in the Earth’s crust, and huge amounts of mantle material gush out. After a bit the wound closes – only to open again after a period. A typical flood basalt event will last a million years or more, before the wound finally closes off for good.

what killed off the dinosaurs? Luis Alvarez

For Deccan Traps & Siberian Traps, scroll down to picture.

Left behind is layer upon layer of solid basalt, amounting to a kilometre or more thick. Rivers then cut valleys through the layers. The sides of the valleys have a step-like appearance, so they are called ‘traps’ which is Dutch for staircase.

With the magma comes devastating quantities of noxious gases, including serious amounts of carbon dioxide. Even a piffling volcano like Mount St. Helens can affect the world’s climate for a year or two. Major eruptions like Krakatoa or Santorini both did serious world-wide mischief. But the worst volcano that you can imagine is as nothing compared with a flood basalt event.

It is no surprise that flood basalt events tend to be associated with mass extinctions.

Basalt outpourings have been found all over the world. However, there being much more ocean than land, most of them are on the ocean floor. The most famous and the most recent is the Deccan Traps, which covers about a third(?) of the area of India. Yes, honestly. That big. This picture of a small part of them comes from the Open University website.

The other famous flood basalt is the Siberian Traps, which occurred 250 million years ago, around the time of the great Triassic extinction (more). The Siberian traps are more mysterious, on account of their much greater age. But a map I’ve seen, published in Scientific American (Oct. 93), suggests that they were even larger than the Deccan Traps. This map also shows large flood-basalt areas in North America, South America and N.E Africa.

What killed off the dinosaurs?

You may think you already know what did the dinosaurs in. It was a meteor strike wasn’t it? The great physicist Luis Alvarez said so, so it must be true.

Not so fast.

The palaeontologists and palaeobotanists already reckoned that they had a perfectly satisfactory explanation. They had charted a long slow deterioration in the World’s climate, and an equally long slow decline in the number of animal and plant (?) species around – including dinosaurs.

Then a catastrophic volcanic or other event provided the final coup de grâce. They may have had the Deccan Traps in mind. Unfortunately nobody had managed to date the Traps accurately, so it was impossible to be sure.

But then a thin line of iridium-rich material was found all round the world. It was right on the boundary – below which there were dinosaur bones, and above which there was nary a one. Meteorites tend to be rich in iridium and so the famous physicist Luis Alvarez declared that it was a meteor strike that did it. End of story.

The palaeontologists weren’t happy, but they were cowed into silence by Alvarez’s fame. (In a way it was a rerun of the earlier famous physicist Lord Kelvin reckoning that he knew better than the geologists how old the planet was (more).)

But the doubts wouldn’t go away. And eventually the iridium layer was discovered in the Deccan Traps – slap bang in the middle.

The basalt had been flooding out for a long time before the meteor strike (no, I can’t tell you how long) and continued to flood out for a long time afterwards (ditto). Its effects would certainly have been making life very difficult indeed for all life around the globe.

I’ve also read the we mammals could have had something to do with it. It seems that many dinosaurs laid eggs, and made nests with stones – on the ground. Now this was fine during their heyday. Who would dare to try and steal a dinosaur egg? But as times got harder, we mammals got bolder. The dinosaurs were under stress anyway. And they had to expend huge amounts of effort trying to protect their precious eggs.

The meteor strike theory predicted worldwide firestorms. But I’ve read that that signs of these firestorms were not being found – and that there was other evidence that the strike itself didn’t have nearly as catastrophic consequences as had been claimed.

The jury is still out. But whatever the final outcome, the story will certainly turn out to be a good deal more complicated than at first thought. But then “‘twas ever thus”!

There’s an irony in all this. The Deccan Traps were certainly implicated in the Dinosaurs’ demise, even if the final straw was the meteor strike. But it was the Siberian Traps, some 150 million years earlier, that cleared out the competition and gave them their big chance in the first place.

And anyway it’s a myth that the dinosaurs died out. There are still plenty of them around today; waking us up with their dawn chorus, and causing great excitement to a large community of bird watchers.