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We all know that a BIG meteor hit the Gulf of Mexico at the end of the Cretaceous period and wiped out the dinosaurs, right? So, big meteor-strikes probably caused the other mass-extinctions too?

Well -- the Chicxulub impact at the KT boundary, 65 million years (my) ago, is indeed well-documented. What's less well-known is that the Deccan Traps, an enormous outpouring of flood-basalts in what is now western India -- over 2 million cubic km(!) of lava, along with billions of tons of SO₂, CO₂, HC_l, and other toxics -- were also in full eruption then. In fact, the famous KT iridium-signature has recently been identified in Deccan interflow sediments.¹  From recent radiometric dating, it looks like all of the Deccan eruptions occurred within a brief, 0.7 my time-span. The biggest and most violent eruptions apparently occurred within a few thousand years of the KT boundary; individual flows of several thousand cubic km of basalt were common.

Compare this to the largest historic 'flood'-basalt eruption: Laki in Iceland produced 12 cu. km of lava in 1783-84. The SO₂ and other gases that Laki erupted, destroyed most of the island's crops and forage. Then 50-80% of the island's livestock, and about one quarter of the Icelandic people, starved to death. Laki lowered global temperatures by about 1 deg. C (from fine-particle ash and sulfur aerosols).

Extrapolating to a 5,000 cu. km flood-basalt eruption, the average global temperature might decrease by around 7 deg. C (13 deg. F). The volcanic HC_l emissions could destroy most of the ozone layer,² dramatically increasing UV at the surface, and injuring or killing many organisms. The familiar volcanogenic "toxics" -- F, As, Sb, Hg, Se etc. -- would poison nearby life. And the volcanic SO₂ and HC_l would cause severe acid-rain damage as they were washed out of the atmosphere. Then, repeat this disaster with the next big eruption, over and over again, a dozen or more times in the next 10,000 years or so. The total 'kill factor' would very likely be greater than that from the Chicxulub impact, albeit spread out over tens or hundreds of thousands of years. And a more gradual die-off is a better fit to the known fossil record.

So it turns out that the volcanists and the meteor-strike proponents were both right, at least for the KT mass-extinction. The combination of the Chicxulub strike with the Deccan mega-eruption turned an 'ordinary' mass-extinction into the second-worst ever. And thoroughly muddied the scientific waters while this was being worked out. Once again, reality trumps fiction -- Nemesis atop Shiva!

But, for the 10 or so "big" mass-extinctions known,³ seven are of the same age as major flood-basalt eruptions, vs. one or two with major same-age impacts. And those two meteor-strikes coincide with massive flood-basalt eruptions -- no major mass-extinction appears to be solely impact-caused. So it's fair to say that flood-basalts are more deadly to Earthly life than meteor-strikes. And a hazard not amenable to any engineering solution that I know of -- except being ready to move off the planet, when the next new hot-spot head nears breakout. Which will come, sure as death.⁴  An unpleasant reminder of our fragility and vulnerability.

Mea culpa: I'd pretty much taken the "KT impact killed off the dinos" theory as proven -- I didn't even bother to read the last volcanist counter-argument I saw. As Courtillot notes, I'm hardly the only one to do so. Hey, those guys are the old fuddy-duddies, right? The stamp-collectors, Luis Alvarez called them. Hence this review, a 'heads-up' to others, and an expiation for me.

Evolutionary Catastrophes is clearly written and is (mostly) accessible to the general reader.⁵  This is the latest chapter in the gradualist vs. catastrophist dialogue that is as old as geologic science. Writing with great good humour, skepticism, and a love for a scientific tale well-told, Courtillot goes a long way towards redressing the balance in the hottest earth-science argument at the turn of the 21st century. Highly recommended.

Pete Tillman has been reading SF for better than 40 years now. He reviews SF -- and other books -- for Usenet, "Under the Covers", Infinity-Plus, Dark Planet, and SF Site. He's a mineral exploration geologist based in Arizona. More of his reviews are posted at www.silcom.com/~manatee/reviewer.html#tillman .