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5. Ultimate volcano
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A fiery flood
like none other Think big. Imagine a flood of lava oozing through an enormous fissure in the crust, and continuing for, say, a million years. That's the basic blueprint for the flood basalts. These rocks form from basaltic lava, whose chemistry allows it to spread quickly into massive "flood basalt provinces." (Thicker lava makes steeper piles -- volcanic mountains.) In Oregon and Washington, flood basalt underlies the 200,000 square-kilometer Columbia River Plateau. In India, flood basalts called the Deccan Traps are up to 3.5 kilometers deep. Big hunk. Really big hunk. Now max out the magma-maker and imagine the Siberian Traps -- a hunk of lava measuring 1.6 million square kilometers. Or larger. Because these floods of lava must have released vast clouds of sulfur dioxide, carbon dioxide, hydrogen fluoride and other gases that reflect or trap solar heat, it's a fair guess that the flood basalts mucked with the climate. The only flood basalt in recorded history (a 12-cubic-kilometer puddle in Iceland in 1783), released so much gas that Benjamin Franklin noticed a dense "dry fog" in Europe. Toxic chemicals from the volcanic gas settled on Iceland's pastures, poisoning sheep and starving 20 percent of Iceland's population to death. Volcanoes clearly can affect climate. In 1991, sulfur dioxide from Mount Pinatubo reflected sunlight back to space, cooling the atmosphere by 0.5 degree Celsius. Although Pinatubo was the largest eruption of the 20th century, it wasn't even a water pistol compared to the flood basalts. And even if these fantastically large lava flows did not poison the landscape or cool the planet, they may have spewed carbon dioxide and cooked the planet through the greenhouse effect. Did volcanic climatic chaos cause extinctions? It's a possibility. Two jumbo flood basalts overlapped with two deadly waves of extinction:
Let's make a date But did the Siberian Traps erupt at the precise moment of extinction? Good question.
How old are you? The researchers looked at volcanic crystals called zircons found in rocks that solidified around the time of the extinction. How do you ask a quarter-billion-year-old rock its age? By looking at the radioactive decay of uranium into lead. As zircon crystals form in magma, uranium enters, but not lead. So all the lead found in a zircon must come from the decay of uranium after the crystal formed. The ratio of uranium to lead indicates how much time has passed since formation. Getting dates from an individual zircon, which weighs less than a thousandth of a gram, is an exacting science, but precision accounting is standard in geochronology. (No, they don't hire folks from Enron and WorldCom.) But when lead leaches from a zircon over the millions of years, the results go astray, and geological daters have to adjust their hard-won dates based on experience and intuition. To increase accuracy, Mundil borrowed a procedure from Jim Mattinson of the University of California at Santa Barbara, and removed radiation-rotted hunks of the zircons before doing the dating. Eliminating parts where lead leakage was likely located produced dates that lined up more closely with each other. Those dates (see "Age and Timing..." in the bibliography) also placed the extinction at precisely the time of the Siberian Traps, 252.6 million years ago (give or take 200,000 years). The coincidence between enormous extinction and excessive eruption may raise a red flag about cause-and-effect, but just because the flood basalt could affect climate doesn't mean it did. The jury is still out, the evidence is mixed, and it's raining cliches like cats and dogs. Still, the cause -- 1.6 million square kilometers of fiery, gassy lava -- was so massive that the new timing does point a finger at extinction, Mundil says. "If you imagine a whole continent, and Siberia was a whole continent, and large parts of it are covered by volcanics within short period, unlike anything we see today, it's not difficult to imagine that it had consequences for the climate ... and severe consequences for life." The link remains a hypothesis, Mundil says. "We can do no better than one part in a thousand [in terms of accuracy]. But we know, within the limits of our uncertainty, that these things happened at the same time." How did life respond to the 1980 eruption at Mount St. Helens? |
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False
color image of Mount Pinatubo in the Philippines, after the 1991
eruptions. The brown color surrounding the volcanic crater shows
the mudflows spreading into the valleys below. Photo: 
The
Deccan traps coincided with the Cretaceous-Tertiary
extinction, the die-off about 66 million years ago that dumped the
dinosaurs in the dust bin of history. However, the extinction may
have been caused by a 
