Geology Reference
In-Depth Information
Such historic eruptions trouble the modern imagination, and for good reason. Sure, their
death tolls pale by comparison with the hundreds of thousands of individuals killed by re-
cent earthquakes in the Indian Ocean and Haiti. But there's an important, terrifying dif-
ference between earthquakes and volcanoes. The size of the largest possible earthquake is
limited bythe strength ofrock.Hardrockcan take onlysomuch stress before it snaps; that
extreme limit can produce an extremely destructive, but localized, earthquake—magnitude
nine on the Richter scale.
Volcanoes, by contrast, have no apparent upper limit in size. In fact, the geological re-
cord holds unambiguous evidence of eruptions a hundred times greater than the largest
volcanic events in recorded human history. Such megavolcanoes would have darkened the
world's skies for years and altered the landscape over
millions
of square miles, not thou-
sands.Themostrecentmegavolcanoexplosion,TaupoontheNorthIslandofNewZealand
26,500 years ago, may have produced more than 200 cubic miles of lava and ash. Toba in
Sumatra, which erupted 74,000 years ago, released an estimated 672 cubic miles of ejecta.
The consequences of another such catastrophe on modern society are hard to fathom.
And yet, even these megavolcanoes, though much greater than any cataclysm in recor-
ded history, were dwarfed by the great flood basalts that contributed to mass extinctions.
Unlike one-off explosions of megavolcanoes, flood basalts represent a sustained interval
with thousands of years of intense volcanic activity. The greatest of these episodes, all of
which coincide with global mass extinctions, produced hundreds of thousands to millions
of cubic miles of lava. The biggest known event, now revealed by more than a half-million
square miles of basalt flows, occurred in Siberia during Earth's greatest mass extinction,
the great dying 251 million years ago. The demise of the dinosaurs 65 million years ago,
so often ascribed to an asteroid impact, is also coincident with immense flood basalts in
India—the Deccan Traps, almost 200,000 square miles in extent, representing more than
120,000 cubic miles of new rock.
These vast surface features could not have come from simple reprocessing of the crust
anduppermantle.CurrentmodelsoffloodbasaltformationenvisionathrowbacktoEarth's
earliest age of vertical tectonics, with giant bubbles of magma slowly rising all the way
from the superheated core-mantle boundary, cracking the crust, spewing out over the cold
surface. Such events are now rare. One scenario posits a roughly thirty-million-year inter-
val between flood basalts, in which case we're somewhat overdue for the next big one.
Ourtechnologicalsocietywillcertainlyreceivefairwarningofsuchanevent.Seismolo-
gists will be able to track the hot, molten plume as it rises. We may have hundreds of years
to prepare for the calamity. But should humanity ever enter another era of megavolcanism,
there would be nothing we could do to stop Earth's most violent paroxysms.
