- It's bad enough when, every few million years, an asteroid
rocks our planet. It's worse if the impact triggers regional or global
volcanic activity, which is not only hazardous to nearby plants and animals
but can choke Earth's atmosphere with deadly gases for months or years.
- But there's also a possible bright side, like the birth
of nice places like Hawaii.
- For more than three decades, scientists have explored
the question of whether an asteroid impact could cause significant volcanic
eruptions, hot spots that spring up out of nowhere and create new landforms
or rearrange old ones. The process might have given birth to the Hawaiian
islands, for example. The idea of linking space rocks and lava goes back
to at least the 1960s, and in recent months the debate has heated up like
a volcano ready to erupt.
- Though no firm answers have emerged, controversial computer
modeling in recent years has shown what might happen, and why.
- Old and buried
- Andrew Glikson from the Australian National University
makes a living by hunting for signs of ancient impact craters, many of
which are not readily visible, some of which are buried under oceans that
didn't used to be where they are now.
- In several recent papers, Glikson maintains that the
large craters found so far on Earth only account for about one-fifth of
the actual number of major impacts predicted over the past 3.8 billion
years. These impact predictions are based on a number of factors, including
the vast numbers of craters on the Moon, which don't disappear on the geologically
dead and atmospherically challenged satellite.
- During these billions of years it is likely that there
were at least 10 impacts producing craters more than 125 miles (200 km)
in diameter and 30 impacts producing craters more than 60 miles (100 km)
in diameter, Glikson figures. And he argues that at least 50 percent of
these impacts would have struck locations in ancient oceans where the Earth's
crust was much thinner than continental crust and, in particular, some
10 percent would have struck the thin crust adjacent to mid-ocean ridges.
- A volcanic eruption is much more likely to occur if an
impact occurs at one of these sites and, paradoxically, evidence of the
impact is likely to be buried under the eruption and lost forever if that
area is later folded, or subducted, into the Earth's crust.
- But Jay Melosh, a crater expert from the University of
Arizona, doubts this and other links between asteroids and volcanoes. Melosh
presented his views last July at a conference on catastrophic events in
Vienna.
- "There is not a single clear instance of volcanism
induced by impacts, either in the near vicinity of an impact or at the
antipodes (opposite side) of the planet," Melosh concluded. "The
possibility of impact-induced volcanism must therefore be regarded with
extreme skepticism."
- Researchers who are independently working on various
impact mechanisms say Melosh's reasoning is flawed.
- Hermann Burchard is a mathematician at from Oklahoma
State University. He notes that there are several examples where either
a volcanic eruption is speculated to be associated with a known impact
or an undiscovered impact is speculated to be associated with a major eruption.
Again the problem is that such eruptions tend to obliterate evidence of
an impact.
- Rocking the other side of the planet
- Mark Boslough and his colleagues at Sandia National Laboratories
have modeled asteroid impacts. In a 1996 paper, they predicted that the
seismic energy from an impact travels through the Earth and is strongly
focussed at the antipode to the impact, near the boundary of the crust
and the hot, molten mantle.
- This, they argue, generates instability in the upper
mantle that can produce a "hot spot" under the surface, like
the one that continues to form the Hawaiian Islands. They also claim that
geologists have no satisfactory explanation for the generation of hot spots
by other means, other than that they arise "spontaneously."
- Melosh expressed skepticism about the focussing idea
after calculating that the seismic energy delivered to the antipode would
be insufficient to melt the rocks. Boslough counters that melting is not
needed to generate an instability -- the increased temperature of the region
may be sufficient to trigger the start of a hot spot.
- Clues on the Moon
- Dallas Abbott from Columbia University, New York has
taken a different approach to the whole question
Using terrestrial evidence and crater evidence from the Moon, which retains
a record of bombardment going back nearly 4 billion years, Abbott has analyzed
the timing of large impacts compared with major volcanic eruptions, or
mantle plumes.
-
- She found a strong correlation between the two and speculates
that large impacts strengthen existing mantle plumes. She describes the
ancient and dormant Deccan Traps volcanism, an area that is presently part
of India. At the time of the Chicxulub asteroid impact, which occurred
in Mexico 65 million years ago and likely led to the demise of the dinosaurs,
the Deccan Trap region was near the antipode of the impact.
-
- Others have suggested this coincidence could be possible
evidence that impact antipode effects initiated the Deccan Traps.
-
- However, Boslough says the Deccan Traps would have been
several thousand kilometers away from the antipode to Chicxulub. Abbott
says the Deccan Traps were active well before the Chicxulub impact, and
so could not have been initiated by that event, but she observes that the
Deccan plume was strongest immediately after the impact and this phase
lasted less than one million years.
At the least, it appears to be a strange coincidence.
-
- Despite advances in computer modeling, there is no clear
physical evidence of a link between space rocks and lava, but the models
are coming up with mechanisms by which an impact could cause, or at least
speed up, a volcanic eruptio
-
- And proponents of the idea are quick to point out that
geologists have not come up with a better explanation for how the Earth's
hot spots got started.
-
- Michael Paine is a member of the Planetary Society Australian
Volunteers. Information related to this story and the threat of asteroids
can be found at his web site.
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