Note: this topic is from 2/04/2003. Thanks Mike Darancette.
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Posted on 02/04/2003 9:54:17 PM PST by Mike Darancette
HOW IMPACTS CAN TRIGGER VOLCANOS
Large asteroid impacts have nasty side effects, as any dinosaur could have told you were she not obliterated by one of these calamity combos 65 million years ago. The ground shakes. Fire arcs across the sky and beyond the horizon. Clouds of debris race around the planet and blot the Sun out for months.
At least that's what theory tells us.
he scenario has never played out in modern times, scientists don't really know exactly what will happen when the next space rock slams into Earth.
One long-supposed incendiary side-effect is enhanced volcanic activity, which can make life pretty miserable for survivors who find themselves on or near the flanks of a newborn plume of molten rock. Some scientists suspect the Hawaiian Islands were born of an asteroid impact.
Volcanoes bring the planet's scorching innards and deadly gases to the surface. A big one would kick choking substances high into the air and make worse the already suffocated sky above a rattled planet.
These multiple effects of an impact are the potential makings of mass extinctions, some scientists have been saying in recent years. Entire species would falter as temperatures plunge and food sources disappear. The overall effects of an asteroid 1 kilometer wide (0.62 miles) or bigger could wipe out crops and bring human civilization to its knees.
There has been some evidence, though just a little, to verify this complex view that extinctions have multiple causes and that asteroids could be at the root of some of them.
Other researchers aren't convinced that asteroids are quite so devastating.
Now a comprehensive review of evidence provides more support for one aspect of the idea, that impacts beget serious volcanism.
Dallas Abbott of the Lamont-Doherty Earth Institute and Ann Isley from the State University of New York at Oswego examined existing impact data going back 4 billion years.
They found 10 major peaks in activity -- stretches of time when asteroids and comets hit the planet in relative flurries. During nine of the 10 peaks, volcanic activity also peaked, as measured by evidence of magma from deep inside Earth, in the mantle, flowing to the surface. Further, two prominent lulls in impact activity also matched up with periods of decreased volcanism.
The volcanic activity is most likely a tertiary effect, Abbott and Isley figure, resulting from the massive earthquakes triggered by the impact. Other studies have shown that the shaking would reverberate through the planet and even cause serious surface earthquakes on the other side of the globe, at a point called the antipode.
"Large impacts generate large earthquakes," Abbott explains. "These earthquakes can trigger volcanic eruptions. If the earthquake is large enough it could do more, it could make the eruption more intense by allowing more magma to escape."
Only about 1 percent of the magma beneath any given volcano is estimated to ever make it to the surface, Abbott said. "So, lots of magma is already there, you just need to make it easier for the magma to get out."
And how does that happen? The answer lies in the surprising fragility of our world.
While the exact mechanism isn't known, Abbott and Isley suggest it involves a cracking of Earth's crust, which allows otherwise trapped magma to flow from deep within the planet. New boundaries may even be created between the major plates that define the continents and which more or less float on the planet's mostly molten interior.
It is also possible, they say, that newly generated cracks allow the planet's molten core to mix with material in the mantle, higher up. The added heat would melt mantle material and intensify existing plumes that push to or near to the surface.
It all sounds as if a bit of Hell itself clamors to the surface to kick the addled Earth while its down.
"Our work means that a combination of effects may cause mass extinctions," Abbott said. "All of these effects could be direct and indirect effects of a large impact."
The work is based on a sketchy history, it should be pointed out. Earthquakes, volcanoes, and the general movement of the planet's dozen or so major crustal plates tend to eliminate ancient evidence. Volcanoes take stuff that used to be on the surface, but was long ago folded inward and melted, and spit it back out in unrecognizable new forms.
So scientists find very little to go on once they reach back more than a few million years.
Abbott points out that only one of about six of all predicted impact craters on the continents have been found. Almost no evidence exists for rocks that struck oceans, which cover two-thirds of the planet.
The Moon, because it is geologically inactive, retains a record of past impacts. The visible craters there help scientists estimate the rate at which impacts must also have occurred on Earth, since we share essentially the same shooting gallery in space as far as incoming objects are concerned.
Almost all researchers agree that prior to about 3.8 billion years ago, the solar system was a tremendously wild place as Earth and other planets and moons were tasked to sweep up the detritus of planetary formation. Impacts were a constant feature. Sporadic busy periods continued thereafter until the bulk of leftover asteroids were confined to a belt between Mars and Jupiter.
The new study involved actual impact craters found on Earth plus evidence of past impacts that show up as specific materials in layers of the planet's crust dated to specific times. The researchers also examined the history of strong plumes of volcanic activity.
Abbott said she's 97 percent confident in the study's results, meaning there is a 3 percent chance that the correlations are instead coincidental.
No asteroids are currently on collision courses with Earth, though hundreds of very large rocks still roam through our neck of the woods, along with thousands or perhaps millions of smaller ones that could cause heavy regional damage. Experts say the odds of a major impact in any given year are low. Eventually, most likely centuries or even many thousands of years from now, another significant impact is probable, however.
The new study is detailed the January issue of Earth and Planetary Science Letters.
Copyright 2003, Space.com
Great. We may be on the verge of 'getting it.'
Note: this topic is from 2/04/2003. Thanks Mike Darancette.
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Note: this topic is from 2/04/2003. Thanks Mike Darancette.
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