BUFFALO, N.Y. -- A new study in the journal Geology is shedding
light on the brief but violent lives of maar-diatreme volcanoes,
which erupt when magma and water meet in an explosive marriage
below the surface of the earth.
Maar-diatremes belong to a family of volcanoes known as
monogenetic volcanoes. These erupt just once before dying, though
some eruptions last for years. Though not particularly famous,
monogenetic volcanoes are actually the most common form of
land-based volcano on the planet.
Despite their number, monogenetic volcanoes are poorly
understood, said Greg A. Valentine, PhD, University at Buffalo
He is lead author of the new
Geology paper, which provides a novel model for describing what
happens underground when maar-diatremes erupt. The research
appeared online Sept. 18.
"The hazards that are associated with these volcanoes tend to be
localized, but they're still significant," Valentine said. "These
volcanoes can send ash deposits into populated areas. They could
easily produce the same effects that the one in Iceland did when it
disrupted air travel, so what we're trying to do is understand the
way they behave."
Previously, scientists theorized that maar-diatreme eruptions
consisted, underground, of a series of explosions that took place
as magma reacted violently with water. With each explosion, the
subterranean water table would fall, driving the next explosion
Taking into account new geological evidence, Valentine and
volcanologist James D.L. White of New Zealand's University of Otago
revise this model.
In Geology, they propose that maar-diatreme eruptions consist
not of ever-deepening explosions, but of explosions occurring
simultaneously over a range of depths.
Under this new paradigm, deep explosions break up buried rock
thousands of feet below ground and push it upward. Shallow
explosions eject some of this debris from the volcano's depths, but
expel far larger quantities of shallow rock.
This model fits well with recent field studies that have
uncovered large deposits of shallow rock ringing maar-diatreme
volcanoes, with only small amounts of deeper rock present. This was
the case, for example, at two sites that Valentine examined at the
San Francisco Volcanic Field in Arizona (see the Journal of
Volcanology and Geothermal Research at http://tinyurl.com/9g4hoq5).
White and Valentine's description of the eruptive process also
corresponds well with White's investigations into the "plumbing" of
maar-diatreme volcanoes, the conduits that carry magma toward the
surface. These conduits become visible over time as a landscape
erodes away, and the main "pipe" -- called a diatreme -- often
shows evidence of explosions, including zones of broken-up rock, at
a range of depths.
Such findings contradict the older model that White and
Valentine argue against.
According to the old model, Valentine explained, ever-deepening
explosions should cause shallow rocks to be ejected from the mouth
of the volcano first, followed by deposits of deeper and deeper
rock fragments. But this isn't what scientists are finding when
they analyze geological clues at volcanic sites.
The old model doesn't account for the fact that even when
scientists find deep rock fragments at maar-diatreme sites, these
bits of rock are mixed mostly with shallow fragments. The old model
also doesn't match with White's observations indicating that
explosions occur at essentially every depth.
The new model uses the strengths of the old model but accounts
for new data. The results give scientists a better basis for
estimating the hazards associated with maar-diatreme volcanoes,