Science News - January 1, 2011

Earth

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Icequakes warn
of avalanche risk

By Marissa Cevallos

Unless you’re eating breakfast, hearing snap, crackle and pop may be an early warning sign of an impending avalanche. Scientists listening in on icequakes that rumble through glaciers have developed a model that can predict a collapse up to 15 days before it happens, the team reports in a study posted at arXiv.org.

With that kind of heads-up, villages could be evacuated and roads closed in avalanche-prone areas.

Though all glaciers groan and creak
under stress, glaciers on an incline are
especially creaky, because the top of the
ice is less well supported against the pull
of gravity than the base— like a book
tilted at a 45-degree angle. Accumulating
snow causes more stress. These forces
cause the glacier to fracture, sending tiny
icequakes throughout. Eventually, if a gla-
cier can’t handle the stress, a large chunk
will fall off, pummeling anything below
with a moving mass of snow and ice.

Studies of the Weisshorn peak reveal
that telltale tremors in a glacier can
alert people to an impending avalanche.

hear some small noise before it breaks.” The team also saw the reflectors accelerate several days before the rupture. Scientists knew that seismic activity increases five days before a break-off, but by combining the motion of the glacier with the icequake activity, the new model can detect a rupture 15 days in advance.

“It’s the first time icequakes have been used as a precursor to these break-offs,” says glaciologist Fabian Walter of the Scripps Institution of Oceanography in La Jolla, Calif.

How a trickle can create a cavern
model of fluid flow helps dissolve puzzle of cave formation

By Alexandra Witze

Spelunkers look at a cave and wonder
how to explore its deepest reaches. But
physicists look at it and wonder how it
got there in the first place. A new math-
ematical analysis solves a long-standing
cave-formation puzzle: how a trickle of
water laced with carbonic acid manages
to quickly dissolve rock to create massive
conduits. The trick, it seems, is that fluid
flow focuses rapidly in certain channels,
which grow at the expense of others and
allow the acid to penetrate deeply.

A new model shows how fluids penetrate rock, forming a cave. At first (left)
channels develop equally, but over time (right) certain channels dominate.

The work could improve understanding of the safety of dams, waste storage sites or anywhere else fluid might be seeping through the ground.

Limestone caves form when a tiny fracture opens in the rock, perhaps due to some internal stress, and water begins percolating through. Most water contains some carbon dioxide, making it a weak acid that can eat away at the calcium carbonate in limestone. It has been unclear how that dissolution can happen fast enough to produce deep penetration and allow long cave systems to form.

The equations describing fluid flow in the rock always contain a mathematical instability, the researchers show. That means that very soon after a fracture opens, fluid flow focuses along tiny ripples, allowing some channels to expand.