11,000
square km
Size of largest
iceberg ever
observed
78
square km
Size of largest
Northern hemisphere
iceberg ever observed
Texas flood carves canyon in days
Torrent is a model for past megafloods on earth and Mars
By Sid Perkins
An unexpectedly large canyon carved
in just three days by a Texas deluge
may help scientists estimate the size
of ancient megafloods believed to have
sculpted terrain on both Earth and Mars.
In July 2002, record flooding in
central Texas caused Canyon Lake, a
dammed reservoir about 55 kilometers
northeast of San Antonio, to overflow.
The water carved a 2.2-kilometer-long
canyon downstream of the dam, Caltech
geologist Michael P. Lamb and colleague
Mark Fonstad, a hydrologist at Texas
State University in San Marcos, report
online June 20 in Nature Geoscience.
Water flowing through the spillway quickly stripped the creek bed of
its trees and soil and began chewing
into the limestone bedrock below, says
Lamb. Over the course of about three
days, the torrent carved a canyon that
ranged between 40 and 60 meters wide
An overflowing Texas reservoir carved a
2.2-kilometer-long canyon in three days.
and averaged 7 meters deep over much
of its length. Overall, the flood excavated
about 460,000 cubic meters of material,
almost half of that being rock.
The torrent was so intense that it
plucked large, flat-sided blocks of limestone up to a meter across from the
bedrock. During the flood’s peak, water
flowed through the canyon at a rate that
would have filled 20 Olympic-sized
pools each minute.
“To be able to document the effects of
this event so soon after it occurred, it’s
an important contribution,” says Jim E.
O’Connor, a geologist with the U.S. Geological Survey in Portland, Ore.
In the lower half of the canyon, the
deluge sculpted streamlined islands
of material resembling those found in
areas stricken by much larger floods on
Earth and presumably on Mars.
The Texas flood “is a potential analog for what we see on Mars,” says Alan
D. Howard, a geomorphologist at the
University of Virginia in Charlottesville who has studied the geology of the
Red Planet. The majority of bedrock on
Mars is basalt deposited by lava flows,
each layer of which would have cracked
into large blocks as it cooled, he notes.
In a raging flood, these blocks could
have been picked up from the Martian
terrain and tossed about like the ones
in the Texas flood, excavating a lot of
material quickly.
“It doesn’t take millions of years to
create an impressive channel,” Howard
notes. “Flowing liquid can do a lot of
work in a short period of time.”
Antarctic shoal
breaks the ice
Breakup of a massive berg
pinpoints an undersea ridge
During the last couple of decades,
scientists poring over satellite images
have noticed several large icebergs
breaking up as they wafted along a particular stretch of the Antarctic coast.
Now, thanks to data gathered in part
by an instrument-laden iceberg, the
researchers know why: The ice masses
were crashing into a previously unreported submarine ridge.
Most icebergs that calve from Antarctica’s Ross Ice Shelf are carried by shore-
hugging currents past Cape Adare, a
section of coast that lies south of New
Zealand. That remote stretch of ocean
is covered with sea ice about 10 months
of the year, says Seelye Martin, an oceanographer at the University of Washington in Seattle. But in October 2005,
iceberg B15A — the largest remnant of
a 300-meter-thick, Connecticut-sized
berg that split off the ice shelf in March
2000— cleared a path as it bulldozed
through the area. Such massive icebergs
break free only once every decade or so
and can float for years if collisions don’t
break them into smaller pieces.
Scientists had installed several
instruments on B15A in 2003 and
2004 — including a seismometer, a compass and GPS equipment — to monitor
the megaberg as it moved north, melted
and broke apart (SN: 5/12/01, p. 298).
Those instruments indicate that in
late October 2005, B15A briefly came
to a stop, rotated back and forth a few
degrees and began cracking up, a splintering also detected by seismometers on
shore and by Earth-orbiting satellites.
Sonar surveys revealed a 9-kilometer-
long, previously unrecognized undersea
ridge where the berg broke up, Martin
and colleagues report online June 18 in
Journal of Geophysical Research–Solid
Earth. The shoal, whose peaks lie at a
depth of about 215 meters, sits about 25
kilometers off Cape Adare, smack dab in
the path of the coast-hugging currents.
“This is a big navigational hazard for icebergs,” Martin notes. The ridge lies too
deep, though, to be a danger to ships that
might navigate these mostly ice-bound
waters, and many small icebergs have
passed over the shoal without incident. s
