About a decade ago, Johns Hopkins University
geologist Bruce Marsh challenged the century-old
concept that the Earth's outer layer formed when
crystal-free molten rock called magma oozed to the
surface from giant subterranean chambers hidden beneath
Marsh's theory — that the deep-seated plumbing
underneath volcanoes is actually made up of an
extensive system of smaller sheetlike chambers vertically
interconnected with each other and
transporting a crystal-laden "magmatic mush" to the
surface — has become far more widely accepted.
This sort of system, known as a "magmatic mush column," is
thought to exist beneath all of the world's
major volcanic centers.
Now, Marsh — using the windswept McMurdo Dry
Valleys of Antarctica as his "walk-in"
laboratory — posits that these channels did more
than simply transport or supply magma and crystals to
form the Earth's surface: As the magma pushed up through
the earth, he says, the pressure fractured
the crust in such a way that it provided a sort of
"template," guiding later erosion in sculpting a series
of valleys and mountain ranges there.
"As the magma made its way to the surface, the
pressure broke the crust up into pieces," said
Marsh, who described his latest findings to fellow
scientists at a recent meeting of the American
Geological Society. "That fracturing reflected a pattern
of stress in the same way that a windshield
put under pressure will eventually fracture and the
pattern of the broken glass would reflect where
the stress was originally applied.
"Magma then seeped in," he said, "and 'welded' the
fractures, sealing them temporarily until
erosion — in the form of snow, rain, ice and wind
— went to work on these weaknesses, carving out
valleys, mountains and other landforms that we see there
today and marking where the solidified
magma originally was."
Marsh said that, in Antarctica, both of these
functions date back at least 180 million years to
the time when the continents split apart. He points out
that this observation brings together the
usually disparate study of deep-seated magmatic processes
and land-surface evolution.
"It's one of those situations where, usually, never
the twain shall meet, but they do in this
case," the earth scientist said. "Having recognized
evidence in this critical process in the McMurdo
Dry Valleys is important because it may allow us to
recognize it in other areas where the geologic
record is scantier and less complete."
The Dry Valleys makes an ideal place to study these
systems because it was eroded into its
present form millions of years ago and has, unlike the
rest of Earth's surface, undergone very little
subsequent erosion. Marsh's colleagues George Denton of
the University of Maine and David Marchant
of Boston University call this region "a relic landscape"
because it is the only known place on Earth
that looks almost exactly as it did millions of years
"The delicacy of the landscape in the Dry Valleys has
preserved for us an unusually rich
collection of geologic evidence of the processes that
formed this terrain," Marsh said.
For more than a quarter of a century, Marsh —
who could be thought of by fans of 1980s
detective television shows as sort of a "Magma P.I."
— has been working to understand the deep
underground systems that bring magma to the Earth's
surface. In 1993, he found the Dry Valleys, a
walk-in "museum" that he calls "the one place on earth
where the plumbing system is exposed in this
"You can stand on shelves of solidified lava that
were deposited by magmatic activity 180 million
years ago," he said. "It's awe-inspiring."