Johns Hopkins University

Spring 2008
Vol. 5, No.2


A Eulogy to FUSE

Pooling Expertise to Combat Pollution

Student Research: From the Field

High-Tech Access for a Centuries-Old Poem

For Female Would-Be Presidents, Press is Hard to Get

>Fractured Origins

adjust type size + -

Fractured Origins

About a decade ago, geologist Bruce Marsh, of the Department of Earth and Planetary Sciences, 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 volcanoes.

Marsh's theory—that the deep-seated plumbing underneath volcanoes is actually made up of an extensive system of smaller sheet-like chambers vertically interconnected 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 magma and crystals to form the Earth's surface: As the magma pushed up through the earth, 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.

Marsh described his latest findings to fellow scientists at a recent meeting of the American Geological Society.

"As the magma made its way to the surface, the pressure broke the crust up into pieces," Marsh says. "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 says, "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."

In Antarctica, according to Marsh, 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 says. "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."