Breaking new ground
Theory suggests magma events fracture earth in such a way as to create template for future erosion
Posted January 3, 2008
It doesn’t get any better than the McMurdo Dry Valleys if you want to get at the deep roots of a volcano’s magma system, to see how the liquid rocks pushes its way through the Earth’s crust.
That’s the judgment by geologist Bruce Marsh at Johns Hopkins University. “It’s the No. 1 place in the world to look at these interactive magmatic processes,” he said. “There’s nothing else really like it.”
The Dry Valleys, located just a short helicopter ride away from McMurdo Station, serves as a living laboratory for a number of scientists each summer season — thanks to its accessibility, unique ecosystem and breathtaking geologic features.
For Marsh, the draw is the “exquisitely displayed” geology stamped across the face of the valley walls. The well-preserved magmatic plumbing from a 180-million-year-old volcanic event has led him to put forth a revolutionary theory about how those past events dictated how erosion would shape the landscape millions of years later.
“The magma probably set up the template for the erosion process,” he said.
The idea begins with another theory that Marsh previously suggested — that the deep-seated plumbing underneath volcanoes consists of an extensive system of small, sheet-like chambers called sills that are vertically interconnected with each other through channels called dikes. These chambers transport what he calls “magmatic mush” to the surface.
Marsh now believes that these channels not only transport magma and crystals to form the Earth’s crust, but fracture it in such a way as to create a template that guides later erosion.
“It turns out that the magmatic plumbing system that came through the crust there, basically lifted and fractured up the crust,” Marsh explained. “That actually made the template for how some of the Valleys eroded.”
Marsh will again lead a team to the Dry Valleys this season, in January, to collect samples and map the spatial relations between the rocks of the four-layered magma system. (See related story Looking at new dimensions.) The dikes and sills collectively are known as the Ferrar dolerites, named after Hartley Ferrar, a geologist on Robert F. Scott’s 1901-1904 Discovery Expedition. Dolerite is basaltic magma that solidifies rapidly in sills and dikes near the surface.
Marsh uses an analogy of cake to make his point: Imagine a stale and dry, four-layered cake that’s been injected with hot paraffin wax. The wax fractures open the cake and fills the fissures. Erosion eventually eats away at the wax in our analogy, though the coated surfaces of the original strata (or cake) are tougher, generally stopping the erosion process.
The vestiges of the magma, though slightly worn down, are still detectable at the surfaces in the Dry Valleys today, Marsh said.
In a press release from Johns Hopkins University following a recent speech he made to the Geological Society of America about the theory, Marsh explained the process this way: “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 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.”
Tom Wagner, program manager for the National Science Foundation’s Antarctic Earth Sciences Division in the Office of Polar Programs, called Marsh “the master plumber of magma.”
Said Wagner, “While the immense exposure of the Dry Valleys and the massive nature of the Ferrar intrusion lent themselves to study, Bruce looked at a problem that people have been puzzling over for hundreds of years and found something new. He is amongst the amazing minds of Earth science.”
NSF-funded research in this story: Bruce Marsh, Johns Hopkins University.