By Natasha Bhuyan
Arizona Daily Wildcat
Wednesday, September 22, 2004
After conducting a yearlong study in California, a team of UA geologists has new information about how mountains are formed, and says this knowledge could help scientists predict when and where volcanic eruptions will occur.
Published in the Sept. 2 issue of Nature magazine, the findings explain the disappearance of Moho, the role of "the drip" and what it all means for volcanoes.
The Moho, or Mohorovicic Discontinuity, is a boundary surface 10 to 70 kilometers beneath the ocean floor that separates the Earth's crust from the underlying mantle.
Most places underneath the Earth's surface have a well-defined Moho, said Mihai Ducea, associate professor of geosciences, but below California's San Joaquin Valley, the Moho is missing.
To solve the missing Moho mystery, Hersh Gilbert, a research associate for the department of geosciences, said the team used recordings of distant earthquakes from a network of seismometers located around San Joaquin Valley. In areas where the Moho was present, Gilbert said, a clear signal was produced on the seismic record.
"We processed this data and used it to image the inside of the Earth in much the same way a CAT scan or MRI can be used by the medical community to image the inside of a body," Gilbert said.
George Zandt, professor of geosciences who led the research, said the team discovered Moho exists underneath most of the Sierra Nevada mountain range, but near the western foothills of that range Moho has vanished.
Zandt blamed the missing Moho on a phenomenon called "the drip."
"The drip" is the location at which the upper part of the Earth's mantle sinks into the rest of the mantle, dragging the crust down with it. When viewed from a cross-section, Ducea said, the activity resembles a drip of paint.
Ducea said the drip in the Sierra Nevada range has destroyed the simple relationship between the crust and mantle.
"We suggest that the drip is pulling down the crust, smearing out the Moho and causing subsidence at the surface," Zandt said.
Ducea said the findings are a "bizarre" departure from what is commonly known about mountain ranges and a step forward in geology and volcanic activity.
By understanding the relationship between the Moho and the drip, Gilbert said the scientific community gains information about the strength of the Earth's crust and how it deforms.
"There have recently been small earthquakes reported near the base of the crust further north in the Sierras," Gilbert said. "An implication of our research may be that these earthquakes result from the shearing-off of dense crustal material, much like what occurred in the southern Sierras nearly 5 million years ago."
Gilbert said he had fun working on the project, which incorporated multiple branches of geology, including seismology, sedimentation patterns and petrology, to describe active processes around the southern Sierras.
The research was funded by the National Science Foundation.
Zandt, Ducea and Gilbert also worked with Jason Saleeby of the California Institute of Technology, Thomas J. Owens of the University of Southern California and Craig H. Jones of the University of Colorado, Boulder.
"It took a lot of effort to figure out what could have made the Moho disappear," Gilbert said. "But once we started to figure that out, all the pieces started to fit together."