May 7, 2012
GPS And Space-based Radar Help Calculate Sierra Nevada’s Age, Growth
Researchers studying the highest mountain range in the continental United States have been able to detect the rate at which it is growing and perhaps how old the mountains that are found there using new technology from space.
The team of researchers from University of Nevada´s Geodetic Laboratory in Reno and University of Glasgow in the UK, said the Sierra Nevada mountains are steadily growing at the rate of about 1 inch every two decades across the entire 400-mile-long range on the Nevada-California border.
Though it has remained unclear in the past how or when the mountains in the region got there, the team believes that with the aid of GPS and space radar technology, they can determine just how old the Sierra Nevada range in fact is. Mt. Whitney, the Sierra´s highest peak at 14,405 feet, is growing at about 1 to 2 millimeters per year. This calculation has led the team to theorize the age of the mountain and its range at just about 3 million years old.
“There´s a surprisingly wide variety of opinions about how and why the Sierra Nevada goes up, and about the ages and timing of all the events that contribute to the uplift,” lead researcher William Hammond, a geophysicist at the University of Nevada, told Crystal Gammon of OurAmazing Planet on MSNBC.com. “These findings suggest that whatever mechanism is at play, it´s acting on the entire range.”
“The exciting thing is we can watch the range growing in real time,” Hammond said in a recent press statement. “Using data back to before 2000 we can see it with accuracy better than 1 millimeter per year. Perhaps even more amazing is that these miniscule changes are measured using satellites in space.”
Hammond and colleagues used satellite-based GPS data and InSAR (space-based radar) data to make their calculations, which show that the crust moves upward compared to Earth´s center of mass and compared to relatively stable eastern Nevada. The data could help resolve a long-standing debate on the exact age of the mountain range.
Geologists have two very different possible theories on the age of the Sierra Nevada range: the first is that the range is 40 to 80 million years old, and the second is that it is about 3 million years old.
That is a big difference, said Hammond. Either the mountains are very old and no longer growing, or they are still quite young and growing at a measurable rate.
By using GPS and InSAR, the team took microwave snapshots of the mountains and surrounding features as the satellites whizzed by overhead. The satellites revisit the same spot every month or so to take more microwave snapshots. Because it can monitor large swaths of landscape over long periods of time, InSAR data is particularly useful for measuring slow and steady changes in the shape of the Earth´s surface.
“Most of the seismic cycle is made up of periods of time where the Earth is not shaking, but it is deforming,” Hammond told Gammon. “We´re getting better at measuring that slow shape change as a way of understanding, for example, where the Earth might break in future earthquakes.”
Hammond and colleagues analyzed the Sierra´s for 18 years using the InSAR images, along with the precise measurements from GPS stations, to zero in on the growth rate. They determined that, based on their analysis of the data received, the Sierra Nevada range, with an average elevation of 6,500 to 8,200 feet, could have been built in less than 3 million years.
While the team can put a possible age on the mountain range, determining exactly how the range was built is still a mystery.
The range lies just west of the Basin and Range Province in Nevada, where east-west tectonic forces are in the process of ripping the Earth´s crust apart. Some geologists believe this stretching might be causing the eastern side of the range to grow upward.
Seismologists believe there is a heavy blob in the Earth´s mantle that may have been attached to the base of the range´s tectonic block. They think this dense blob used to weigh the block down, like a keel on the bottom of a ship. Then, sometime between 3 million and 10 million years ago, this keel peeled off and sank down deeper into the Earth, and the Sierra Nevada block popped up.
Whichever it was, Hammond said the process affected nearly the entire range and is still building up the Sierra Nevada today.
The research team includes Hammond, Geoff Blewitt, Hans-Peter Plag and CornÃ© Kreemer from the University of Nevada; and Zhenhong Li of the Centre for the Observation and Modeling of Earthquakes, Volcanoes and Tectonics, School of Geographical and Earth Sciences, University of Glasgow in the UK.
GPS data for the research was collected through the team´s MAGNET GPS Network in Reno, and more than 1200 stations from the national Science Foundation (NSF) EarthScope Plate Boundary Observatory and more than 10,000 stations from around the entire world. The InSAR radar data came from the European Space Agency with support from NASA.
The research was funded in the US by the NSF and NASA, and in the UK by the Natural Environment Research Council. Findings of the study were published in the April 27 issue of the journal Geology.