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Geophenomena

Research out of the ashes
Seafloor steam carries lava flows


Research out of the ashes

The devastating fires that ravaged Southern California this fall burned more than 1,100 square miles of land, leaving a stark landscape in their wake. The fires killed 20 people and destroyed thousands of homes and hundreds of thousands of trees. Fire also interrupted data flowing from seismic and GPS networks operated by the U.S. Geological Survey (USGS) and partner organizations, temporarily halting part of the region’s earthquake monitoring system. In the months following the fires, people are beginning to rebuild, but the hillsides remain largely bare.

On Oct. 26, the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) onboard the Terra satellite captured this image of the Old Fire/Grand Prix fire burning on either side of Interstate 15 near the Cajon Pass in the San Bernardino Mountains, roughly 80 kilometers east of Los Angeles, Calif. The image is simulated natural color, showing how the scene would look from above to a human eye. Image courtesy NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team.


The charred landscape is presenting an unprecedented research opportunity, however, for geoscientists studying fault zones, debris flows and flooding, and ecosystems in the region. USGS researchers are at the forefront, hoping to use this opportunity to plan for and mitigate hazards in the future, says Susan Cannon, a geologist with USGS in Denver.

The fires burned off all the vegetation covering miles of fault-ridden hillsides, including hazardous sections of the San Andreas, Sierra Madre and Cucamonga faults. Researchers can now observe the faults and past earthquake deformation scars and obtain “bare earth” images like never before, says Ken Hudnut, a USGS geophysicist in Pasadena, Calif.

Light detection and ranging aerial mapping (LIDAR) can provide digital, high-resolution topographic data through dense groundcover and foliage. “However, LIDAR works even better without any obscuring vegetation,” Hudnut says. If geologists could take aerial photographs and LIDAR images of the faults before vegetation regrows, then they could better map subtle geomorphic features along the faults. These features would provide timing constraints, slip histories and rupture lengths of past fault ruptures, all needed to produce seismic hazard assessments along the heavily populated sections of these faults.

The denuded hillsides are also presenting research opportunities related to flood and debris-flow risks, Cannon says. Vegetation ordinarily holds soils in place, so once winter rains set in, the slopes are at high risk for flash flooding and debris flows. And with a small El Niño predicted this year by the National Oceanic and Atmospheric Administration, mountainous Southern California can expect heavier than usual rains. The counties already have rain-gauge networks in place, and USGS has stream gauges in many of the hazardous channels. Researchers can plug real-time rain- and stream-gauge information into a rainfall intensity/duration threshold computer model to help forecast flood and debris-flow events, Cannon says. They can then implement a real-time flood and debris-flow warning system.

Using data from throughout the West, Cannon and colleagues have also developed models for assessing the probability of debris flow from individual basins and the magnitude of the response. Adding data from the recently stripped hillsides to these models, Cannon wants to refine the models to make them specific to Southern California. Doing so will help predict which basins are most susceptible to debris-flow activity and which communities have the greatest risk for debris flows and flooding the next time fire strikes.

The scope and magnitude of the fires have also created a unique opportunity to study the effects of fire on the natural environment. One such study involves looking at the effect of fire suppression policies on the forests. The fires burned right through major stretches of landscape that were only a few years old just as readily as they burned through the older shrublands with high fuel loads, says Jon Keeley of USGS in Three Rivers, Calif. The Cedar Fire in San Diego tore through several prescription burns of a thousand acres or more each. “And over 40 percent of the 60,000 acre Otay Fire burned through a landscape that had burned completely in 1996,” Keeley says.

The Healthy Forests Restoration Act, passed into law on Dec. 3, aims to protect old-growth forests and communities from catastrophic wildfires by reducing the fires’ natural fuel. The legislation dictates that at least half of the funding for fuel reduction projects must be used in wildland-urban border areas, such as the San Bernardino Mountains. But the majority of acreage burned in the fires this fall was in shrublands.

Research has clearly demonstrated that landscape-level fuel treatments have no ability to stop these catastrophic events, Keeley says. More research is needed to figure out how to best protect people and property in the region.

Meanwhile, nature plugs on: Winter rains come, hillsides try to maintain their ground, and vegetation begins to sprout over the faults.

Megan Sever

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Seafloor steam carries lava flows

Right now, thousands of meters beneath the ocean surface, at near-freezing temperatures and extremely high pressures, lava is erupting somewhere along the mid-ocean ridge system, the 56,000-kilometer-long volcanic mountain chain that encircles the globe.

Geologists studying the seafloor near mid-ocean ridge volcanoes have found strange features, presumed to have formed from molten lava, as much as a few kilometers away from the eruption sites. These features include jagged-edged holes, collapsed pits and open cavities. Some pits contain lava pillars — short hollow tubes rising through the cavity from a floor scattered with pieces of fractured basalt.

Scientists study rocks from the mid-ocean ridge, hoping to better understand how steam helps to create lava-related features, such as this collapsed pit, many kilometers away from eruption sites. Image courtesy of Mike Perfit.

Now, scientists have discovered curvy, or cuspate, drip-like features on the undersides of pieces of lava from the roofs of cavities on the East Pacific Rise, a ridge 700 kilometers southwest of Acapulco, Mexico. The discovery has led scientists to identify a mechanism that not only explains the seafloor features, but also how still-molten lava could form these features so far from the vent. That mechanism is steam.

The cuspate drip features on the samples, taken from depths of about 2,500 meters, exhibit the work of “bubbles and cavities of briny vapor at magmatic temperature that existed during formation of the drip,” says Mike Perfit, a marine geologist at the University of Florida, who published these findings with colleagues in the Nov. 6 Nature.

Previously, researchers did not think that seawater could boil at the high pressures found on the mid-ocean ridge. Thus, the new finding is very interesting, says Bill Chadwick, a marine geologist with Oregon State University, “and they document it very well.”

Steam also helps the lava travel many kilometers from the vent without being quenched by ice-cold seawater, Perfit says. As the molten lava rolls over the seafloor, water trapped in fractures below is able to boil, making a carpet of steam on which the lava can more easily flow. The cavities insulate the lava, allowing it to flow farther. The rolling process may also explain the extreme smoothness of some lava-flow surfaces, despite flowing over uneven ground.

“These lavas come out and skate around on top of this very expanded vapor phase,” Perfit says. “This may be a really significant volcanological aspect of how you create the upper part of the seafloor.”

Steam bubbling up through the lava creates the hollow pillars and pockets under the lava shell, allowing lava to form the drips and cuspate features found on the East Pacific Rise samples, Perfit says.

Historically, most geologists have agreed that the features were due to molten lava flowing beneath a hardened glassy shell of lava. But they attributed the features to “lava drain-away,” in which lava flows down a hill, draining the area it just traveled through and causing the glassy shell to collapse.

“The problem with that was, we didn’t ever see that much drained-away lava,” Perfit says. “If you have a large volume of it that just erupted, and it drains away, you need to be able to find all that material.” Another problem is that drain-away would cause a vacuum beneath the hardened shell, which would then cause the entire sheet to collapse instantaneously, he says.

Some researchers have suggested that magmatic gases bubbling out of the lava would cause the vapor pockets. However, Perfit says, magmatic gases couldn’t account for the size of the cavities produced or their distances from the vent.

Also, scanning electron microscope investigations of the samples have shown a variety of unusual minerals and the presence of once-molten salt fused to the drip features, Perfit says. Salt would come from seawater and not magmatic gases. If the salt had precipitated out of seawater long after the drip features formed, it would be present as salt crystals. The lack of crystals means the seawater reached temperatures high enough to melt salt (around 800 degrees Celsius), which, in turn, means it went through a vapor phase, he says.

However, many questions remain to be answered, including why the molten salt has not dissolved despite exposure in undersaturated seawater. Perfit and colleagues also hope to further understand the influence of steam on lava chemistry and its role in the formation of new seafloor.

“I agree with the evidence they cite that says this actually happens,” Chadwick says. “But where there’s room for different interpretations is exactly how it happens, under what circumstances and to what extent.”

Sara Pratt
Geotimes contributing writer

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