July 24, 2012
Pre-eruption Activity In Conduits Affect Volcanic Ejections
Brett Smith for redOrbit.com - Your Universe Online
In addition to being fairly unpredictable, volcanoes can eject a wide range of material, from mile-high plums of black ash to a deadly hail of fist-sized pumice. These ejections travel extremely fast and can reach internal temperatures between 750 to 1,500 degrees Fahrenheit.
The prevailing theory has been that the difference in particle size determined when bubbling magma deep below the volcano converts into a rising stream of gas and rock bits during a process known as fragmentation.
However, a recent study published in Nature Geoscience indicates that the size of the ejected particles depends more on how often those particles collide as they race up the volcano´s conduits on their way to the surface. This means that the farther rock particles have to travel–the more likely they will be pulverized into a fine ash.
“The longer these particles stay in the conduit, the more often they collide with each other,” said volcanologist Josef Dufek of Georgia Tech University in a press release.
“These high-energy collisions break the volcanic particles into fractions of their original size. That´s why deeper fragmentations produce small particles. Particles that begin closer to the surface with less energy don´t have time for as many collisions before they exit the volcano. They stay more intact, are larger and often contained in pyroclastic flows.”
While larger chunks of pumice can be more deadly, ash plums can threaten wider areas. Winds can spread the falling particles across thousands of miles, posing a risk to plants, animals, and air travelers. Ash from the 1980 Mount St. Helens eruption fell as far away as Montana.
Using high-energy experiments and computer models to study how these rock particles might disintegrate, Dufek and his team, which included two University of California, Berkeley scientists, determined that a volcano is more likely to give off ash plumes if fragmentation begins miles underground.
In their research, the team gathered geologic material from California´s Medicine Lake volcanic deposit for use in collision experiments. Scientists believe that the Medicine Lake Volcano has been active for 500,000 years. They also obtained glass spheres to simulate pumice that also hardens before crystals are able to form.
Using a compressed gas-powered gun, the volcanologists found that particles must collide at almost 100 feet per second to fragment into pieces. They then calculated evidence that showed large pumice particles will not maintain their integrity unless the fragmentation occurs at a shallow depth, within about 1600 feet.
Unfortunately, due to nature and inaccessibility of the fragmentation process, scientists have very little data surrounding how the mechanism works and acknowledge that much of the eruptive dynamics and some composition of the ejected material are determined in this process.
According to Dufek, future research will build on this recently published study and focus on understanding the dynamics behind super volcanoes, whose activity produced the features in the current Yellowstone National Park.
“We know very little about the eruption processes during super eruptions,” said Dufek. “Indications of their fragmentation levels will provide important clues to their eruptive dynamics, allowing us to study them in new ways.”