17 December 2015

Volcano pressure guns show how rocks spew and eruptions ensue

Posted by lhwang

by Brendan Bane

The vinegar volcano is a stale science experiment. But Italian geologist Valeria Cigala takes the tired demonstration to a violent new level: She builds artificial volcanoes—essentially high-pressure guns packed with volcanic rock—to erupt in her lab.

Cigala, who studies volcanic blasts at the Ludwig Maximilians University in Munich, fires off fake volcanoes so she can predict how real ones spew gas and rock into the atmosphere. The shape of a volcanic vent as well as the temperature and size of the particles packed inside determine how fast and far its contents launch. Cigala found the most violent eruptions came when vents were funnel-shaped, packed full of small particles and hot. But surprisingly, she found that low-temperature eruptions from narrow openings shot rocks at the widest angles. Cigala presented her research at the 2015 American Geophysical Union Fall Meeting.

Cigala said she has long held a fascination with volcanoes. She said she’s inspired by two questions: “What comes out and how can we reconstruct what was inside?” To find the answers, she built an assortment of artificial volcanoes: steel tubes packed with varying amounts of basaltic rock and pumped full of argon gas to boost their pressure. The tubes were fit with four openings: two wide-open funnel vents, a straight cylinder, and a narrow nozzle. The openings mimic simplistic versions of real volcanic vents.

Basaltic particles burst forth from pressurized volcano guns. 
Each of these guns, or artificial volcanoes, is packed full of 
volcanic rocks 1-2 millimeters (0.04-0.08 inches) in diameter and 
erupts shortly after exceeding 150 bars (2175 psi) at a room 
temperature of 25 degrees Celsius (77 degrees Fahrenheit). 
  Credit: Valeria Cigala

Each tube was sealed at one end and closed on the other by three iron diaphragms. The diaphragms were designed to fail when a tube’s internal pressure exceeded 150 bars (2,175 psi), Cigala said. The tubes were either heated to 500 degrees Celsius (932 degrees Fahrenheit) or left at a room temperature of 25 degrees Celsius (77 degrees Fahrenheit). Cigala and a high-speed camera peered from behind a plexiglass window. When she wanted an eruption, Cigala simply turned a valve to pump more argon into the pipe. The diaphragms broke, gas spewed, and an eruption ensued.

“It’s kind of noisy. It’s a nice boom,” Cigala said while describing the eruptions, which tended to last 20-30 milliseconds. Her high-speed camera, shooting at 10,000 frames per second, captured the action. “First you see gas coming out, and you can already see that it’s influenced by the geometry of the vent,” Cigala said as she narrated the violent footage. Surprisingly, rocks spread the farthest from the nozzle-shaped vent, which had the smallest opening, and when the tube was at room temperature. But when Cigala packed the funnel tubes full of smaller rocks and heated them, the rocks shot out far faster, some at nearly 300 meters per second (660 miles per hour).

Each eruption blasted into a 3-meter (9.8-foot) chamber, which captured all the rocks and helped Cigala measure their breakage. The nozzle vent broke up the most rocks. More fragmentation means a greater chance of another violent phenomena, she said: lightning.

When enough small rocks bump and clash inside the eruption chamber, lightning can strike. Cigala said she wants to devote her next round of blasts to quantifying the lightning effect and to producing more realistic volcanoes. She wants to simulate eruptions using asymmetric vents, which more closely mimic real volcanoes.

— Brendan Bane is a graduate student of the Science Communication Program at UC Santa Cruz. You can follow him on twitter at @brendan_bane