Violent volcanic eruptions may be caused by sealed vents rather than by zones of water and volatile gases in magma as previously thought, say scientists who studied California's 600-year-old Obsidian Dome.
"This changes the way we think about magma chambers and suggests magmas may be more uniformly gas rich and not as zoned as we thought before," said geologist John C. Eichelberger of Sandia National Laboratories.Violent eruptions have been explained by the influence of water vapor and volatile gases in the upper zone of a magma chamber, deep beneath the Earth's surface. The theory says trapped gases expand as magma is forced into lower-pressure regions near the surface, causing explosions.
The violent eruption creates a funnel-shaped opening which collects debris, filling the vent through which subsequent magma must pass. The porous rubble is like a sand pile, keeping pressure on the magma but letting gas leak out before drier magma reaches the surface, where its lower pressure results in a flow.
Sandia's study suggests magma acts as a sponge that can leak gas as it rises to the surface. Fractured rock in the volcanic vent allows gas to escape from the magma, releasing pressure and causing less violent eruptions.
The Obsidian Dome volcano contains a magma tube within a broader vent. The vent contains debris from the initial explosive eruption and the dome was built of lava that flowed over several weeks. Chemical studies showed the same magma caused the explosion and the dome-building, and that the material that built the dome lost most of its gas before reaching the surface.
Magma that is 65 percent to 75 percent silica has silica-rich molecules that tend to chain together and drastically increase the resistance to flow, or viscosity. Silica-rich magma can be up to 100,000 times more viscous - thicker - than basalt magma that is 40 percent to 50 percent silica.
Charles R. Carrigan of Sandia said if gas can't escape, magma with as little as 3 percent gas will expand several hundred times when it reaches the surface, resulting in an explosion.