WHY ARE SOME ERUPTIONS SO DESTRUCTIVE?
By Homer Montgomery
0254 Zulu (GMT) - R/V Melville, Southeast of Guam
Nineteen ninety one was a banner year for the edge of the Philippine plate. On June 15, Mount Pinatubo let loose with one of the largest eruptions of any volcano this century producing tremendous pyroclastic flows and lahars (mudflows). The 40 km high ash cloud was mighty impressive. Of great interest to everyone studying this eruption, Pinatubo had last been active 600 years before the 1991 eruption. The other great eruption was Unzen in Japan.Unzen had been quiet for 200 years. Unzen has remained active in the memory of Japan because the last time it blew up 15,000 people perished.
Lahars are roiling, ugly milkshakes of water and volcanic ejecta. They may be thin or as thick as wet concrete. Remember, composite volcanoes have considerable slope. Mudflows rip down from above scouring and burying everything in their paths. The most graphic example I can recall is Nevado del Ruiz in Columbia. The eruption melted all the snow and ice atop the peak. Melt water immediately mixed with volcanic ash and other debris.Twenty five thousand people died when they were ripped up and shot away in a mad sea of mud.
Of decidedly extreme danger is the pyroclastic (from the Greek pyro for "fire," and clastic for "broken") material produced in eruptions. Ash, blocks, and volcanic bombs fly out of the vent and may devastate the nearby surroundings. The most spectacular type of eruptive display is unquestionably the nuee ardente (pronounced noo-ay are-dant). A nuee ardente (French for "glowing cloud") is an incandescent cloud more dense than air that moves on a cushion of hot, nearly frictionless, expanding gas. As the gas continues expanding, explosions also continue. When viewed during the day the clouds are black, but at night they glow an eerie red like something one might expect in a Stephen King movie. Along with the associated volcanic material, pyroclastic flows collect all manner of material including tree trunks, parts of houses, bodies, and even automobiles. All of this debris ends up strewn across the landscape, wrapped around immovable objects, or snagged in trees that were too robust to be carried away.
What is the mechanism behind these fearsome eruptions? Why are they so destructive as compared to the relatively safe, fluid, shield volcano eruptions, like those in Hawaii? Pinatubo and Unzen are subduction zone volcanoes. As such they are located at convergent boundaries. Composite volcanoes have stiff magma that makes it difficult for gases to escape. Stiff magma has a higher percentage of SiO2 than does fluid magma.Great pressures are produced. As the magma ascends the gases expand rapidly producing incredibly violent, explosive eruptions. You know the process. Open a bottle of Coke. Put your finger over the top.Ê Shake it up and then let her rip. What you have is subduction zone volcanism in a Coke bottle courtesy of escaping gases. Adults love this trick. Have you ever seen a bottle of Champagne at a celebration that was not fired off with great gusto? Moet Pinatubo.
The Unzen eruption killed people and tore up lots of stuff. The Pinatubo eruption, aside from killing people and tearing up lots of stuff, affected the entire planet. Ash and gases ejected into the upper atmosphere lowered the temperature in some places by as much as 0.5¡ C. The process is poetic, but evil chemistry. SO2 combines with H2O producing sulfuric acid. The sulfuric acid blocks some sunlight and thus alters the earth's heat balance.
The price of living on many beautiful, mountainous coastlines is having ill-tempered neighbors. Volcanoes.