On this day in 2006, a sudden explosion of ash, gas, and steam erupted from the long since dormant Fourpeaked Mountain in Alaska, kicking off months of volcanic activity and excitement.
Fourpeaked Mountain is located 200 miles southwest of Anchorage, the United States’ northernmost major city. The mountain is made up of a series of small, isolated volcanic exposures along its ridges and cliff faces. The orientation of the various lava flows that once spilled out from these exposures suggests that the Fourpeaked Mountain’s summit–which is almost completely covered by the icy Fourpeaked Glacier–is also the vent for the volcano that formed it.
Like Vesuvius (which was responsible for the sudden destruction of Pompeii and Herculaneum in 79 CE) and Krakatoa (which exploded in 1883, killing thousands and also producing the loudest sound ever heard in modern history), Fourpeaked Mountain is a stratovolcano. This common type of volcano, also known as a composite volcano, has a composite layered structure created over the ages out of repeated outpourings of eruptive materials–ash, hardened lava, pumice, tephra–which rarely flow far from the mountaintop because of their high viscosity. A stratovolcano is typically tall and steep and characterised by highly explosive eruptions.
Until very recently, however, Fourpeaked Mountain was actually thought to be extinct. Certainly it had lain dormant for a very long time, at least 10,000 years and possibly much more, prior to the unexpected phase of volcanic activity that started on 17 September 2006. On this day local pilots and other civilians reported a distinct pair of vast steam plumes, visible from as far away as the city of Homer across the Cook Inlet, rising into the sky. Subsequent scientific flights and air samples confirmed that volcanic gases were venting “vigorously” from Fourpeaked Mountain, and melting its glacier-coated ice cap, and soon afterwards the Alaska Volcano Observatory confirmed its official classification as an “explosive eruption.”
Because so little was known about this ancient volcano, scientists were unsure of exactly what to expect, but, although some explosions and minor seismic disturbances continued into November that year, nothing has occurred since. At the time of writing, the Alaska Volcano Observatory has studied Fourpeaked Mountain and declared its Aviation Alert Level as “green” and its Volcanic Alert Level as “normal.” In other words, the likelihood of another explosion is thought to be very low.
Credit: Cyrus Read / U.S. Geological Survey
Caption: Volcanic gases escape through the glacier on Fourpeaked Volcano on 24 September 2006.