Mount Vesuvius and Mount St. Helens
By Frank Reckendorf, Engineering Geologist, Salem, Ore.
Both Italy's Mount Vesuvius and Washington State's Mount St. Helens are made up of layers of lava flows and blown-out fragmental material like ash, little stones, and volcanic bombs. In the past, both volcanoes have produced unusually sudden sliding or flowage of unsorted masses of rock and other materials called debris avalanches. Most of the May 18, 1980 Mount St. Helens eruption in the upper valley of the North Fork Toutle River and in the vicinity of Spirit Lake are debris avalanche deposits. Mount Vesuvius also has extensive debris avalanche deposits on top of blown out fragmental material that buried Pompeii and Herculaneum in 79 A.D. and later. Both volcanoes also produced mudflows that originated on the slopes of the volcano that are commonly known as lahars. Mount St. Helens (more than 40,000 years old) is somewhat older than Vesuvius (27,000 years old), and started with ash deposits falling on older volcanic and sedimentary deposits. Mount Vesuvius is a volcano within a volcano (Mount Somma), which has its roots in calcareous (limestone) sedimentary rock that, when intruded by magma, produces extremely violent eruptions.
The Mount Vesuvius part of Mount Somma is thought to have been about 3,000 meters high at the time of the A.D. 79 eruption. Roughly 2,000 meters is thought to have been blown off the top of Mount Vesuvius in the A.D. 79 eruption and then the mountain was mostly quiet for hundreds of years with only small eruptions until 1631. There were numerous eruptions between 1631 and 1944, which raised the mountain to about 1,200 meters. The 1944 eruption and ash fall came as quite a surprise to the Allies, who had been preoccupied with World War II. However, the Italians were well aware of the eruptive potential, as they had been monitoring Vesuvius since 1841. The 1944 eruption created much of the 300-meter deep and 600-meter wide crater we see today. The Allied military government worked hard to bring help to the local people that had already suffered great hardships from the war.
It takes about a half-hour to an hour to walk up the serpentine path from the parking lot (at about 1,000 meters) to the edge of the spectacular multi-colored crater of Mount Vesuvius (at about 1,165 meters). One can view into the crater at the contrasting colorful red, buff, and gray layers of lava flows and blown-out fragments of several eruptions, particularly that of March 1944. There is a 20-meter layer of the red scoria part of a lava advance that is visible near the top of the eastern wall of the crater. The red scoria looks just like what one commonly sees in the Cascade Mountains of Washington State. The red cinders found along the highways in the Cascades is often ground up red scoria. One can view the steam rising in the Mount Vesuvius crater and just feel the great energy in the ground waiting to be released. The view outward from the rim of the crater is also spectacular as one looks down on old lava flows, debris avalanches, and mudflows. One can see for about ten miles, and out in the distance are cities like Ercolano (of which the buried city of Herculaneum is a part) and Naples. Vesuvius has been essentially dormant since 1944, and is well worth the effort to visit. It is far more spectacular in its grandeur, color, and view than Mount St. Helens.