Volcanic eruptions take place when magma, which is the molten rock found underneath the earth’s surface, spews out of a fissure on the earth’s crust. When magma reaches the surface in an eruption, it is then called lava. Eruptions can be explosive or slow-moving.
Volcanologists have determined different types of volcanic eruptions, including Hawaiian, Strombolian, Vulcanian, Plinian, and Surtseyan eruptions. Of these, Plinian eruptions, also termed Vesuvian eruptions, are widely known, thanks to historical documentation of Mount Vesuvius’ eruption in 79 A.D. Interestingly enough, Plinian eruptions are characteristic of composite volcanoes, or stratovolcanoes. Some of the more famous stratovolcanoes in history are Mount Vesuvius and Mount Pinatubo. Meanwhile, a subcategory of the stratovolcano is the caldera, which can lead to more cataclysmic eruptions, such as those that took place with Krakatoa and Mount St. Helens.
Plinian eruptions are among the most violent of eruptions, and at times, lead to decapitated volcanoes. They often start suddenly after a long dormancy, and once begun, unleash megatons of energy.
A Plinian climax lasts for less than two days, but wreaks enormous destruction in that time. They are associated with rapidly-developing columns of gas and ash, outbursts that expel thick debris at accelerated speeds, and with broad dispersal of dust and ash. For instance, ash from Mount St. Helens was shoveled like snow in Washington state, dust from Krakatoa gave brilliant sunsets around the world, and the hapless people of Pompeii were buried beneath debris from Vesuvius. Plinian eruptions like those of Vesuvius, Krakatoa, and Mount St. Helens continue to capture our imaginations and linger still, either in memories or in historical documents.
Plinian eruptions are named after the ancient Roman statesman, Pliny the Younger. He wrote a now-famous account of how Mount Vesuvius’ eruption in 79 A.D. had devastated the Roman towns of Pompeii and Herculaneum. Historians have gathered data showing evidence of earthquakes taking place prior to the fateful day of August 24, 79 A.D. Apparently, the Romans of the day did not recognize events like earthquakes, dried-up wells, turbulent seas, and unusual animal behavior as warning signs that the dormant volcano was waking up. Within two days of the eruption, Pompeii and Herculaneum were buried, and for a time, they were forgotten. Centuries later, while an aqueduct was being reconstructed, Pompeii was rediscovered.
On May 20, 1883 a German vessel’s captain first noted the plumes of smoke emanating from the Indonesian island of Krakatoa, that signaled an end to the volcano’s slumber. Three months later, on August 26, the first eruption took place. Within hours, it created tsunamis that battered coastal communities in the surrounding Indonesian islands. But the worst part would not arrive until around 5:30 a.m. the next day, on August 27, when the volcano erupted with such power that it blew the island of Krakatoa apart, leaving only 30 percent of the island’s mass intact. The blast was heard thousands of miles away, even in far-off India. Moreover, the ash released into the air by both eruptions significantly impacted planetary climate, with average temperatures across the globe dipping by as much as 1.2 degrees Celsius (34.16 degrees Fahrenheit). The force of the eruption was estimated at a magnitude of over 12,000 times the power of an atomic bomb.
Before its 1980 eruption, Mount St. Helens was recognized as a symmetrical Cascade range beauty that stood serene and quiet for almost 130 years. In March 1980, some minor ground-shaking signaled the volcano’s reawakening. The northern face of the volcano began to visibly swell over the next couple of months, earning the nickname “The Bulge.” By May 18, Mount St. Helens exploded back to life. A 5.1 magnitude earthquake occurred at 8:32 a.m. triggering a chain-reaction that led to the collapse of the bulge and a subsequent debris avalanche. The entire northern slope slid away and exposed the volcano’s super-heated core, resulting in an explosive lateral blast that ripped the volcano summit open. The sound from the blast could be heard hundreds of miles away. A massive Plinian column of ash shot sky-high and released volcanic ash into the upper atmosphere. The pressure wave from the volcano blast flattened trees in 200-square-mile area around the volcano and set off mudslides that took the lives of 57 people and thousands of animals. Eventually the ashfall came down in the form of black rain that coated residents in Washington, Idaho, and Montana. Ash deposits were found as far away as the Great Plains.
** All images utilized in this article are from the public domain.