Mount Vesuvius formed 17,000 years after the collapse of the Somma Rim, a caldera-like structure. This volcano is located above a subduction zone in which the African plate is being subducted by the Eurasian plate. The African plate is subducted at about one inch every year ( Vesuvius, Italy). The oldest dated rock from Mount Vesuvius is approximately 300,000 years old. Mount Vesuvius itself is around 17,000 years old, but formed from a previous volcano. This is why that rock is around 300,000 years old. Since Vesuvius is so old, it has had a long history of eruptions. The first known eruption was in 5960 B.C. and its latest eruption was April 4, 1944.
The one eruption that Vesuvius is most famous for is that of Pompeii. On the eve of August 24, 79 Vesuvius blew its top. The eruption happened in two phases; The Plinian phase and the Pelean phase. The word Plinian derives from Pliny the Younger. His written observations of Pompeii's eruption is an important historic record. The term Pelean derives from Mount Pelee. This volcano, located on the island of Martinique, was the first documented pyroclastic flow in its 1902 eruption.
In the Plinian phase, tephra was ejected in to a high column. This 20 km tall column of ash and pumice fell onto Pompeii like rain. After eighteen hours of ash fall, 2.5 m of pumice stones fell onto Pompeii. This phase buried many low structures and caused roofs to collapse (Martini).
The most damaging phase of the eruption began one the morning of August 25, 79. The Pelean phase brought pyroclastic flows and surges. Pompeii was first hit by a pyroclastic surge which consisted of a cloud of volcanic ash and hot gas. By hugging the ground and traveling at speeds exceeding 100 km per hour, the first surge left a deposit of 10- 20 cm high. A second surge also occurred after the first. This surge left 70 cm of airfall ash and accretionary lapilli along with 10- 20 cm of limestone and volcanic rocks. After the two pyroclastic surges occurred a high temperature avalanche swept across the land. With a thickness varying from 200-50 cm , hot pumice, ash, and gas moved at high speeds to its destination of Pompeii (Martini). If any of Pompeii's 20,000 residents were still alive, this pyroclastic surge and flow surely finished them.
These bodies were buried under 75 feet of solidified ash for nearly 1700 years. It wasn't until the excavations of Pompeii, Herculaneum, and Stabaie that there was a real understanding of the 79 eruption. Archaeologist, Mario Pagano, has been leading an 18 year dig of Herculaneum. This excavation not only uncovered 300 bodies of Herculaneum's 5,000 residents, it also allowed researchers to construct a diary of the eruption (Lorenzi):
August 24, 79 A.D.
1 pm: When residents of Herculaneum took an afternoon stroll, they were able to see a column of smoke rising nine miles into the sky above Mount Vesuvius. Ash fall began to fall onto the city causing some people to run.
12 am: A cloud of ash and pumice soared 19 miles into the sky. This was the time when Mount Vesuvius unleashed molten lava.
August 25, 79 A.D.
1 am: A hot gas and ash cloud moved into the town of Heculaneum. Vesuvius' hot breath killed Herculaneum's residents in a fraction of a second. Anthropologist Paolo Petrone studied the skeletal remains and concluded that they experienced an instant death from the 750 degree Fehrenheit cloud.
1-6 am: Surges continued to make their way to Herculaneum.
7 am: A surge finally reaches Pompeii and kills its residents. Their final resting place is on a thick carpet of pumice.
8 am: There is no more laughter, music, or life in these once lively
towns. Mount Vesuvius had the power to silence them and succeeded.
Sources of Information:
Lorenzi, Rossella. "Newest Findings." Pompeii, A Lost City Revealed. March 31, 2000. <http://www.discovery.com>(21 October 2001)
Martini, Kirk. "Brief Chronology of the 79 Eruption." Volcanic Phenomena at Pompeii. July 10, 1997. <http://urban.arch.virginia.edu> (21 October 2001)
Vesuvius, Italy. (1996). <http://volcano.und.nodak.edu>
(21 October 2001)