The ad79 Eruption of Mt. Vesuvius

This is a drawing of the 1631 eruption of Mt. Vesuvius. Click on image for full size version (128K GIF) Image courtesy of: Dr. Boris Behncke. Artist: Giovan Batista Passaro This account of the eruption of Mt. Vesuvius was modified from the Vesuvio website.

Around 1300 on 24 August a tall eruption column rose above Mt. Vesuvius and tephra began to fall. In the area around Pompeii the thickness of the tephra layer increased by 6 to 8 inches per hour. The pumice clasts which comprised the tephra were up to 3 inches in diameter. Because these mini-boulders arrived with speeds of up to 100 miles/hour, they may have caused injuries and isolated deaths, and should have, after a few hours, caused the collapse of roofs. Soon the city was enshrouded in complete darkness, a phenomenon that is familiar to other populations involved in eruptions such as those at Mount St. Helens, and Mt. Pinatubo. The residents probably did not even know what kind of event was striking them, and waited in their homes, hoping that the shower of rock would sooner or later come to an end.

After seven hours of continuous explosive activity, the eruption tapped a lower, less evolved portion of the magma chamber, and the composition of the eruption abruptly changed, implying a sharp interface between two magma types in the magma chamber. The eruption siginficantly gained in vigor, and the ash column rose much higher in altitude. For five more hours, pumice fell. By the early morning (about 0100) of 25 August, about 1.5 yards of pumice covered the Pompeii area.

A second change in the eruptive dynamics then occurred. The eruptive vent widened such that local gas pressure could no longer push up the tall eruption column. This phenomenon is what scientists call collapse of the eruption column. The mixture of gas and ash no longer rose up, but immediately fell back onto the slopes of the volcano, forming glowing avalanches of pyroclastic flows which rushed rapidly down slope, destroying everything in their paths. This change in the eruptive dynamics proved fatal to the thousands of people around the volcano. The Roman towns of Pompeii and Herculaneum were destroyed in a matter of minutes.

Pompeii and Herculaneum were rediscovered in the 18th century with many treasures intact. The reconstruction of these cities gives a vivid idea of what Mt. Vesuvius is able to do.

This is a drawing of the 1631 eruption of Mt. Vesuvius. Click on image for full size version (128K GIF) Image courtesy of: Dr. Boris Behncke. Artist: Giovan Batista Passaro This account of the eruption of Mt. Vesuvius was modified from the Vesuvio website.

Around 1:00 pm on 24 August a tall cloud of steam and ash rose above Mt. Vesuvius and debris began to fall. In the area around Pompeii the thickness of falling debris increased by 6 to 8 inches per hour. The rocks which comprised the debris were up to 3 inches in diameter, and fell with a speed of up to 100 miles/hour. They may have caused injuries and isolated deaths, and should have, after a few hours, caused the collapse of roofs.

The city was soon covered in complete darkness, a phenomenon which is familiar to other populations involved in eruptions such as those at Mount St. Helens, and Mt. Pinatubo. The residents probably did not even know what kind of event was striking them, and waited in their homes, hoping that the shower of rock would sooner or later come to an end.

After 12 hours of continuous explosive activity, a change in the eruptive dynamics occurred. The mouth of the volcano widened such that local gas pressure could no longer push up the tall ash column. The mixture of gas and ash no longer rose up into the sky, but immediately fell back onto the slopes of the volcano, forming glowing avalanches of hot flowing material (perhaps 800 degrees) which rushed rapidly down slope, destroying everything in their paths. This change in the eruption proved fatal to the thousands of people around the volcano. The Roman towns of Pompeii and Herculaneum were destroyed in a matter of minutes.

Pompeii and Herculaneum were rediscovered in the 18th century with many treasures intact. The reconstruction of these cities gives a vivid idea of what Mt. Vesuvius is able to do.

This is a drawing of the 1631 eruption of Mt. Vesuvius. Click on image for full size version (128K GIF) Image courtesy of: Dr. Boris Behncke. Artist: Giovan Batista Passaro This account of the eruption of Mt. Vesuvius was modified from the Vesuvio website.

Around 1:00 pm on 24 August a tall cloud of steam and ash rose above Mt. Vesuvius and debris began to fall.

The city was soon covered in complete darkness, something which is familiar to people involved in eruptions such as those at Mount St. Helens, and Mt. Pinatubo. The people waited in their homes, hoping that the shower of rock would sooner or later come to an end. By the early morning (about 1:00 am, about 1.5 yards of debris covered the city of Pompeii.

Then, a fatal change in the eruption occurred. The cloud of ash no longer rose up into the sky out of the mouth of the volcano, but fell down the slopes of the volcano, forming glowing avalanches of hot flowing material which rushed rapidly down slope, destroying everything in their paths. With this change, the towns of Pompeii and Herculaneum were destroyed in a matter of minutes.

Pompeii was rediscovered in the 18th century with many treasures intact. The reconstruction of the city gives a vivid idea of what Mt. Vesuvius is able to do.