Other than causing tsunamis, creating lava flows, and pouring ash and gases into the air, volcanoes can be deadly in another way. Pyroclastic flows sometimes result from an eruption and can cause massive damage and loss of life. These flows can move at incredible speeds, and many people are not able to get away from the deadly flows. There have been many examples of this type of disaster throughout recorded history, and when a populous region is nearby, pyroclastic flows are often associated with a large loss of life.
Basically, pyroclastic flows are fluidized masses of gases, rocks, and ash that can exit out of a volcano and travel away from the volcano across the land. The materials in the flow can be heated from as low as 100 degrees all the way up 800 degrees Fahrenheit. As the flow moves over the land from the eruption, it destroys all vegetation and will kill all of the people that are engulfed by it. Some flows move as slowly as 30 to 100 feet per second or as fast as 600 feet per second, and can travel 30 to 100 miles away from the volcano across the land before dispersing. Some of the most notable historic examples of pyroclastic flows are as follows.
In 79 AD, Mt. Vesuvius erupted without warning. It had been considered extinct, as there had been no eruption or notable activity for over a hundred years. Two pyroclastic flows from the volcano buried the cities of Pompeii and Herculaneum. Several thousand people were killed when they were engulfed and buried in the flows. The ash flows actually preserved the cities as well as many of the items and even artwork present.
Viewed as the largest volcanic disaster in recorded history, the Tambora volcano, which is located on Sumbama, Indonesia erupted on the 10th and 11th of April in 1815. The pyroclastic flows were responsible for part of the loss of life associated with the disaster and claimed the lives of 12,000 people. Though the volcano exhibited three years of seismic activity before the eruption, local people were not prepared for any eruption.
Similar to Tambra, Krakatau had several years of seismic activity before the eruption took place in 1883. During the final eruption, much of the island was destroyed in an explosion that was said to be the loudest noise in history and heard farther than 2,500 miles away. Pyroclastic flows during the eruption were responsible for the deaths of 5,000 people, but only accounted for a portion of the total deaths associated with the disaster.
St. Pierre in Martinique was devastated by a volcanic eruption of the nearby Mont Pelle in 1902. The eruption caused a pyroclastic flow to surge towards the town at nearly 160 kilometers per hour (100 miles per hour). The entire city was devastated and only two people managed to survive, while 30,000 were killed by the flows. One person was able to get away from the flows, while another was locked away in the poorly ventilated jail and spared.
The eruption of the Nevado del Ruiz in Columbia occurred on November 14, 1985. Though seismic activity was being monitored, a storm obscured any indication that volcanic activity was occurring and only an earthquake was registered. Those living nearby did not receive proper warning of the eruption which sent pyroclastic flows down the mountainside. These flows actually melted glaciers on the side of the mountain and caused the formation of a lahar, which is just a pyroclastic flow with more water in it. Traveling at around 50 kilometers per hour, the lahar moved into the valley and reached the town of Armero causing the deaths of 23,000 people.
The 1980 eruption of Mount St. Helens in Washington featured many pyroclastic flows. These flows occurred and continued to occur for about 5 hours during the eruption. It is known that at least seventeen different pyroclastic flows occurred as a result of the eruption on May 18. Two weeks after the eruption, the temperature of the deposits made by the flows was still between 570 and 785 degrees Fahrenheit. People were evacuated from the area and the pyroclastic flows were not responsible for any deaths.
On June 12 of 1991, Mt. Pinatubo in the Philippines began erupting, however it was not until June 15 that an explosion occurred that triggered a much larger eruption. Pyroclastic flows rushed down the mountain and filled valleys with as much as 600 feet of volcanic deposits. The nearby area was safely evacuated before the eruption so there were no casualties, but the entire area was affected and will likely continue to be affected by the eruption and the flows. The volcanic material deposited by the pyroclastic flows may retain heat for decades and still had temperatures around 900 degrees Fahrenheit 5 years after the eruption.
These are just a few of the many historical pyroclastic flows that have resulted from a volcanic eruption. A trend can be seen that in recent decades, newer detection technology and knowledge of eruptions has helped to give warning to areas. With this advanced warning, the areas can be evacuated before the eruption and before the pyroclastic flows have a chance to claim lives.