Lahars and Pyroclastic Flows
Active volcanoes are known to be dangerous places. History gives plenty of examples of them causing mass destruction and loss of live, for example Vesuvius destroying Pompeii and Herculaneum in 79AD, Krakatoa exploding in 1883 and Mt St Helens in the Cascade Range (USA) erupting in 1980.
It's common to think of the greatest danger coming from lava, liquid rock at temperatures of about 750°C to 1250°C, in the form of lava flows, and splatter thrown out of erupting craters. In fact, flowing lava kills relatively few people, and unless you somehow become surrounded by flowing lava it is usually possible to run, or even walk away from most lava flows!
The real killers are much more frightening. They can travel at over 100 miles per hour, move across land and sea, flow uphill as well as down, rip trees up by their roots, flatten buildings and kill people and animals instantly. These deadly processes are called pyroclastic flows and lahars.
Pyroclastic flows are clouds of hot volcanic gases, ash and volcanic bombs that can sweep down the volcano's sides and other steep hills at speeds over 100 miles per hour. When a volcano erupts violently its common for large volumes of rock to be pulverised in the explosion and reduced to tiny particles. These are mixed with high temperature gasses, ash and larger pieces of rock, forming a red hot cloud that is very dense. Imagine a cloud of sand and air straight from a blast furnace, and you are getting close to what a pyroclastic flow looks and feels like.
A pyroclastic has an added killer element; poisonous gasses at temperatures hot enough to burn your lungs away.
On 24th and 25th August AD79 the volcano, Vesuvius, erupted in Italy. On the 24th, smoke,ash and falling pumice caused a mass evacuation of the town, but left most buildings still standing. Then, on the 25th, a pyroclastic flow hit the town of Pompeii, damaging walls of buildings, ripping roofs off and killing all living things in its path. About 2000 bodies have been found in the excavations at Pompeii, and most seem to have died from choking on poisonous or hot gasses.
When Krakatoa erupted, a massive pyroclastic flow blasted out from the volcano and traveled across the sea to cause damage on other islands. It managed to do this because of its great heat. The blast turned the sea water to steam as it passed over it, creating an almost frictionless cushion over which the ash,gas and pumice rock flowed.
The initial eruption of Mt St Helens included a pyroclastic flow that swept down the mountain, flattening forests, overtaking escaping vehicles and killing several people who stood absolutely no chance of moving out of its path, including a research scientist, David Johnston, who was working at an observation station about 8km away from the main crater. He had been monitoring gas emissions and watching the mountain whilst in contact with his base,Vancouver, via a two way radio. He transmitted a short message "Vancouver!,Vancouver!,This is it". Moments later he died as over 4km³ of rock, gas and steam exploded from the northern side of the mountain. Pyroclastic flows moved away at speeds up to 130km per hour. These ash clouds, at 500°C, burnt everything in their path, before settling out around Spirit Lake. As the deposits built up they blocked the exit from the lake causing the water level to rise by a massive 60m and trigger fears of another disaster waiting to happen.
Lahars are volcanic mud flows created when pyroclastic flows become mixed with water. The ash and water mix and can form a type of mud that sets like concrete once it stops flowing. Water can come from within the volcano or surrounding area as the ground explodes and is pulverised, or from the melting of snow and ice on the flanks of the volcano. In the case of Mt St Helens the pyroclastic flow melted the snow and ice on the upper slopes forming fast flowing mud flows which swept down the mountain into the Toutle River, which at one stage was heated to 90°C.
The mud flows moved through houses picking up cars, trucks, trees and even whole buildings. The collected debris was carried downstream where it smashed into river defences and bridges. As the mud settled in the Columbia river it reduced the depth in places from over 12m to neared 4m, trapping large boats upstream of the shallows.
When Vesuvius erupted in AD79, Pompeii was buried under ash, but the nearby town of Herculaneum was hit by a lahar that totally buried the town. When it stopped flowing it set, literally as hard as rock. Today, both towns have been partially excavated, but Herculaneum is surrounded by high rock walls on top of which is a modern town. The rock walls are the exposed lahar into which archaeologists have dug to expose the ruins.
Lahars do not always occur at the same time as volcanic eruptions, and can occur at anytime where suitable deposits of ash and loose rock exist. This makes it almost impossible to predict a lahar. Volcanic eruptions are preceded by hints such as earthquakes, steam and ash, changes in water table and ground height, but a lahar can be triggered by a single day of heavy rain. This makes them a real danger in populated areas.
In 1998 at the Casita volcano in Nicaragua a lahar claimed over 2,500 lives when 200,000 m3 of rock and loose soil on a very steep section of the volcano subsided with no warning. As it swept over the towns of El Porvenir and Rolando Rodriguez the only warning was a noise like helicopters or thunder, and ground tremors that some thought were earthquakes. The towns were totally covered by the mud in less than 3 minutes, killing almost everybody.