Lahars, Surges and Pyroclastic Matter Help

By — McGraw-Hill Professional
Updated on Sep 4, 2011

Lahars And Surges

Eruptions of snow-capped volcanic mountains with their super-heated ash melt peak snow and ice causing a mudflow of melt water and volcanic ash called a lahar . Lahars are extremely dangerous as they travel and kill quickly, burying everything in their path. Even when minor lahars flow downhill and clog existing streams, they cause a lot of flooding downstream.

The worse disaster caused by a lahar happened in 1985 when the Columbian volcano, Nevado del Ruiz erupted. The melting and cracking of its summit snow and ice caused a 40-m lahar traveling around 25km/hr to surge over the town of Armero. Approximately 23,000 people were killed instantly. The mass and force of the lahar leveled buildings, vehicles, and trees in its way. The violent mudflow covered the town to a depth between 2 and 5 meters thick. The same thing happened along the same path in 1595 and 1845. Not a lucky place to live!

Although Mount St. Helens’ volcanic blast was amazing, there have been larger blasts in recent history. For example, the 1815 eruption of Tambora in Indonesia ejected around 30km 3 of debris – roughly 30 times the volume of the pyroclastics of Mount St. Helens.

Pyroclastic surges are the most violent of all volcanic activities. A pyroclastic surge can travel with the speed of an atomic blast in the form of low-density, extremely hot clouds of rock particles and gas flows. They travel at over 161km (100 mi) per hour and create huge turbulences.

In 1902, Mont Pelée erupted near the town of St. Pierre, Martinique located 8km away. It sent a fast-moving, veil-like cloud of hot volcanic ash and gases over the town and leveled it. The cloud destroyed all plant life, wiped out the town and killed 30,000 people.

Pyroclasts are fragments of crystallized volcanic rock and previously solid lava of any size that are forcibly ejected from a volcanic vent during an eruption.

A pyroclastic flow levels and chars everything in its path including trees and buildings. Like the Mount St. Helens eruption, an eruption’s pyroclastic flow can be more deadly than traveling lava. These flows are of higher density and follow the contours of the land more closely than surges. Their combined heat and speed make them lethal. It is nearly impossible to get out of the way of a pyroclastic flow. The few people, who have survived such eruptions, were able to find shelter behind a ridge or other natural feature and were very lucky to survive the heat.

A pyroclastic flow is the denser-than-air mixture of fine ash and hot gases (temperatures over 1000°C) resulting from an eruption.

Pyroclastic Matter

A pumice fall is made up of bubbling magma blown out of a volcanic vent or fissure by superheated gases. The change in pressure and expansion of gases in the magma, when it reaches the surface, creates cavities throughout the rock. This magma-type hardens into pumice , a light volcanic glass containing many small air bubbles that allow it to be carried great distances on the volcanic wind.

When pyroclastic fragments cement together to form rock, they are named according to the size of the fragments. When particles are small (less than 2 mm across) they are called ash and the rock is called an ash tuff . Particles that are bigger, between 2 and 64 mm in diameter, are called lapilli and the rock formed is called lapilli tuff . The largest pyroclastic particles, larger than 64 mm across, are called bombs and the rock formed is called an agglomerate .

The particles of ash in some volcanic eruptions are very hot. When they land, the extreme heat is so hot that it melts particles together. This glassy, fine-grained rock formed by the fusion of settling volcanic ash is called welded tuff . Some welded tuff natural glasses look a lot like lava flows. Volcanic rocks in the western United States that look like cooled lava are really welded tuffs. Some tuffs and agglomerates are layered and look a lot like sedimentary rock. When more than one layer of ash is spread by wind and rain, it may become solid in individual layered volcanic beds. If loose volcanic fragments are washed down the steep slopes of a mountain as mudflows and debris flows, the settling rock might be partly igneous and partly sedimentary.

Another type of pyroclastic flow makes a type of glassy, melted ash called ignimbrite . This type of flow is very large and covers hundreds of square kilometers of land to depths of several feet. When ancient Mount Vesuvius erupted in August of AD 79, it covered the towns of Pompeii and Herculaneum with chunks of hardened lava and thousands of pounds of ignimbrite ash. It did not cover it by a lava flow. Vesuvius has been active off and on over the past 300 years with the last eruption coming as recently

Igneous rock is formed from the cooling and hardening of magma within the earth’s crust. Over 95% of the top 16km of the crust is made up of igneous rock formed from lava eruptions. That is a lot of volcanic activity! Let’s see how all that happens.

Practice problems of this concept can be found at: Volcano Practice Test

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