Pyroclastic Flows

A pyroclastic flow is a fast-moving avalanche of hot gas, fluidized lava fragments, and ash that is ejected from an erupting volcano. In some cases reaching speeds of 160 kilometers (100 miles) per hour or higher, pyroclastic flows can reach temperatures of more than 540 degrees Celsius (1,000 degrees Fahrenheit), are extremely destructive, and can cover large areas surrounding an eruption.

Principal Terms

caldera: an underground magma chamber

fluidization: process whereby the rocks and minerals contained in a volcanic eruption are vaporized within a pyroclastic flow

ignimbrite: a pumice-filled pyroclastic flow caused by the collapse of a volcanic column and accelerated by gravity

lahar: combination of water, ash, and mud that creates tremendous mudslides in a pyroclastic flow

nuée ardente: a type of dense pyroclastic flow that occurs after a volcanic dome collapse; also known as block and ash and Peleen flow

Plinian eruption: volcanic eruption that sends ash and debris high above the volcano in a column

pumice: light, frothy volcanic rock formed when lava is cooled above ground following a violent eruption and pyroclastic flow

vesiculated: gas-infused rock within a pyroclastic flow

Basic Principles

A pyroclastic flow occurs when superheated gas and rock are ejected forcefully from a volcano during eruption. Pyroclastic flows may occur when, after a volcanic eruption sends a plume of ash and rock fragments into the sky, gravity causes the cloud to collapse and the superheated material to flow to the surface. In other cases, pyroclastic flows can occur when a volcano explodes outward, sending a pyroclastic flow along the side of the mountain and along the ground.

Pyroclastic flows comprise hot gas, rock, and ash. When such a flow is released from an active volcano, it travels downhill at extreme speed, causing enormous destruction along its path. As a key volcanic hazard, pyroclastic flows have been the most destructive components of some of the most violent eruptions in history, including the eruptions of Mount Vesuvius, Krakatoa, and Mount St. Helens.

Pyroclastic flows combine with a great deal of other material as they travel after an eruption. Flows are so destructive that they can, for example, disrupt lakes and ponds that are located on the volcano’s surface. Additionally, the intense heat that accompanies the flows is enough to instantly melt the snow and ice packs at summits. Furthermore, volcanic eruptions release a tremendous amount of water vapor into the sky, along with ash and soot, causing intense rainstorms. All of this water combines with mud and falling ash to form a slurry called a "lahar." Lahars add to the pyroclastic flow’s overall destructiveness.

There are two general components of a pyroclastic flow. One is basal flow, which comprises dense rock fragments and material. Basal flow moves along the contours of the land as it travels down and away from the volcano. Another component of a pyroclastic flow is the cloud of ash, gas, and other lighter material, which surges above the basal flow.

In addition to causing extreme damage to the area surrounding a volcano, pyroclastic flows deposit a large volume of volcanic material. Pyroclastic flow deposits, which are sometimes found many miles from the eruption site, occur in three general layers. The bottom layer (the basal or sillar level) contains fine grains of pumice and volcanic glass. The middle level usually comprises welded pumice containing a combination of unsorted fragments of pumice and other volcanic minerals (such as silicon). Atop the welded middle level is a layer of ash. In many cases, the pyroclastic flow deposit is mixed with the water and debris of lahars. Scientists often look to flow deposits for information about the type of activity that has occurred in a volcanic area.

Fluidization and Flow Velocity

A central element of the velocity of a pyroclastic flow is its fluidization. A pyroclastic flow contains large quantities of rock and ash in addition to gas. However, substances therein are superheated to the point in which the particles have little friction. Combined with the water and gas from the eruption and with the vaporized water and snow that the flow encounters as it moves downslope, the flow travels rapidly and unobstructed, moving along the contours of the mountain.

Fluidization is, in turn, dependent on the types of material contained in the flow. The granular minerals and rocks are major contributors to how the flow moves, playing a role in the flow’s density. The more dense the flow, the faster and farther it travels. Pyroclastic flows have been known to travel great distances (sometimes over large bodies of water). In Japan, for example, an eruption in 4000 BCE sent a pyroclastic flow more than 60 km (37 mi) from its source, including 9.5 km (6 mi) over water. Researchers found that the flow deposit was about 2 meters (6.5 feet) thick only, a testament to the flow’s low density.

Scientists are still attempting to study the natural process of fluidization and how pyroclastic flows carry fluidized minerals from volcanic sites. Increased research on the natural process of fluidization will help researchers understand the nature of pyroclastic flows and how they transport minerals across the earth’s surface.

Ignimbrites

There are two general classifications of pyroclastic flows. The first of these is the ignimbrite or "pumice flow." Ignimbrites are volcanic clouds that consist largely of pumice, a light, gaseous rock that is formed from frothy lava. These clouds are vesiculated (the rock contained within the cloud contains a large amount of gas), which means that they have a low density. The grains of pumice contained in the flow vary in size, particularly as they proceed from their volcanic source.

Because they are low in density, ignimbrites are expelled rapidly. During a Plinian eruption, wherein the eruption is vertical, a cloud is expelled high into the air above the volcano. However, as the eruption continues to send denser debris into the sky, the debris causes the cloud to collapse. The main component of ignimbrite flow, containing large concentrations of pumice, stays on the ground; the cloud of ash hovers above it. Meanwhile, the fluidized pumice flow rushes down the slope and outward, accelerated by its fall from high above the volcano.

Pumice flows emanating from another type of eruption—the caldera collapse—can be some of the most devastating types of pyroclastic flows. Here, a caldera (an underground magma chamber) is drained in a volcanic eruption. The collapse sends a sheet of pumice outward rather than upward, filling adjacent valleys and causing widespread destruction. In some cases, ignimbrites emanating from a caldera collapse have been known to wipe out an area hundreds of kilometers wide, leaving extremely thick deposits of pumice far from the volcano.

Nuée Ardente Flow

The second type of pyroclastic flow is called the "nuée ardente" ("glowing cloud"), so named by French geologist Antoine Lacroix. Nuées ardentes are frequently associated with Peleen eruptions, after the deadly 1902 eruption of Mount Pelée in Martinique. That eruption was marked by two massive, fiery pyroclastic flow clouds. The vertical column’s intimidating luminescence captured observers’ attention, although the surface level of the flow was far more destructive.

Nuées ardentes are considerably denser in composition than are ignimbrites. Nuées ardentes occur when the dome (a mass of viscous lava that cools and forms a bulbous mass around the volcano’s vents) of a volcano collapses explosively in an eruption. The release of fluidized ash and rock in the pyroclastic flow is too dense to travel into a volcanic column; it therefore spills rapidly from the vents and along the contours of the mountain at incredible speeds. Nuées ardentes are nonvesiculated because they contain blocks of fragments from the dome and ash and other particles (hence, the more accurate name provided to these occurrences: "block and ash" flows).

When a dome collapses, the flow surges from the vent, upward or outward, or both. The dense cloud then bends down as the pyroclastic flow begins to travel along the surface of the volcano. A second component of the nuée ardente, the ash cloud surge, moves above the flow, masking it. Because they are so dense, nuées ardentes are not as extensive as ignimbrites. Still, they move with tremendous speed and, as was the case of St. Pierre, Martinique, in 1902, can cause widespread damage to the nearby environment.

Historic Examples of Pyroclastic Flows

Pyroclastic flows have been the main force behind some of the most awesome and destructive volcanic eruptions in recorded history. For example, 7,700 years ago, a 3.6-km (12,000-foot) volcano dubbed Mount Mazama erupted in what is now central Oregon. The caldera feeding the volcano collapsed with such force that the resulting ignimbrite flow sent a sheet outward for nearly 32 km (20 mi) in every direction. The eruption destroyed a large portion of the mountain itself and created what is now Crater Lake, the deepest freshwater lake in North America.

The origin of the term "Plinian" (named for Pliny the Younger, who recorded his observations of the subsequent devastation) comes from the eruption of Mount Vesuvius in 79 CE, one of the most infamous volcanic eruptions in human history. This Plinian eruption created a devastating ignimbrite pyroclastic flow, which quickly engulfed the nearby Roman cities of Pompeii and Herculaneum, leaving both communities (and the people and animals who did not escape the rapidly advancing flow) buried in pumice.

On May 7, 1902, Mount Pelée erupted near St. Pierre in Martinique. Witnesses report having seen two clouds emitted from atop the 1,400-m (4,600-ft) mountain: A black, glowing column spewed vertically and another rushed down the sides of the mountain. Only two of the thirty thousand people living in St. Pierre survived the nuée ardente flow.

One of the most famous examples of pyroclastic flows in modern history is the 1980 eruption of Mount St. Helens in Washington State. This event featured both types of pyroclastic flow. On May 18, the mountain erupted violently, sending a Plinian column more than 19 km (12 mi) into the sky. When the massive column collapsed, the resulting ignimbrite flows, coupled with a significant lahar, destroyed millions of trees and caused massive steam explosions in nearby lakes and rivers.

In the following few weeks, a lava dome formed at the mountain’s vent. During subsequent eruptions in June, the dome’s collapse produced several nuée ardente flows, triggering more avalanches and mudslides that contributed to the major alteration of the area around the mountain.

Effects of Pyroclastic Flows

Pyroclastic flows are major events that can substantially alter the landscape and environment in and around a volcano. Many of the effects of pyroclastic flows are destructive in nature, devastating all forms of life in their paths.

Pyroclastic flows also can significantly alter the topography of the mountain and surrounding areas, creating new crevasses and surface features along the volcano’s slopes and, therefore, new routes for descending lava and debris in future eruptions. In the long term, nuées ardentes and ignimbrites also can alter the shapes and courses of rivers, lakes, and streams.

Studying Pyroclastic Flows

One of the difficulties scientists encountered while attempting to study the volcanic activity at Mount St. Helens in 1980 was that so much of the mountain was obscured by the massive ash clouds that the eruption produced. Indeed, even when dormant a volcano presents a number of challenges for scientists, not the least of which includes traveling up steep, dangerous terrain to the cone.

Modern technology has greatly enhanced scientists’ ability to study pyroclastic flows and other aspects of volcanism. For example, the use of computer models has enabled scientists to analyze potential flow courses and velocities. Such data enable them to isolate hazard zones and potentially save lives in the event of an eruption.

In other cases, scientists may utilize radar systems, based on the ground and mounted on aircraft and satellites, to study the paths of pyroclastic flow surges. For example, scientists have successfully analyzed flow deposits and mapped flow paths on an active volcano by using synthetic aperture radar. The use of such technology precludes the need to climb up a potentially dangerous slope in unpredictable weather and atmospheric conditions. Other scientists have used ground-penetrating radar systems to map flow paths and layers in the topography of a volcano, making it easier to create models of volcanic activity.

In addition to radar and computer modeling, another invaluable technological tool in the study of pyroclastic flows is the global positioning system (GPS). Utilizing this satellite-based technology, scientists can analyze pyroclastic flow patterns, movement, and composition despite cloud cover and other visual obstructions.

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