What Are the Effects of Pyroclastic Flows?

Pyroclastic flows come from a volcanic eruption of solid or molten rock fragments or a combination of both. This discharge of dry, hot fragments and gases move at great speed from the volcanic vent. Pyroclastic flows may also come from a non explosive eruption when a lava flow or dome collapses down the slope of the volcano.

Pyroclastic flows usually contain two types of discharge. The first is a basal flow, this is a flow of heavy coarse material that will move along the ground. The second type of discharge is ash and gas that vent upward. The ash may then fall covering a wide area around the eruption. A pyroclastic flow is extremely damaging and dangerous to anything in its path.

A pyroclastic flow that contains light materials is referred to as an ignimbrites. A flow that contains dense materials is called a nuee ardent. A pyroclastic surge is a lower density flow that is less concentrated than a pyroclastic flow but just as dangerous and deadly.

Impact damage:

Rock fragments of all sizes can be vented in a pyroclastic flow. This material can travel at speeds of 80 kilometers per hour (49.7 mph) and knock down, bury or even carry away anything caught in the path of the flow. Impact can also damage and then block roadways that need to be used for evacuation.

Fire damage:

The temperature of the gas and rock in a pyroclastic flow can vary from 200 C (392 F) to 700 C (1292 F). Even a slow moving flow will destroy land and buildings in its path. The flow does not even need to come into direct contact with buildings or vegetation as the extreme heat will cause combustion without actual contact. People and animals are also in danger from the potential inhalation of hot gas and ash in the air.

Flood potential:

The flow itself is very hot and the heat can melt ice and snow caps on or around the volcano. This sudden melt can send quick and large amounts of water into adjacent valleys and overrun rivers or nearby streams.

Cooled debris:

Once the pyroclastic flow has stopped and the rock fragments have cooled it forms hardened black rock. This material can block streams and rivers forming dams or creating lakes where they did not exist before. The debris can also bury and cover farmlands, homes or scenic areas. Restoring the damaged land is extremely difficult unless the amount of debris is minimal.

Landslides:

A large pyroclastic eruption can break off part of the volcano dome itself causing a large landslide and possibly the collapse of the volcanic mountain itself. What was once a mountain and valley can become a more level area with the volcanic potential to begin again.

References:

http://volcanoes.usgs.gov/hazards/lava/index.php

http://mivo-sys.tripod.com/pyroclastic.html

http://volcanoes.usgs.gov/hazards/pyroclasticflow/index.php