A pyroclastic flow is a dense combination of extremely hot gases and solid matter that issues from a volcano during an eruption or collapse. These flows are the most dangerous aspect of volcanic activity since they travel at high speeds, can cover miles of ground, are extremely hot, and contain toxic gases. Although they are composed of both gases and solid fragments, pyroclastic flows behave closer to a liquid than a solid or a gas. When this phenomenon occurs at low density, it is called a pyroclastic surge.
As a whole, a pyroclastic flow behaves much like a liquid, except, unlike a fluid, the density of the flow changes as it travels. The flow appears to be a thick billowing cloud but contains both solid and gaseous materials. When a flow finally stops, deposits, sometimes over 328 feet (100 m) thick, are created.
There are two general classifications of pyroclastic flows. A nuée ardente flow occurs when part of the lava dome, or a volcanic vent, collapses. A pyroclastic flow of this nature contains solid lava fragments and gases. The first recorded instance of a nuée ardente flow was in 1902 during the eruption of Mount Pelée.
Pumice flows occur when the eruption column collapses. The resulting solid material is vesiculated, or covered in tiny holes, and is therefore less dense than the fragments in a nuee ardente flow. These lighter fragments are called pumice.
Since it is so dense, a pyroclastic flow follows the ground terrain, usually spilling down the volcano and into valleys. When it hits water, it will sink below water level and usually continue moving forward. These flows may stretch as far as 124 miles (200 km), and can travel over 62 miles per hour (100 km/hour) covering as many as 7,722 square miles (20,000 km2) before they stop. Extremely hot, pyroclastic flows can reach temperatures of 1,967°F (1,075°C).
There are two main sections to a pyroclastic flow. The heavier solid particles travel near the ground, creating the basal flow. Above the basal flow are lighter particles and gases which create the ash cloud. Pumice flows have an additional component called a ground surge, which consists of clouds of ash that jump ahead of the main flow. The ash from ground surges actually glows from the heat produced.
When a pyroclastic flow is made up of dilute materials, it is called a pyroclastic surge. Pyroclastic surges are much lighter than flows, though they still contain both gases and solid particles. Since they are low-density, they span out in the air above the ground rather than following the terrain.