Pyroclastic flows are the mixture of solid to semi-solid fragments and gases that flow down the sides of a volcano. They are in a fluidized form. They are heavier than air emulsions (suspension of a liquid in another liquid). When they are falling, they look somewhat like a snow avalanche. The difference is of course, that they are extremely hot and contain deadly toxic gases and move at hurricane force speeds of over 100 kilometers (around 62 miles) per hour. They are the deadliest part of any volcanic eruption. The fluidization of the pyroclastic flow contributes to its speed abilities.
The mobility of the pyroclastic flow comes from the absence of inter-particle friction .This type of flow is a dispersion of large fragments in a material made up of fine fragments. The smaller fragments are kept in constant suspension by the continuous stream of hot, expanding gases. This solid gas mixture can now support the larger fragments that join the mixture.
This ever expanding gas component is obtained from a combination of the constant exsolution or bubble formations of the volcanic gases that are released from the hot pyroclasts (fragments) and the ingestion, heating and rapid expansion of air during movement of the flow.
Fragments in the pyroclasic flows are divided into three sizes. The smallest are simply called Ash and are no larger than 2 millimeters. Lapilli, the second size of the fragments range from 2-64 millimeters in size and the largest fragments are known as Blocks or Bombs and are 64 plus millimeters.
The terminology of pyroclastic flow deposits can be confusing. There are two end-member types of flows. Nuee ardents are dense fragments that are created from the collapse of a growing lava dome. The resulting deposits are called block and ash. Pumice flows have low density pumice stones that are obtained from the collapse of an eruption column. The deposits from this type of flow are called ignimbrites.
Even though both the Nuee ardent and the pumice flows are fluid, the pumice flow is much a faster flow. The pumice flows have a tripartite (three part) division. The main body of the flow hugs the ground surface. Pumice fragments in an ash mixture make up this main body. An ash cloud lies over this avalanche. The ground surge is an additional part of a pumice flow. These are forward moving jets of incandescent ash that occur in the head of the pumice flow. Fluidization is created when air is ingested in the front of the pumice flow. Materials in the pumice flow are hurled forward when the air becomes heated up.
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