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Pyroclastic flows sweep down the flanks of Mayon Volcano, Philippines, in 2018. A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) [1] is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of 100 km/h (30 m/s; 60 mph) but is capable of reaching speeds up to ...
The most important characteristic of a Peléan eruption is the presence of a glowing avalanche of hot volcanic ash, called a pyroclastic flow. Formation of lava domes is another characteristic. Short flows of ash or creation of pumice cones may be observed as well. The initial phases of eruption are characterized by pyroclastic flows.
Earthly pyroclastic flows can travel at up to 80 km (50 mi) per hour and reach temperatures of 200 to 700 °C (392 to 1,292 °F). The high temperatures can burn flammable materials in the flow's path, including wood, vegetation, and buildings.
Once clasts have fallen to the ground, they remain as tephra unless hot enough to fuse into pyroclastic rock or tuff. When a volcano explodes, it releases a variety of tephra including ash, cinders, and blocks. These layers settle on the land and, over time, sedimentation occurs incorporating these tephra layers into the geologic record.
The deposit has a maximum thickness of 6 m and ash flow layers are interbedded at the top. Phase 2 has ash and lapilli beds that are cross stratified with mega-ripples and dune-like structures. The deposit thicknesses vary from 10 cm to 12 m. Phases 3 and 4 are pyroclastic density current deposits. Phases 1 and 3 were phreatomagmatic. [8]
Fumaroles may occur along tiny cracks, along long fissures, or in chaotic clusters or fields. They also occur on the surface of lava flows and pyroclastic flows. [11] A fumarole field is an area of thermal springs and gas vents where shallow magma or hot igneous rocks release gases or interact with groundwater. [12]
The maximum thickness of the pyroclastic debris comprising the rim of a typical tuff ring is generally thin, less than 50 meters (160 ft) to 100 meters (330 ft) thick. The pyroclastic materials that comprise their rim consist primarily of relatively fresh and unaltered, distinctly and thin-bedded volcanic surge and air fall deposits.
The total dense-rock equivalent volume of the eruption, including pyroclastic flows and ash fall, was between 216 cubic kilometers (52 cu mi) and 550 cubic kilometers (130 cu mi), with the larger estimate placing the eruption in the low end of the supereruption range (VEI 8). [17]