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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 resulting deposits formed a fan-like pattern of overlapping sheets, tongues, and lobes. At least 17 separate pyroclastic flows occurred during the May 18 eruption, and their aggregate volume was about 0.05 cu mi (0.21 km 3). [9] The flow deposits were still at about 570 to 790 °F (300 to 420 °C) two weeks after they erupted. [9]
These can often climb over obstacles, and devastate human life. [2] 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.
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]
The emplacement temperature range of the first surge was 180–220 °C (360–430 °F), minimum temperatures; of the second, 220–260 °C (430–500 °F). The depositional temperature of the first was 140–300 °C (280–570 °F). Upstream and downstream of the flow it was 300–360 °C (570–680 °F). [30]
Each process generates different rock; mid-ocean ridge volcanics are primarily basaltic, whereas subduction flows are mostly calc-alkaline, and more explosive and viscous. [ 55 ] Spreading rates along mid-ocean ridges vary widely, from 2 cm (0.8 in) per year at the Mid-Atlantic Ridge , to up to 16 cm (6 in) along the East Pacific Rise .
Monitoring mass movements and failures uses techniques lending from seismology (geophones), deformation, and meteorology. Landslides, rock falls, pyroclastic flows, and mud flows (lahars) are example of mass failures of volcanic material before, during, and after eruptions.
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.