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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 ...
Initially moving about 220 mph (350 km/h), the blast quickly accelerated to around 670 mph (1,080 km/h), and it may have briefly passed the speed of sound. [9] [32] Pyroclastic flow material passed over the moving avalanche and spread outward, devastating a fan-shaped area 23 miles across by 19 miles long (37 km × 31 km). [32]
It is similar to a pyroclastic flow but it has a lower density or contains a much higher ratio of gas to rock, [1] which makes it more turbulent and allows it to rise over ridges and hills rather than always travel downhill as pyroclastic flows do. The speed of pyroclastic density currents has been measured directly via photography only in the ...
The material collapses upon itself, forming a fast-moving pyroclastic flow [31] (known as a block-and-ash flow) [33] that moves down the side of the mountain at tremendous speeds, often over 150 km (93 mi) per hour.
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.
The main extremely fast moving pyroclastic flow travelled at close to the speed of sound and devastated the surrounding area, climbing more than 1,500 m (4,900 ft) to overtop the nearby Kaimanawa Ranges and Mount Tongariro, and covering the land within 80 km (50 mi) with ignimbrite. Only Ruapehu was high enough to divert the flow.
As pyroclastic flows erupted from the volcano's crater, they melted the mountain's glaciers, sending four enormous lahars down its slopes at 60 kilometers per hour (37 miles per hour). The lahars picked up speed in gullies and coursed into the six major rivers at the base of the volcano; they engulfed the town of Armero , killing more than ...
If it does happen, then material reaching the bottom of the convective thrust region can no longer be adequately supported by convection and will fall under gravity, forming a pyroclastic flow or surge which can travel down the slopes of a volcano at speeds of over 100–200 km/h (62–124 mph). Column collapse is one of the most common and ...