Pyroclastic flows are hot avalanches of rock, ash, and gas that sweep down the flanks of volcanoes at high velocities. This photo shows a relatively small pyroclastic flow at Mayon volcano in the Philippines on September 23, 1984. These hot, ground-hugging flows can travel at velocities to about 100 km/hr and reach areas well beyond the flanks of a volcano. Their high temperatures make them lethal to anything in their path. Billowing ash clouds rise above the denser basal portion, which can consist of vesiculated pumice or dense lava clasts.
Mayon volcano in the Philippines is one of Earth's best examples of a classic, conical stratovolcano. Its symmetrical morphology is the exception rather than the rule, and is the result of eruptions that are restricted to a single central conduit at the summit of the volcano. Eruptions are frequent enough at Mayon, the most active volcano in the Philippines, to overcome erosive forces that quickly modify the slopes of most volcanoes.
A nighttime view from Legaspi City on September 14, 1984, shows incandescent lava flows descending the SW flank of Mayon volcano in the Philippines. The flows traveled about 4 km to the lower flanks of the volcano, adjacent to previous flows from eruptions in 1968 and 1978.
A hot lahar sweeps down a channel on the SW flank of Mayon volcano in the Philippines on September 14, 1984, five days after the onset of an eruption. The water temperature of this lahar was about 80 degrees Centigrade. Note the large block in the center of the channel that is being transported by the lahar.
A pyroclastic flow sweeps down the SE flank of Mayon volcano in the Philippines on September 24, 1984. A thick column of ash rises above the surface of the moving pyroclastic flow, which was the largest of a series of pyroclastic flows that occurred during an eruption that began on September 9. Flow velocities of 50 m/sec were estimated from timed 35-mm photographs. The pyroclastic flow traveled 7 km from the summit vent, which is hidden behind the far left side of the ash column.
Ash clouds rise above a pyroclastic flow traveling down the Buang valley on the upper NW flank of Mayon volcano in the Philippines on September 12, 1984. The toe of the advancing pyroclastic flow is visible at the lower right. These pyroclastic flows traveled down to 100 m elevation at rates of about 20 m/sec.