Pyroclastic flow
Pyroclastic flow
Pyroclastic density currents (PDC) are hot mixture of pyroclasts and volcanic gases which move at very high speed down the slopes of a volcano. These density currents form when an eruption column can't maintain its upward momentum and the column collapses and flows down the slopes of the volcano. Another way for these density currents to form is from a collapse of a lava dome where it generates a hot avalanche of volcanic materials rapidly moving down the slopes of the volcano.
Because eruption column density is lower than the atmosphere, the column in most cases ascends many kilometres above the volcano. Finally, it spreads out and forms a cloud and the ash coming from the column is dispersed with the wind. However, when the eruption column can't intake enough air it cools down and becomes denser than the surrounding air. When this happens the column collapses due to gravity force and forms a PDC. PDCs are made from tephra (ash, pumice, scoria, rocks, lava) and hot gases. The tephra and the gases mix together creating a very mobile mass which can flow over the surface of the ground similarly to water. The PDCs can be extremely hot and move at very high speed. The temperature is caused by the fact that the tephra, rocks, and gases, have only lost a small amount of their heat to the atmosphere when the currents form. The temperature of the current can vary from 100°C up to 800°C. Depending on the density PDCs can be categorized into two groups. If the column contains a lot of solid material while it collapses it is called pyroclastic flows, however, if the quantity of solid material is low compared to the gases it is called a pyroclastic surge. Pyroclastic flows can flow up to several hundred kilometres speed per hours, while pyroclastic surges flow significantly slower.
Pyroclastic flow
Magma that reaches the surface during explosive eruptions is usually rich in volcanic gases which have the same temperature as the liquid flowing magma. For most part the gas is water vapor (H2O). Felsic magmas are richer in gas than mafic magmas, and this explains why it is more common for pyroclastic flows to form during felsic or intermediate eruptions. Pyroclastic flows are however also known to have formed during mafic eruptions, an example would be the mafic pyroclastic flow which formed during the beginning of the eruption of Hekla in 2000.
These flows occur in two ways:
- The eruption column collapses during and explosive eruption. This happens when the column can't intake enough air to become less dense than the atmosphere and keep raising due to buoyancy. This happens especially during large eruptions.
- A lava dome collapses and tumbles down the slope of the volcano due to gravity.
Pyroclastic flows destroy everything in their path and the temperature is so hot that people and animals die.
Pyroclastic surges
Pyroclastic surges are more common than pyroclastic flows, also usually smaller. Nevertheless, they can still be as lethal as pyroclastic flows. In 1902, in the city St. Pierre on the island Martinique in the Carribean, a pyroclastic surge was the cause of the death of 29.000 residence in the city. Pyroclastic surges which travel 1-2 km away from the eruption site form in most phreatomagmatic eruption, where the magma interacts with water. These types of eruptions are quite common in Iceland, for examples in Grímsvötn volcano, Katla volcano and in eruptions in the sea, for example when the island of Surtsey was formed in 1963–1967. These pyroclastic surges related to phreatomagmatic eruptions are considerably colder than in other explosive eruptions because of the cooling effect of the external water.
