User:Hutvh/sandbox/Pyroclastic Density Currents
Fast-moving current of hot gas and volcanic matter that moves away from a volcano
From Wikipedia, the free encyclopedia
A pyroclastic density current or PDCs ( the umbrella term for pyroclastic flow or pyroclastic surge)[1] is a fast-moving mixture of hot gas and volcanic matter (collectively known as tephra) that flows along the ground. Pyroclastic density currents have caused more fatalities than any other volcanic hazard [2]. They can move at velocities of 100 km/h (62 mph) on average but is capable of reaching speeds up to 700 km/h (430 mph).[3] The gases and tephra can reach temperatures of between 200°C and 700°C (390-1300°F). [4]
PDCs form as byproducts of certain explosive eruptions. Their speed depends upon the density of the current, the volcanic output rate, and the gradient of the slope. They are extremely hazardous due to their high mobility. PDCs are able to flow over large topographic obstacles[5][6] and travel for tens to hundreds of kilometers[1].
A pyroclastic density current is a type of gravity current.[7][8] It flows due to negative buoyancy (it is more dense than the surrounding air) until it reaches buoyancy reversal where it become less dense than the surrounding air and floats away as a pyroclastic cloud or phoenix cloud.
Currently, over 100 million people world wide live in areas with a risk of PDCs.[9]
Origin of term
An early description of the eruption of Vesuvius between May 14 and June 4, 1736 by Fransesco Serao introduced the word pyroclast is derived from the Greek πῦρ, meaning "fire", and κλαστός, meaning "broken in pieces".[10][11] Later work surrounding the disastrous 1902 eruption of Mount Pelée on Martinique coined the a new term for flows which glow red in the dark: nuée ardente (French, "burning cloud") [12][note 1].
Two additional terms arose in pyroclastic surge and pyroclastic flow to refer to to currents with vastly different densities. Pyroclastic surge was used to describe dilute turbulent currents while pyroclastic flow referred to dense granular flows of ash and rock.
Branney and Kokelar (2002) suggested the term pyroclastic density current as umbrella term for many different types of flow used in scientific literature [1]. This change in terminology reflects the current understanding is that surges and flows represent two end-members of flow regime but a single PDC can span many regimes of flow transport during it's existence.
Generation of PDCs
There are several mechanisms that can produce a pyroclastic flow:
- Column collapse occurs due to full or partial collapse of an eruption column from a Plinian eruption or Vulcanian eruption (e.g. Mount Vesuvius' destruction of Herculaneum and Pompeii). In such an eruption, the material forcefully ejected from the vent heats the surrounding air and the turbulent mixture rises, through convection. If the erupted jet is unable to heat the surrounding air sufficiently, convection currents will not be strong enough to carry the plume upwards and it falls, flowing down the flanks of the volcano.
- Boiling over occurs when material is erupted from the vent without forming a column and flows down the slopes of the volcano (e.g., Novarupta in 1912).
- Gravitational collapse of a lava dome or spine (e.g., Montserrat's Soufrière Hills volcano, which caused nineteen deaths in 1997).
- The directional blast (or jet) when part of a volcano collapses or explodes (e.g., the eruption of Mount St. Helens on May 18, 1980). As distance from the volcano increases, this rapidly transforms into a gravity-driven current.
Size and effects
Flow volumes range from a few hundred cubic meters (yards) to more than 1,000 cubic kilometres (~240 cubic miles). Larger flows can travel for hundreds of kilometres (miles), although none on that scale has occurred for several hundred thousand years. Most pyroclastic flows are around 1 to 10 km3 (about ¼ to 2½ cubic miles) and travel for several kilometres. Flows usually consist of two parts: the basal flow hugs the ground and contains larger, coarse boulders and rock fragments, while an extremely hot ash plume lofts above it because of the turbulence between the flow and the overlying air, admixing and heating cold atmospheric air causing expansion and convection.[13]
The kinetic energy of the moving cloud will flatten trees and buildings in its path. The hot gases and high speed make them particularly lethal, as they will incinerate living organisms instantaneously or turn them into carbonized fossils:
- The cities of Pompeii and Herculaneum, Italy, for example, were engulfed by pyroclastic surges on August 24, 79 AD with many lives lost.[14]
- The 1902 eruption of Mount Pelée destroyed the Martinique town of St. Pierre. Despite signs of impending eruption, the government deemed St. Pierre safe due to hills and valleys between it and the volcano, but the pyroclastic flow charred almost the entirety of the city, killing all but two of its 30,000 residents.[citation needed]
- A pyroclastic surge killed volcanologists Harry Glicken and Katia and Maurice Krafft and 40 other people on Mount Unzen, in Japan, on June 3, 1991. The surge started as a pyroclastic flow and the more energised surge climbed a spur on which the Kraffts and the others were standing; it engulfed them, and the corpses were covered with about 5 mm (0.2 in) of ash.[15]
- On 25 June 1997, a pyroclastic flow travelled down Mosquito Ghaut on the Caribbean island of Montserrat. A large, highly energized pyroclastic surge developed. This flow could not be restrained by the Ghaut and spilled out of it, killing 19 people who were in the Streatham village area (which was officially evacuated). Several others in the area suffered severe burns.[citation needed]
Interaction with water
Testimonial evidence from the 1883 eruption of Krakatoa, supported by experimental evidence,[16] shows that pyroclastic flows can cross significant bodies of water. However, that might be a pyroclastic surge, not flow, because the density of a gravity current means it cannot move across the surface of water.[16] One flow reached the Sumatran coast as far as 48 km (30 mi) away.[17]
A 2006 BBC documentary film, Ten Things You Didn't Know About Volcanoes,[18] demonstrated tests by a research team at Kiel University, Germany, of pyroclastic flows moving over water.[19] When the reconstructed pyroclastic flow (stream of mostly hot ash with varying densities) hit the water, two things happened: the heavier material fell into the water, precipitating out from the pyroclastic flow and into the liquid; the temperature of the ash caused the water to evaporate, propelling the pyroclastic flow (now only consisting of the lighter material) along on a bed of steam at an even faster pace than before.
During some phases of the Soufriere Hills volcano on Montserrat, pyroclastic flows were filmed about 1 km (0.6 mi) offshore. These show the water boiling as the flow passed over it. The flows eventually built a delta, which covered about 1 km2 (250 acres).
A pyroclastic flow can interact with a body of water to form a large amount of mud, which can then continue to flow downhill as a lahar. This is one of several mechanisms that can create a lahar.[citation needed]
On the Moon
In 1963, NASA astronomer Winifred Cameron proposed that the lunar equivalent of terrestrial pyroclastic flows may have formed sinuous rilles on the Moon. In a lunar volcanic eruption, a pyroclastic cloud would follow local relief, resulting in an often sinuous track. The Moon's Schröter's Valley offers one example.[20][non-primary source needed]
