Our Beautiful World
Photograph by W.E. Scott on July 1, 1991., USGS
|Picture to the right, above:
Pyroclastic flows from the eruption of Mount Pinatubo on June 15th, 1991, which buried the Marella River Valley with dust, ash and debris. The valley was covered by a 50 to 200m thick layer. This eruption was one of the biggist in the 20th century. It placed ab out 5.5 cubickilometers of volcanic mass over an area of almost 400 squarekilometers. The pyroclastic flow moved as far as 12 to 16 kilometers in all directions from the volcano.
A pyroclastic flow is a fluidized mixture of solid to semi-solid fragments and
hot, expanding gases that flows down the flank of a volcanic edifice. These
awesome features are heavier-than-air emulsions that move much like a
snow avalanche, except that they are fiercely hot, contain toxic gases, and
move at phenomenal, hurricane-force speeds, often over 100 km/hour.
They are the most deadly of all volcanic phenomena. FLOW FLUIDIZATION
The extraordinary velocity of a pyroclastic flow is partly attributed to its
fluidization. A moving pyroclastic flow has properties more like those of a
liquid than a mass of solid fragments. Its fluid behavior can only be described
as spectacular, as evidenced by the 6000-year-old Koya flow in southern
Japan, which traveled more than 60 km from its source, ten of which were
over open water! The Koya flow left a deposit that was only two meters
thick over its 60 km extent. Such mobility comes from the disappearance
of inter-particle friction. A fluidized flow is best described as a dispersion of
large fragments in a medium of fluidized fine fragments. A constant stream of
hot, expanding gases keeps the smallest of the fragments (ash and lapilli size
particles) in constant suspension. This solid-gas mixture can then support
larger fragments that float in the matrix. The expanding gas component is
derived from a combination of (1) the constant exsolution of volcanic gas
emitted by the hot pyroclasts, and (2) from the ingestion, heating, and rapid
expansion air during movement of the flow.
The terminology of pyroclastic flows and pyroclastic flow deposits can be
complex and confusing. In general, there are two end-member types of flows:
NUÉE ARDENTES -- these contain dense lava fragments derived
Pyroklastisk flom fra utbruddet på Mount
Pinatubo 15 juni 1991, som begravde Marella River Valley med støv,
aske, og annet vulkansk
En av de verste flommene var den som drepte en hel by da Mt.Pelèe
Pyroklastisk flom = slamstrømmer, løse utbrudds-produkter (tefra) og (oftest) kokende hett vann.
1. Kan dannes ved at utblåst damp tettes til regnskyer og blander seg med asken.
2. Før utbruddet kan det ha samlet seg vann i krateret. (Som ved Kelut)
3). Snø og is rundt toppen smelter og blandes med vulkansk
aske ( Ruiz)
4. Regnskyll efter askenedfall.
5. Overflatevann/grunnvann trenger ned i sprekker og fører
Da asken her er våt, dannes ofte askelumper store som snøballer,
Pyroklastiske flommer KAN bevege seg nedover vulkansidene ofte med
Sky som inneholder gasser og pyroklastiske bergarter, dvs. bruddstykker
Dataene 1-5 er hentet fra