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PYROCLASTIC FLOW  / PYROKLASTISK FLOM




    
 Bilde: USGS



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 from
the collapse of a growing lava dome or dome flow, and
PUMICE FLOWS -- these contain vesiculated, low-density pumice
derived from the collapse of an eruption column.


http://www.geology.sdsu.edu/how_volcanoes_work/Pyroflows.html


Pyroklastisk flom fra utbruddet på Mount Pinatubo 15 juni 1991, som begravde Marella River Valley med støv, aske, og annet vulkansk
materiale. Dalen ble dekket av et 50 til 200 m.tykt lag. Dette utbrudd
var et av de største i det 20.århundrede. Det la fra seg ca 5.5 kubikkilometer steinmasse over et område på nesten 400
kvadratkilomter.Den pyroklastiske flommen dro så langt som
12 - 16 km fra vulkanen i alle retninger.

En av de verste flommene var den som drepte en hel by da Mt.Pelèe i
Vest-India eksploderte i 1902.

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 til phreatiske
(freato-magmatiske) eksplosjoner. Slynger aske og sand til værs sammen
med store mengder vanndamp. (Taal, Philippinene)

Da asken her er våt, dannes ofte askelumper store som snøballer, som
treffer bakken (eller?) med stor kraft.

Pyroklastiske flommer KAN bevege seg nedover vulkansidene ofte med
enorm hastighet, over 100 km/time, og med temperaturer over 400° C.
(Merapi). Dette er det mest dødelige fenomen ved vulkanutbrudd.

Sky som inneholder gasser og pyroklastiske bergarter, dvs. bruddstykker
slynget ut i atmosfæren. Dette kan være lavabiter, lavadråper eller
sønderrevne deler av krateret.

Dataene 1-5 er hentet fra
Egil Sæther: Jordskjelv, vulkaner og eruptiver bergarter, Universitetsforlaget 1987.



bukkm.gif
ANIMALS

over 250

birdm.jpg
BIRDS

over 500

flower.jpg
FLOWERS

over 225
Google
 
Web www.vulkaner.no





Pinatubo. Better get away very, very fast..... / Her gjelder det å komme seg unna....
(National Geographic 12/1992) Instituto Geofysico, Ecuador)

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