[meteorite-list] pyroclastic density currents: Rich Murray 2010.10.27

Wed Oct 27 23:56:37 EDT 2010

pyroclastic density currents: Rich Murray 2010.10.27
http://rmforall.blogspot.com/2010_10_01_archive.htm
Wednesday, October 27, 2010
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Mark Boslough's supercomputer simulations in 2005 at Sandia National Lab 
proved that the momentum of a meteor arriving at an angle at speeds of about 
30 km/s causes the air burst a few kilometers high to produce an 
incandescent plasma jet, not the spherical fireball of a bomb explosion:

"...Even then -- and this is the chief difference between Boslough's and 
Crawford's simulation and previous ones -- the fireball continues speeding 
towards the ground, driving a massive shockwave before it.
At this point the fireball is moving much slower than the asteroid had been 
prior to the explosion, but it is still traveling at supersonic speeds.
And it is the fireball and its accompanying shockwave, say the article's 
authors, not the initial bomb-like explosion, which cause most of the damage 
on the ground...."

http://74.125.155.132/scholar?q=cache:YY6MFUns_CkJ:scholar.google.com/+%22Mark+B\
oslough%22,+impacts&hl=en&as_sdt=10000000000
[ Extracts ]

"......Recent work by Dr. Mark Boslough 4 shows that the impact physics of 
NEOs in the 30-100 meter range has been misunderstood due to a process he 
calls a Low-Altitude Airburst (LAA), which is a newly recognized threat 
regime that has been previously underestimated.
In an LAA event the main body of the NEO comes apart at high altitudes (~80 
km to ~10 km), but the object's mass and kinetic energy are conserved as a 
fast moving, loosely aggregated, collection of particles which entrain a 
column of air reaching the ground in what might be termed an "air hammer."
Dr. Boslough's work shows that the "air hammer" from NEOs as small as 30 
meters inflicts significant damage, as was seen in the 30-meter-class
Tunguska event.
Dr. Boslough has also shown that an LAA from a ~100 meter diameter NEO 
melted sand into glass across a region about 10 km in diameter during Libyan 
Desert Glass impact ~35 million years ago.
During this event the LAA's fireball settled onto parts of Egypt and Libya 
for about a minute with temperatures approaching 5,000K.
Its hypersonic blast wave extended radially for about 100 kilometers...."

I haven't yet found detailed public information about temperatures, 
pressures, and durations of the complex turbulent blast jet on the surface.

Many physicists could probably calculate useful first order estimates and 
write software simulations that would give valuable information, enough to 
estimate the area and depth of geoablation of the ground, and the transport 
of ejecta in all directions.

Including angular momentum from the spin of the meteor would require some 
specialized working experience in using hydrodynamic codes, such as for 
tornados and hurricanes.

Existing studies of debris laden tsunamis, underwater turbidity currents, 
and volcanic pyroclastic density currents are very suggestive:

http://en.wikipedia.org/wiki/Pyroclastic_flow

"A pyroclastic flow (also known scientifically as a pyroclastic density 
current[1]) is a fast-moving current of extremely hot gas (which can reach 
temperatures of about 1,000 °C (1,830 °F)) and rock (collectively known as 
tephra), which travel away from the volcano at speeds generally as great as 
700 km/h (450 mph).[2]
The flows normally hug the ground and travel downhill, or spread laterally 
under gravity.
Their speed depends upon the density of the current, the volcanic output 
rate, and the gradient of the slope...

Pyroclastic flows that contain a much higher proportion of gas to rock are 
known as "fully dilute pyroclastic density currents" or pyroclastic surges.
The lower density sometimes allows them to flow over higher topographic 
features such as ridges and hills....

"Volumes range from a few hundred cubic meters to more than a thousand cubic 
kilometres.
The larger ones can travel for hundreds of kilometres, although none on that 
scale have occurred for several hundred thousand years.
Most pyroclastic flows are around one to ten cubic kilometres 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, admixes and heats cold atmospheric air causing expansion and 
convection. [5]...

Testimonial evidence from the 1883 eruption of Krakatoa (see the article), 
supported by experimental evidence,[8]shows that pyroclastic flows can cross 
significant bodies of water.
One flow reached the Sumatran coast as much as 48 km (30 mi) away. [9]..."

http://www.geo.mtu.edu/volcanoes/hazards/primer/pyro.html

"Pyroclastic flows are fluidized masses of rock fragments and gases that 
move rapidly in response to gravity.
Pyroclastic flows can form in several different ways.
They can form when an eruption column collapses, or as the result of 
gravitational collapse or explosion on a lava dome or lava flow (Francis, 
1993 and Scott, 1989).
These flows are more dense than pyroclastic surges and can contain as much 
as 80 % unconsolidated material. The flow is fluidized because it contains 
water and gas from the eruption, water vapor from melted snow and ice, and 
air from the flow overriding air as it moves downslope (Scott, 1989)...

Ignimbrites and nuees ardentes are two types of pyroclastic flows. An 
ignimbite contains mostly vesiculated material whereas a nuee ardente 
contains denser material (Francis, 1993). ...

Pyroclastic flows can move very fast.
Small pyroclastic flows can move as fast as 10 to 30 m/s while larger flows 
can move at rates of 200 m/s (Bryant, 1991).
Nuees ardentes have been known to extend 50 kilometers from their source, 
and Ignimbrites, because of the lighter weight material that they carry, can 
extend 200 km from their source (Bryant, 1991 and Scott, 1989).
At Mount Pinatubo in the Philipines, pyroclastic flow deposits were 220 m 
thick in some valleys but averaged 30 to 50 m thick in others (Wolfe, 1992).
Pyroclastic flows have been known to top ridges 1000 m high (Bryant, 
1991)....

Pyroclastic flows can be very hot.
In fact, pyroclastic flows from Mount Pelee had temperatures as high as 1075 
degrees C (Bryant, 1991)!..."

3 times more downward energy from directed force of meteor airburst in 3D
simulations by Mark B. E. Boslough, Sandia Lab 2007.12.17: Rich Murray
2010.08.30
http://rmforall.blogspot.com/2010_08_01_archive.htm
Monday, August 30, 2010
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Rich Murray, MA
Boston University Graduate School 1967 psychology,
BS MIT 1964, history and physics,
1943 Otowi Road, Santa Fe, New Mexico 87505
505-501-2298  rmforall at comcast.net

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