Geology 309 - Lecture 17
Pyroclastic flows: nuees ardentes
Name -
Nuee ardente means glowing cloud. Is a confusing term, glowing avalanche
would be more accurate and descriptive.
Often easier to use block and ash flow as a synonym.
Definition - pyroclastic flow whose magmatic component is
dense rock, in contrast to the vesiculated pumice that forms
ignimbrites. Typically poorly
vesiculated andesite and dacite, sometimes basalt.
Gravity controlled rather than fluidized by juvenile magmatic gases
Nuees are hot avalanches not cold. How tell?
Deposits are often spherical boulders with outward radiating
cracks known as Prismatic jointing.
Can fit pieces together like a jigsaw puzzle.
Wouldn't have survived if rock was transported in a cold avalanche, had to
have happened after avalanche came to rest (i.e., a cooling phenomenon of
a hot rock).
I. Process
Three types of nuees ardentes:
- Merapi type
- Peleean type
- Soufriere type
Merapi type
Gravitational collapse of lava flows and domes
Lava domes get oversteepened as they
grow. Gravity is in charge.
Peleean type
Explosive events on growing lava domes
Powerful explosions within dome trigger eruption and collapse of dome.
Get some juvenile pumice mixed in with unvesiculated lava
Soufriere type
Not related to lava domes, but to eruption column collapse.
In this case, it is not a Plinian column that collapses, but a
smaller column, typically from a Vulcanian explosion. Thus, magma
that is erupted is less vesiculated than pumice and hence forms a block
and ash deposit.
Example: Mt. Mayon in the Philippines in 1968.
II. Deposits
-
Block and ash deposits.
Low volume (<1 km3); thus much smaller than pumice flows
-
Clasts of juvenile unvesiculated magma in ash matrix. Confined
to pre-existing valleys
-
Clasts can be huge (low vesiculation) - sometimes greater than a
meter and often
bigger than the thickness of the entire deposit
-
Reverse grading - same reason as all the raisins come to the top of your
granola. Void spaces filled by small particles.
-
High density blocks in main part, marginal deposits are like surges, plus
associated ash cloud on top