Geology 309 - Lectures 13-15
Mixtures of ash, gas, and larger particles (pumice, solidified lava, or lithics)
1. Two endmembers (see Fig 10.1 in text):
- Flows that contain vesiculated, low density pumice - called Pumice
Flows.
Associated deposits are called ignimbrites
- Flows that contain unvesiculated, dense lava clasts - called Nuees
Ardentes or Block and Ash Flows.
Associated deposits called block and ash deposits
2. A third type of pyroclastic flow is called a pyroclastic surge.
This flow is very low density and contains mostly ash and fine material.
- will cover this
later. See Fig 10.2 in text.
Pyroclastic flows are transient, dynamic phenomena; deposits are what they
leave behind. Generally we don't get to see the event, only the
deposits left behind.
B. How do Ignimbrites/Pumice flows form? - by Plinian column collapse
-Talked about convective region of Plinian column - could only rise if less
dense than surrounding atmosphere.
-Once Plinian column loses that density advantage, will collapse into
pyroclastic flows. Initial Plinian phase will be followed by pyroclastic
flow.
-How do you lose the density advantage? Increase the mass eruption rate
(mass/time) by too much
Mass eruption rate depens on:
(affects eruption velocity)
- vent radius
(allows more material through)
Bigger each of those, taller the column
(until a certain point)
1. Why would volatile content decrease? Progressively tapping deeper part
of magma chamber:
Click on this image for expanded view
2. How does vent radius change as eruption proceeds?
Vent erodes with time and eruption column gets taller. Eventually too much
mass comes out and the column collapses.
Click on this image for expanded view
At some point, increased mass eruption rate can no longer sustain
Plinian column. So much pyroclastic material comes through that
column becomes denser than air and collapses.
Thus potential energy from elevation gained is converted to kinetic energy.
Travel at high velocity over the ground (>100 km/hr)
How to do this? (use Fig. 10.6):
- Exit velocity stays the same (% volatiles stays the same), but
vent radius increases due to erosion. Cross the line at some critical
volatile content
- Vent radius perhaps increases, but then magma taps deeper level
of magma chamber which has less volatiles so that exit velocity decreases.
Cross the line at some critical volatile content.
This column collapse explains commonly observed phenomenon - pyroclastic flow deposits
often overlie pyroclastic fall deposits
Computer Simulation of the AD 79 eruption of Vesuvius (collapsing
Plinian column)
C. Emplacement of Pyroclastic Flows
Pyroclastic flows are made of solid lumps of pumice and some lithics, plus ash
and gas
They are not liquids, but they do have properties of fluids. They
are amazingly mobile. Why?
- So much kinetic energy from collapsing columns - potential
energy is converted to kinetic energy
- Flows are fluidized by gas so they have very low viscosity
What is fluidization?
Gas and ash moving vertically through a particle aggregate (e.g.
pumice lumps) exerts sufficient upward force approximately equal to or
greater than gravity.
Basically, gas and particles have lower density than particles alone.
- Large clasts (pumice) float in a medium of fluidized fines.
Coarsen upwards.
- Not all parts are fluidized - high density particles (lithics) sink
downwards through fluidized mass. Coarsen downwards.
- Thus segregation by density. Pumice clasts float upwards, lithics
downwards. Can also get pumice concentration zones with increasing
fluidization.
See figure below:
(Click on this image for expanded view)
Small dark-shaded particles are lithics, large unshaded particles
are pumice fragments. Note the how the grading (coarsening
upwards or downwards)
changes with increasing fluidization
Click here for these same features, but
in a real rock
(look at middle flow - lithics concentrated
at base, pumice clasts coarsen upwards to top)
Where does gas come from to maintain fluidization?
- Gas exsoliving from magma and juvenile clasts (most is lost early)
- Air ingested during movement of the pyroclastic flow
D. Standard Ignimbrite Deposits
Divided into flow units that are products of individual flows (there are
usually multiple flows in a single eruptive event)
See Fig 10.19 of your text
Layer 3 - rarely preserved. Thin, extremely fine-grained ash deposits.
Finest ash winnowed out from flow and later settles. Called
Co-ignimbrite ash. Has lost crystals, which are dense enough to say
behind in Layer 2.
Layer 2b - thickest part, poorly sorted mix of pumice clasts, ash, lithics,
and crystals. Reverse grading (coarsening upwards) of pumice,
normal grading of lithics
Layer 2a - fine-grained basal unit - <1 m thick, pumice and lithics are
reversely graded
Layer 1 - pyroclastic surge deposit as bottom layer. Finely laminated,
fine grained, crystal rich. Sometimes shows cross bedding. Commonly
missing!
Plinian fall deposits at base
Click here for an example of an ignimbrite that has
lithic concentration at base of 2b (missing fall and layer 1) overlain
by coarsening upward pumice clasts.
Click here
for a typical poorly sorted ignimbrite (pumice clasts surrounded by fine
ash and some dark colored lithics)
Excellent examples discussed in text:
- Tambora, Indonesia (1815) - 50
cubic km of dense rock equivalent erupted. 44km high Plinian column,
7 massive ignimbrite flows
- Katmai, Alaska (1912) - 15 cubic km
of dense rock equivalent erupted
E. Distinction between ignimbrites and air fall
-
Ignimbrite - basin fill, very low initial dips (<2 degrees), poorly
sorted (ash to block sized particles), only
largest particles show grading
-
Air fall tuff - mantles topography, dips up to angle of repose,
very well sorted, commonly reversely graded (coarsens upwards)
F. Welding in ignimbrites
Cohesion (sintering and flattening) of glassy fragments due to heat
and compaction
Occurs in both glass shards (ash-sized) and pumice lumps. Pumice
lumps lose their vesicles and glassy matrix becomes compressed
so that lumps look like flattened obsidian (called fiamme
Unwelded ignimbrite from Cerro Galan, Argentina.
Densely welded ignimbrite from Japan. Note flattened pumice
lumps that look like obsidian. Fiamme include crystals.
Welding can range from none, to incipiently welded, to
densely welded
Generally, the most densely welded part of a tuff is in the middle
(click on this image for enlarged version)
G. Other features in ignimbrites