Geology 309 - Lecture 16
Incredible
Video footage
of a pyroclastic flow from Soufriere Hills
Also, take a look at the
sequence of still images
that shows the collapse of a Plinian column to produce
a pyroclastic flow.
You can download a video clip of this by going
here and scrolling down about 3 page lengths until you
get to that same still
image. Click in the caption on the "mpeg animation". In the clip,
the column collapses,
produces the flow, then rejuvinates to continue the Plinian column.
Pyroclastic flows - PYROCLASTIC SURGES
Pyroclastic surges:
Definition
Surges are a low density, dilute type of pyroclastic flow.
They are turbulent, as opposed to laminar flow regime (pumice
flows and nuees ardentes are typically laminar). They
don't travel very far since they are low density (not as much
momentum).
Three types of pyroclastic surges:
1. Base surge
Typically a ring-shaped shock blast that is associated
with a hydrovolcanic eruption.
Process
Most well known in small basaltic eruptions, but can occur in any water-
magma interaction.
Can accompany initial blast of eruption (especially hydrovolcanic)
May accompany initial collapse of eruption column if water is involved
Deposits
-
Resembles Surtseyan deposits
- Poorly sorted like pyroclastic flows
- Wedge shaped (thick at vent, thin rapidly away, rarely >5-6 km)
- Can have asymmetrical cross bedding, erodes channels, can have
planar bedding.
- Distinguish from air falls in that surges thin over topographic
highs and thicken in lows)
2. Ground surge
Process
-
Probably derived from a pyroclastic flow itself rather than an
eruption column because (unlike base surges) they travel just
as far as pyroclastic flows
- Can occur in or at front of advancing head of flow - jets of ash cloud
- Injection of air at head of flow heats up and gets strongly fluidized.
Turbulent surge, immediately overrun by laminar pyroclastic flow
Deposits
- Typically found underneath pyroclastic flows (Layer 1 of ignimbrite
sequence)
- <1 m thick
- Finely laminated, sometimes cross bedded
- Pinch and swell
- Medium sorting (intermediate between pyroclastic flow and fall)
- Fine grained (like medium sand, 2 mm median diameter)
- Distinguishing characteristic: enriched in lithics and especially
crystals (often looks like sugar). Thus density segregation.
3. Ash cloud surges
Same deposits as ground surge, but within or on top of pyroclastic
flow, or occur as lateral extension
(a) Lateral extension - if somehow pyroclastic flow can't go over
a topographic obstacle because it is too dense, sometimes the co-ignimbrite
ash cloud can. Thus the flow segregates, and becomes turbulent over
topography, creating an ash cloud surge.
(Below is image used in class)
If this is hard to see, you can try to click
here
for a better source of the same image.
(b) Within pyroclastic flow - Abortive switch from Plinian
column to pyroclastic flow sometimes results in a short-lived
ash cloud surge and pyroclastic flow. This happened a couple of
times at Vesuvius
during the 79AD eruption (read story in text - excellent).
4. Blast surge
- Associated with initial explosive blast from catastrophic
removal of overburden on magma (i.e., Mt. St. Helens 1980)
- Shock wave (plus particulate matter)
-
Mt. St. Helens example