Pyroclastic transport and
deposition
Christoph Breitkreuz,
TU Bergakademie Freiberg
1
Two main types of pyroclastic
transport and deposit:
1. Fallout > deposit = tephra
2. Flow
2
Eolian fractionation!
Mt. St. Helens, 1980
3
Phonolitic fallout deposit,
Meidob Hills, NW Sudan
4
Fallout Lage
zwischen Ignimbriten
des Bandelier Tuffs,
Jemez Mtns, New
Mexico
5
Diagenetically compacted fallout deposit, Permian, N Chile
6
Laacher See, Eifel, W Germany, ballistic block
7
Fallout upon water - water-lain fallout deposits
Lacustrine fallout deposit, Permian, N Italy
Various types of grading!
8
Formation of pyroclastic flows:
- lava(-dome) collapse
- eruption column collapse
Branney & Kokelaar 2002
9
Mt. Pelee, 1902/3, Martinique
10
Mt. Unzen 1991-1995
11
Block-and-ash-flow deposit:
result of an explosive lava dome eruption
Meidob Hills, NW Sudan
12
Parameters controlling eruption column collapse:
Fig. 2.5 Stability fields for convecting and collapsing columns in terms of vent radius and
magmatic volatile content. Magma discharge rate (right side) is largely a function of vent
radius (left side) whereas exit velocity (bottom) is largely a function of volatile content (top)
(Orton 1996, from Wilson, Sparks & Walker, 1980).
13
Ngauruhoe 1978, New Zealand
14
What drives Pyroclastic flows?
Kinetic energy, gravity, turbulence and fluidisation!
Large spectrum of processes and deposits:
Cool
Hot
dilute turbulent
often phreatomagmatic
dense
hot
weakly turbulent to laminar
Surges and
cool pyroclastic flows s.s.
Hot pyroclastic flows s.s.
15
Surge deposits: pyroclastic cross stratification!
G. Wörner
Laacher See, Eifel, W Germany
16
Lithofacies types of
surge deposits
Fig. 5.10 Classification
of base surge bedform
and
internal
crossstratification
variations
related to depositional
rate (relative to transport
rate; vertical axis) and
surge temperature and
moisture
content
(horizontal axis); flow
comes from left (From
Cas & Wright 1987, after
Allen 1982).
Fig. 5.11 Proximal to distal (LFS) and vertical (VFS) facies variation in pyroclastic
surge beds (i.e. single depositional units) on Songaksan tuff ring (after Chough &
Sohn, 1990). The lateral facies sequence (LFS 1) was distilled from common
downcurrent facies transitions in flank deposits whereas vertical facies sequences
(VFS) are distilled using Harper´s (1984) method of facies sequence analysis. VFS1
and VFS2 are from proximal near-vent deposits, probably where short-lived pyroclastic
surges were overladen by suspended sediment fallout (compare with Lowe, 1988).
LFS1, and its vertical expression (VFS3) indicates downcurrent decrease in particle
concentration, grain size, and suspended-load fallout rate with a resultant increase in
traction and sorting processes (from Orton 1996).
17
Fig. 5.14 Schematic
diagram showing the
structure and idealised
deposits
of
one
pyroclastic flow (From
Cas & Wright 1987).
Ground surge deposit,
E California
18
Low-angle cross stratification
19
Deposition by aggradation
(Branney & Kokelaar (2002)
20
Branney & Kokelaar (2002)
Se genera cuando la parte inferior
de la CDP es lo suficientemente
diluida (escasa a nula interacción
de partículas)
y la velocidad es muy baja
(procesos de tracción o saltación).
Las partículas caen directamente.
Características de los depósitos
Macizos y sin estratificación, aunque una débil fábrica
direccional puede formarse si la velocidad es suficiente para
orientar las partículas.
21
Pyroclastic fractionation!
elutriation
22
Pyroclastic
flow deposits
Gradación
may show various types of
grading:
Brohltal, Laacher See, Eifel, W
Germany
23
Accretionary
Lapilli
Schumacher & Schmincke 1991
hailstone-like ashaggregates from wet
eruption clouds and
co-flow-ash clouds
24
Vesiculated Tuff: surge/fall deposits from wet fine ash clouds
25
Professor!
Don‘t forget to make some drawings about…
Proximal – distal facies variation
Lateral facies variation
26
Can pyroclastic flows climb mountains?
- kinetic energy from column collapse
(e.g. Taupo, New Zealand)
- „expanded flow“ concept (Branney & Kokelaar 1992)
(e.g. Campanian Ignimbrite, Italy,
Fisher et al. 1993)
27
Post-depositional processes in hot pyroclastic flow deposits:
1. welding compaction, in extreme cases: rheomorphism
2. gas + fluid escape >> vapor phase crystallisation
28
Fig. 5.17 Model of sedimentation and initiation of
rheomorphic flow during the early accumulation phase of
ignimbrite D on a slightly inclined basal topography. Note
that pure flattening (compaction) is the first step of
deformation affecting the freshly deposited pyroclastic
material in both cases. Shear flow, in this configuration, is
always secondary and depends on a critical load pressure
as well as on the speed of the upward prograding
cooling/lithification front (From Kobberger & Schminke
1999).
Rheomorphic ignimbrite
Fig. 5.16 Steps of the depositional and rheomorphic
flow history of ignimbrite D by sketching the major
stages of deposition including the successive vertical
changes of pyroclast strain, the development of fabrics
crucial to define the related deformation mechanism,
and the progression in ascending and descending
lithification fronts. The model is based on the
configuration of a gently but evenly inclined basal
topography during deposition and rheomorphic flow
(From Kobberger & Schminke 1999).
29
Diagenetic compaction
non-welded ignimbrite,
N Chile, Permian
>> Branney and Sparks 1990
Welding-compaction
Teplice ignimbrite, Late Carboniferous,
Czech Republic
30
From Marcelo Arnosio
Eutaxitic texture
fiamme
31
Blue Creek
ignimbrite,
Idaho, USA
Parataxitic texture
Botzen Porphyry,
Permian N Italy
In hot thick deposits:
Adsorption of volatiles back
into the glass >>
disappearance of vesicles
and interclast pores
(Sparks et al. 1999)
32
Post-depositional processes in hot pyroclastic flow deposits:
1. welding compaction, in extreme cases: rheomorphism
2. gas + fluid escape >> vapor phase crystallisation
Concept:
- Depositional unit
- Cooling unit
Smith 1961
Fig. 5.15 Cross section through part of the Upper
Bandelier Tuff, showing welding and crystallisation zones.
The ignimbrite is a compound cooling unit, and shows an
upward increase in the degree of welding in cooling units
I-III. Recognisable flow units are much thinner in units IV
and V, and nearer the source they pass into densely welded
tuff, continuing the trend towards higher temperature of
emplacement of successive pumice flows that is more
clearly shown by units I-III. Note, the topography that the
ignimbrite fills in is cut into older ignimbrites and
basement, including Precambrian (cross section is
approximately normal to movement direction of the
pumice flows) (From Cas & Wright 1987, after R. L.
Smith & Bailey 1966).
33
Bishop Tuff, Owens Gorge, E
California
34
Non-welded ignimbrite
„Sillar“ Facies: lithification by
vapor phase crystallisation
35
Degassing pipes
fines-depleted
From Marcelo Arnosio
36
Fig. 5.21 Occurences of gas segregation structures in pyroclastic flow deposits. 1, Pipes and pods
generated initially or formed entirely by intraflow gas sources during emplacement; 1a, formed by
continued post-emplacement gas flow; 2, formed from heated ground water and incorporating
fluviatile pebbles; 3, formed above burnt vegetation and logs (From Cas & Wright 1987).
Partially eroded degassing pipes, Bishop Tuff, E California
37
Fig. 5.20 Zones of vapour-phase
crystallisation and devitrification in the
Bishop Tuff. Fumarole mounds project from
top of vapour-phase zone through nonwelded ash (From Cas & Wright 1987, after
Sheridan 1970).
Sillar Facies with
fumarole mounds and
curved cooling joints
38
Hydroclastic flow deposit
(from collapse of a
phreatomagmatic eruption
column)
Szentbekkalla,
Hungary
39
How to distinguish a pyroclastic flow deposit from a
sedimentary mass flow deposit?
Criteria for a hot emplacement
- welding-compaction (careful with diagenetic
compaction (Branney and Sparks 1990)
- high T crystallisation domains: spherulites and
lithophysae
- degassing pipes
- in-situ cooling joints in large lava clasts and lithics
- AMS
Problem: there are hot Lahars (e.g. Arguden and Rudolfo 1990)!
40