6 Pyroclastic Transport&Deposition 2008 Eng

8/18/2019 6 Pyroclastic Transport&Deposition 2008 Eng

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Pyroclastic transport and

deposition

Christoph Breitkreuz,

TU Bergakademie Freiberg


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Two main types of pyroclastictransport and deposit:

1. Fallout > deposit = tephra

2. Flow


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Mt. St. Helens, 1980

Eolian fractionation!


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Phonolitic fallout deposit,

Meidob Hills, NW Sudan


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Fallout layer betweenignimbrites;

Bandelier Tuffs,

Jemez Mtns, New

Mexico


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Diagenetically compacted fallout deposit, Permian, N Chile


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Laacher See, Eifel, W Germany, ballistic block


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Lacustrine fallout deposit, Permian, N Italy

Various types of grading!

Fallout upon water - water-lain fallout deposits


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Formation of pyroclastic flows:

- lava(-dome) collapse

- eruption column collapse

Branney & Kokelaar 2002


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Mt. Pelee, 1902/3, Martinique


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Block-and-ash-flow deposit:

result of an explosive lava dome eruption

Meidob Hills, NW Sudan


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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 ventradius (left side) whereas exit velocity (bottom) is largely a function of volatile content (top)

(Orton 1996, from Wilson, Sparks & Walker, 1980).

Parameters controlling eruption column collapse:


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Ngauruhoe 1978, New Zealand


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What drives pyroclastic flows?

Kinetic energy, gravity, turbulence and fluidisation!

Large spectrum of processes and deposits:

Cool

dilute turbulent

often phreatomagmatichot

Hot

dense

weakly turbulent to laminar

Surges andcool pyroclastic flows s.s.

Hot pyroclastic flows s.s.


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Surge deposits: pyroclastic cross stratification!

Laacher See, Eifel, W GermanyG. Wörner


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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).

Fig. 5.10 Classification

of base surge bedform

and internal cross-

stratification variations

related to depositional

rate (relative to transport

rate; vertical axis) and

surge temperature andmoisture content

(horizontal axis); flow

comes from left (From

Cas & Wright 1987, after

Allen 1982).

Lithofacies types of

surge deposits


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Fig. 5.14 Schematic

diagram showing the

structure and idealised

deposits of one

pyroclastic flow (From

Cas & Wright 1987).

Ground surge deposit,

E California


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Low-angle cross stratification


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Branney & Kokelaar (2002)

Deposition by aggradation


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Se genera cuando la parte inferior

de la CDP es lo suficientemente

diluida (escasa a nula interacciónde partículas)

y la velocidad es muy baja

(procesos de tracción o saltación).

Las partículas caen directamente.

Características de los depósitosMacizos y sin estratificación, aunque una débil fábrica

direccional puede formarse si la velocidad es suficiente para

orientar las partículas. B r a n n e y & K

o k e l a a r ( 2 0 0 2 )


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elutriation

Pyroclastic fractionation:

>> important compositional

changes in crystal-rich flows!


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GradaciónPyroclastic flow deposits

may show various types of

grading:

Brohltal, Laacher See, Eifel, W

Germany


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Hailstone-like ash-

aggregates from wet

eruption clouds and

co-flow-ash clouds,

Trigger by rain

possible!

Accretionary

Lapilli

Schumacher & Schmincke 1991


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Vesiculated Tuff: surge/fall deposits from wet fine ash clouds

Osteifel


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Proximal – distal facies variation

Lateral facies variation

Professor!Don‘t forget to make some drawings about…


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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)


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Post-depositional processes in hot pyroclastic flow deposits:

1. welding compaction, in extreme cases: rheomorphism

2. gas + fluid escape >> vapor phase crystallisation


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Fig. 5.16 Steps of the depositional and rheomorphicflow history of ignimbrite D by sketching the majorstages 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).

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


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Welding-compactionTeplice ignimbrite, Late Carboniferous,

Czech Republic

Diagenetic compactionnon-welded ignimbrite,

N Chile, Permian

>> Branney and Sparks 1990

E t iti t t From Marcelo Arnosio


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Eutaxitic texture

fiamme

From Marcelo Arnosio


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Parataxitic textureBotzen Porphyry,

Permian N Italy

Blue Creekignimbrite,

Idaho, USA

In hot thick deposits:

Adsorption of volatiles back

into the glass >>

disappearance of vesicles

and interclast pores(Sparks et al. 1999)


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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).

Post-depositional processes in hot pyroclastic flow deposits:

1. welding compaction, in extreme cases: rheomorphism2. gas + fluid escape >> vapor phase crystallisation

Concept:- Depositional unit

- Cooling unit

Smith 1961


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Non-welded ignimbrite„Sillar“ Facies: lithification by vapor

phase crystallisation; typical

crystals in SiO2-rich systems:cristobalite, tridymite, sanidine

Vapor phase crystals on ash fragment, SEM

image (experiment by Grunder et al. 2005)

Degassing pipes fines depleted


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Degassing pipes

From Marcelo Arnosio

fines-depleted

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


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Partially eroded degassing pipes, Bishop Tuff, E California

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).

Fig. 5.20 Zones of vapour-phase


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crystallisation and devitrification in the

Bishop Tuff. Fumarole mounds project from

top of vapour-phase zone through non-

welded ash (From Cas & Wright 1987, after

Sheridan 1970).

Sillar facies with

fumarole mounds andcurved cooling joints

H d l ti fl d it


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Hydroclastic flow deposit

(from collapse of a

phreatomagmatic eruptioncolumn)

Szentbekkalla,

Hungary


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6 Pyroclastic Transport&Deposition 2008 Eng