Pyroclastic transport and deposition

Transcription

1 Pyroclastic transport and deposition Christoph Breitkreuz, TU Bergakademie Freiberg

2 Two main types of pyroclastic transport and deposit: 1. Fallout > deposit = tephra 2. Flow

3 Eolian fractionation! Mt. St. Helens, 1980

4 Phonolitic fallout deposit, Meidob Hills, NW Sudan

5 Fallout layer between ignimbrites; Bandelier Tuffs, Jemez Mtns, New Mexico

6 Diagenetically compacted fallout deposit, Permian, N Chile

7 Laacher See, Eifel, W Germany, ballistic block

8 Fallout upon water - water-lain fallout deposits Lacustrine fallout deposit, Permian, N Italy Various types of grading!

9 Formation of pyroclastic flows: - lava(-dome) collapse - eruption column collapse Branney & Kokelaar 2002

10 Mt. Pelee, 1902/3, Martinique

11 Mt. Unzen

12 Block-and-ash-flow deposit: result of an explosive lava dome eruption Meidob Hills, NW Sudan

13 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).

14 Ngauruhoe 1978, New Zealand

15 What drives pyroclastic flows? Kinetic energy, gravity, turbulence and fluidisation! Large spectrum of processes and deposits: Cool dilute turbulent often phreatomagmatic Surges and cool pyroclastic flows s.s. Hot hot dense weakly turbulent to laminar Hot pyroclastic flows s.s.

16 Surge deposits: pyroclastic cross stratification! G. Wörner Laacher See, Eifel, W Germany

17 Lithofacies types of surge deposits Fig 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 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).

18 Fig Schematic diagram showing the structure and idealised deposits of one pyroclastic flow (From Cas & Wright 1987). Ground surge deposit, E California

19 Low-angle cross stratification

20 Deposition by aggradation Branney & Kokelaar (2002)

21 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. Branney & Kokelaar (2002) Características de los depósitos Macizos y sin estratificación, aunque una débil fábrica direccional puede formarse si la velocidad es suficiente para orientar las partículas.

22 Pyroclastic fractionation: >> important compositional changes in crystal-rich flows! elutriation

23 Pyroclastic Gradación flow deposits may show various types of grading: Brohltal, Laacher See, Eifel, W Germany

24 Accretionary Lapilli Schumacher & Schmincke 1991 Hailstone-like ashaggregates from wet eruption clouds and co-flow-ash clouds, Trigger by rain possible!

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

26 Professor! Don t forget to make some drawings about Proximal distal facies variation Lateral facies variation

27 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)

28 Post-depositional processes in hot pyroclastic flow deposits: 1. welding compaction, in extreme cases: rheomorphism 2. gas + fluid escape >> vapor phase crystallisation

29 Fig 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 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).

30 Diagenetic compaction non-welded ignimbrite, N Chile, Permian >> Branney and Sparks 1990 Welding-compaction Teplice ignimbrite, Late Carboniferous, Czech Republic

31 Eutaxitic texture From Marcelo Arnosio fiamme

32 Blue Creek ignimbrite, Idaho, USA 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) Parataxitic texture

33 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 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).

34 Bishop Tuff, Owens Gorge, E California

35 Vapor phase crystals on ash fragment, SEM image (experiment by Grunder et al. 2005) Non-welded ignimbrite Sillar Facies: lithification by vapor phase crystallisation; typical crystals in SiO 2 -rich systems: cristobalite, tridymite, sanidine

36 Degassing pipes fines-depleted From Marcelo Arnosio

37 Fig 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

38 Fig 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

39 Szentbekkalla, Hungary Hydroclastic flow deposit (from collapse of a phreatomagmatic eruption column)

40 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)!