IAVCEI/CEV/VMSG Field Workshop,
Tenerife, Canary Island
Pyroclastic Density Currents and Topography


22-29 March 2007

Judy Fierstein,
Volcano Hazards Team,
U.S. Geological Survey


This eight-day field workshop to Tenerife, the largest and highest ocean island volcano in the Canary archipelago, examined the nature of density currents-how they travel and how they are deposited-focusing in particular on several well-exposed Tenerife ignimbrites that swept radially across the island over the last 2 Ma. Rebecca Williams (U. Leicester) ensured smooth logistics in her role as workshop administrator for this field workshop sponsored by IAVCEI and its Commission on Explosive Volcanism and the VMSG (UK Volcanic and Magmatic Studies Group). We stayed in the pleasant beachside “Hotel Playa Sur” in the quiet fishing/kite-surfing town of El Médano on the southeastern coast of the island, which provided easy access to each day's daily field destinations. Leaders Mike Branney (U. Leicester) and Rich Brown (U. Bristol) provided the 18 participants representing seven countries and 16 different institutions a general geologic background of the island, describing the oldest basaltic shield lavas, the explosive caldera-producing pyroclastics, and the more recent Teide stratovolcano and cinder cones and lava fields. Most days, however, were filled with beautifully exposed ignimbrite and fallout sequences that provided the stratigraphic context in which to interpret the depositional features described in great detail by both leaders.

A main goal of this workshop was for modelers, experimentalists and geologists to examine deposits in the field together and to discuss from different perspectives eruptive, transport, and depositional factors that are reflected in the deposits. Exposures show ignimbrite sheets far more complex than simple, single, idealized coherent flows. The 273 ka Poris ignimbrite (Brown and Branney, 2004 a, b) was examined in most detail because the zoning of the pumice compositions, lithic types and abundances, and distribution of accretionary lapilli have all been used to divide the ignimbrite into a time-stratigraphic framework. This time stratigraphy permits evaluation of the influence of topography on a depositing current, and the competing influences of vent-derived characteristics on the resulting deposits. There were frequent and lengthy discussions of en-masse emplacement of a flow versus progressive aggradation of a sustained density current (Branney and Kokelaar, 1992) and of which emplacement mechanisms could best explain the sorting, grading, and bedform structures so well-displayed in these deposits. Discussions frequently came back to the degree of density sorting within a current, how marked is the density stratification within the current, whether it is gradational or abrupt, and how that moving current interfaces with the topography over which it is traveling. Such discussion was enhanced by evening videos of flume experiments and modeling simulations presented by workshop participants, each providing insights in controlled situations to the evidence 'on the ground'. Participant contributions also made it clear that ignimbrite deposits elsewhere show variably graded and stratified deposit characteristics, many of which are similar to those at Tenerife (e.g., Valley of Ten Thousand Smokes, Alaska; Fierstein and Wilson, 2005), which also show that simple en-masse eruption and deposition for an entire ignimbrite sheet is not the norm. Also shown well on Tenerife and not uncommon elsewhere (Laacher See, Valley of Ten Thousand Smokes, Bishop Tuff) is that time-equivalent deposits can vary greatly from one location to another within the same eruptive sequence, such as a thick pyroclastic flow deposit in one place being represented by a thin fine ash layer in another, or, a thick flow deposit in one place is a multi-component flow+fall deposit in another. Brown and Branney emphasized the role of topography in developing these deposit differences on Tenerife, especially considering how the density currents filled fluvially carved slopes.

In summary, this participant sensed general agreement that large ignimbrite sheets are not deposited as single en-masse flows, that density currents are graded-at least to some degree-during transport and that deposit characteristics are in part dependent on where along the transport line, from source to final stopping point, they are exposed. The detailed work done on the ignimbrites on Tenerife underscores that a time-stratigraphic framework is critical to unraveling details of ignimbrite emplacement.

Group

Participants of the workshop

References:

Branney, M., Kokelaar, P. (2002) Pyroclastic density currents and the sedimentation of ignimbrites. Geol Soc London, Memoirs 27, 152 pp.
Brown, R., Branney, M. (2004) Event-stratigraphy of a caldera-forming ignimbrite eruption on Tenerife: the 273 ka Poris Formation. Bull Volcanol 66:392-416.
Brown, R., Branney, M. (2004) Bypassing and diachronous deposition from density currents: Evidence from a giant regressive bed form in the Poris ignimbrite, Tenerife, Canary Islands. Geology May 2004; v. 32; p. 445-448.
Fierstein, J., Wilson, CJN. (2005) Assembling an ignimbrite: Compositionally defined eruptive packages in the 1912 Valley of Ten Thousand Smokes ignimbrite, Alaska. GSA Bull 117; p. 1094-1107.




Last Update: Sep. 4, 2008