On the emplacement of ignimbrite in shallow-marine environments
Introduction
Pyroclastic flows are a common product of volcanoes in marine settings (Cas and Wright, 1987). However, exactly what happens when a pyroclastic flow enters the sea is poorly understood. Small-volume block-and-ash flows formed by dome collapse are known to travel as highly concentrated avalanches that are probably denser than water. Lacroix (1904) observed examples of such flows entering the sea at Montagne Pelée in 1902, and their submarine equivalents broke cables nearly 20 km offshore. At Montserrat in 1996, the basal avalanches of some dome-collapse flows entered the sea, generating a zone of intense boiling, while the overriding ash cloud passed over the surface (Cole et al., 1998).
The case of the voluminous pyroclastic flows that generate ignimbrite is more controversial Fisher, 1984, Cas and Wright, 1987, Cas and Wright, 1991. The occurrence of welded ignimbrites in marine sedimentary successions has led to the assertion that pyroclastic flows can enter the sea and travel at high temperatures for considerable distances under water (e.g., Francis and Howells, 1973, Sparks et al., 1980). However, the evidence has been reviewed critically by Cas and Wright (1991) using strict criteria for water depth and high-temperature emplacement (welding, columnar jointing, elutriation pipes, thermal remnant magnetism). They concluded that most ignimbrites in ancient marine successions were emplaced in shoreline or shallow-water settings. The best examples include the Capel Curig Tuffs Howells et al., 1979, Fritz and Howells, 1991, Orton, 1991, McArthur et al., 1998, the Pitts Head Tuff (Wright and Coward, 1977), the Upper Donzurobo Formation Kato et al., 1971, Yamazaki et al., 1973, the Koura Formation (Kano, 1990), and the Shirahama group (Tamura et al., 1991).
In these cases, the sedimentary facies associations suggest water depths no more than a few tens of metres when the ignimbrite was laid down. A modern example is the 1883 Krakatau ignimbrite, which is known from measurements of thermal remnant magnetism to have been emplaced at about 500°C in 40 m of water 10 km from the caldera centre Mandeville et al., 1994, Mandeville et al., 1996. It is therefore well established that ignimbrite can in some circumstances deposit and even weld in water depths up to a few tens of metres, although the mechanism is not well understood.
Most pyroclastic flow deposits in deep-marine environments are inferred to have been emplaced as water-saturated mass flows, possibly in some cases the cold, lateral equivalents of subaerial ignimbrites Yamada, 1973, Niem, 1977, Carey and Sigurdsson, 1980, Wright and Mutti, 1981, Cas, 1983, Kano et al., 1989. There are fewer occurrences of ignimbrites emplaced hot below wave base Schneider et al., 1992, Kano et al., 1994, Fritz and Stillman, 1996, White and McPhie, 1997. Some of these may have been laid down close to source during subaqueous eruptions, as demonstrated for the Lower Rhyolitic Tuff (Howells et al., 1986) and at Mineral King Caldera (Kokelaar and Busby, 1992).
Other developments have further complicated this debate. Large pyroclastic flows have been widely assumed to travel as highly concentrated flows, like their smaller-scale equivalents (e.g., Sparks, 1976, Wilson, 1985). However, there is good evidence that some flows such as those of the Koya Ignimbrite (Ui, 1973), the Campanian Ignimbrite (Fisher et al., 1993), and the Kos Plateau Tuff Keller, 1970, Allen, 1998 travelled many kilometres or tens of kilometres over the sea, showing that parts of the flows were significantly less dense than water. Parts of the Krakatau pyroclastic flows also travelled across water, laying down hot deposits on nearby islands (Carey et al., 1996). If many large pyroclastic flows are light enough to travel across water, a mechanism needs to be found to explain how hot ignimbrite comes to be deposited on the seabed. In this paper, we use the term pyroclastic flow in a general sense for any pyroclastic density current that lays down ignimbrite, no matter what the particle concentration.
Fig. 1 shows four possible situations when a pyroclastic flow enters the sea:
(1) The flow is denser than water and travels beneath it (ρc>ρw≫ρa). The bulk densities of block-and-ash or scoria flow deposits typically lie in the range 1500 to 2000 kgm−3, so these flows, if poorly expanded during transport, are most likely to behave in this manner. Pyroclastic flows that travel in a non-turbulent mode and that are protected by a steam carapace Francis and Howells, 1973, Sparks et al., 1980, Kokelaar and Busby, 1992 are most likely to travel furthest before transforming into debris flows and turbidity currents by water ingestion.
(2) The flow is slightly lighter than water and forms a surface current (ρw>ρc≫ρa). Many ignimbrite deposits have bulk densities only slightly greater than 1000 kgm−3 (e.g., Mandeville et al., 1996); thus, the parent flows are likely in many cases to be less dense than water, as supported by the known ability of some such flows to cross open sea.
(3) The flow is much lighter than water and mainly passes over it (ρw≫ρc>ρa). This corresponds to flows that are highly expanded or to the upper parts of density-stratified flows.
(4) Irrespective of flow density, ingestion and vaporisation of water is so rapid that destruction at the flow front exceeds supply rate and the flow is destroyed explosively at the shoreline Carey and Sigurdsson, 1980, Whitham, 1989.
In cases 1 to 3, there is a region in which the flow totally displaces the sea, irrespective of whether the flow is lighter or denser than water. Provided that the eruption is sustained long enough, it is possible for the flow to lay down hot ignimbrite on this region of the seabed. Once the eruption ceases, the sea will then return to cover the ignimbrite. There are therefore two mechanisms by which hot ignimbrite can be emplaced below sea level: (1) by dense laminar flows that travel under the sea without significant mixing, and (2) by flows (laminar or turbulent) that simply push back the shoreline and deposit ignimbrite on the seabed. We do not address the first mechanism any further here, but focus on the ability of pyroclastic flows to displace the sea. We refer to this mechanism as shoreline displacement.
In this paper we constrain the extent of shoreline displacement theoretically and experimentally with simple analogue systems. The ability to push back the sea is a function of discharge rate, flow density, and sea floor bathymetry. We assume that the pyroclastic flow spreads radially and is sustained over a long time period. Our model is only applicable to sustained eruptions in which there is enough time for the hot pyroclastic flows to flush the seawater and cold, water-saturated flow front out of the displacement zone. We also assume that the flow behaves as a homogeneous, turbulent fluid. This is perhaps most appropriate for fast-moving flows, although even small pyroclastic flows at Mount St. Helens have been successfully modelled as turbulent flows (Levine and Kieffer, 1991). Experiments were carried out both with simple saline currents and with saline currents that produced gas by mixing with the ambient fluid. The latter were designed to place constraints on the effects of water ingestion and steam production on the extent of shoreline displacement.
Section snippets
Pyroclastic flows denser than seawater
We consider first the case of a pyroclastic flow denser than seawater (case 1; Fig. 1). A gravity current on a horizontal surface is driven by the difference in hydrostatic pressure between the current and the ambient fluid in the far field. The Bernouilli equation states that, for the inviscid turbulent case, the difference in hydrostatic pressure must equal the dynamic pressure Benjamin, 1968, Simpson, 1987:so that the velocity of the current is:where ρc and ρw
Saline currents without gas production
Two sets of experiments were carried out (Fig. 3). The first involved light currents of fresh water in denser salt water. We used a sectorial Plexiglass tank with a 2.8-m-long triangular base, an apex angle of 5.9°, and a height of 30 cm (Fig. 3). A reservoir with a removable gate was added near the apex of the tank. The tank was filled with salt water up to a height between 1.2 and 9 cm. The reservoir was then fed by a constant flux of coloured fresh water. Flow rates varied between 61 and 755
Discussion
We have presented equations to describe the entry of a pyroclastic flow into the sea and have tested those equations using a simple laboratory system. The equations and experiments suggest that, given steady-state conditions, the pressure exerted by a pyroclastic flow might be capable of temporarily displacing the sea, enabling hot ignimbrite to be deposited on the seabed.
The experiments with gas production show that buoyancy generation in the flow does not necessarily affect the displacement
Acknowledgements
Thoughtful reviews by Steven Carey and Brian Dade led to improvements to the manuscript. Financial support from the French Centre de Recherche Volcanologique (CRV) is acknowledged.
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