PYROCLASTS · The formation, preservation and lithification of low viscosity magma pyroclasts

The formation, preservation and lithification of low viscosity magma pyroclasts

PYROCLASTS – Publishable summary Explosive volcanic eruptions produce pyroclastic deposits comprising a diverse array (in size range and shape) of particles of disrupted or fragmented magma (juvenile pyroclasts) and subordinate proportions of lithic fragments (e.g. wall rock). The properties of pyroclastic deposits, and the pyroclasts themselves, are used by volcanologists to reconstruct the styles and intensity of unobserved explosive volcanic eruptions. Present day volcanism produces a range of magma types characterized by large variations in melt composition. At the point of eruption, the effective viscosity of these magmas can vary by at least 10 orders of magnitude. This range in magma viscosity results mainly from variations in eruption temperature, melt composition, abundances of dissolved and exsolved volatiles, and crystal contents. Felsic magmas such as rhyolite and dacite generally feature high effective viscosities (>109 Pa s), high volatile contents, and commonly erupt explosively. Explosive magmatic eruptions cause intense fragmentation of these magmas into ash- (< 2mm), lapilli- (2 - 64 mm), and block-sized pyroclasts; lapilli-sized and larger pyroclasts, called pumice, are characterised by irregular shapes and an abundance of trapped gas bubbles or vesicles. Basalt magmas are substantially less siliceous in composition and have higher eruption temperatures; the consequence is that basaltic magmas have effective viscosities that are orders of magnitude less viscous than the common felsic magmas (102 to 103 Pa s). The explosive eruption of basalt also causes fragmentation of the magma into irregular vesicular clasts (called scoria). The lower viscosity of basalt, however, also allows for more fluidal pyroclast shapes and the formation of smooth solidified droplets called achneliths or Pele’s tears (Figure 1). These tears indicate that viscous relaxation of the melt droplets under the influence of surface tension occurred after magma disruption, whereas pumice and scoria record the brittle fragmentation of the magma with no subsequent relaxation. The least viscous (~10 Pa s) magmas found on Earth include carbonatite, natro-carbonatite and kimberlite. To date there has been little reliable examination of the nature of the the pyroclasts produced by the explosive eruption of very low-viscosity magmas. Consequently, there is little physical understanding of pyroclast formation processes (e.g. magma fragmentation or disruption) in such magmas. My studies of kimberlite pyroclastic deposits suggest at least three main differences between the pyroclasts produced by fragmenation of low viscosity melts versus high viscosity melts: i) kimberlite pyroclasts have little or no vesicularity ii) kimberlite pyroclasts tend to be spherical, and iii) the pyroclastic deposits commonly contain free crystals which have been entirely liberated from the melt. The PYROCLASTS project is investigating the behaviour of low viscosity magmas during explosive eruptions; how they fragment or break-up, what in-flight processes might occur (Figure 2), and what are the main influences on the final pyroclast morphology. Research is carried out through both field and experimental studies. To date we have explored the ability of kimberlite melts to produce glassy ash (Porritt and Russell, 2012. Physics and Chemistry of the Earth, v.45-46: 24-32) and the formation of basaltic Pele’s tears (Porritt, Russell and Quane, 2012. Earth and Planetary Science Letters, v 333-334: 171-180). On-going research is examining the break-up of analogue liquids into droplets and the influence of both viscosity and ejection velocity on this process. Figure 1. Fluidal basalt pyroclasts from the 1969 Kilauea Iki eruption, Hawaii. These achneliths have smooth shiny surfaces, unbroken by vesicles, and show a progression from irregular shapes to tears and then spheres as size decreases (ruler divisions = 0.5 mm) Figure 2. Model of in-flight modification of low viscosity pyroclasts dependant on drop size, which influences cooling rate. Whilst still above the glass transition temperature (in the relaxed ‘hot’ regime) processes such as relaxation and surface tension shaping, and bubble growth and nucleation can occur. Below the glass transition temperature (unrelaxed ‘cool’) these processes are prohibited. If bubble growth is faster than the relaxation timescale of the melt brittle fragmentation will occur. Figure taken from Porritt, Russell and Quane (2012)

Data: CORDIS, © European Union