Estimation of Maximum Effusion Rate for the Pillan 1997 Eruption on Io: Implications for Massive Basaltic Flow Emplacement on Earth and Mars

Introduction: In 1997 the Galileo spacecraft observed a huge eruption at Pillan, on the jovian satellite Io, which emplaced at least 56 km of lava which covering more than 5000 km in less than 200 days [1-3]. By considering varying effusion rate QF following the treatment of Wadge (1981) [4], we estimate peak values of QF in the range 1.87 x 10 to 5.94 x 10 m s. This is in the range proposed for rubbly pahoehoe flood lava flows on Mars and the Earth [5,6]. Pillan 1997 eruption: The 1997 eruption of Pillan on Io was characterized by active lava-fountaining and the rapid emplacement of extensive flows of mafic or ultramafic composition that covered over 3000 km of Io’s plains and the floor of Pillan Patera, another 2500 km. If all of these flows are 10 m thick [2], the flow volume is 56 km, making Pillan Patera one of the largest eruptions ever documented by humankind. By comparison, the Laki eruption of 1783-1784 emplaced a dense rock equivalent of ~15 km [7] and the 1815 Tambora eruption involved ~30 km of magma [8]. This volume does not include the extensive low-albedo pyroclastic deposits that covered ~4 x 10 km. These lavas were emplaced in a minimum of 52 days and a maximum of 167 days [1, 2]. The best estimate, based on the cooling of the lava flows, yielded a duration of ~99 days [1]. A review of the thermal and visible activity at Pillan is given in [1,2]. QF verses QE: Estimates of the rate of eruption in previous studies calculated mean eruption rate, QE, the average rate at which lava was emplaced, yielding values of 2000-7000 m s with ~3000 m s being the preferred value [1,2]. As shown by Wadge [4], while QE is useful for providing a comparison between eruptions, it is not particularly illustrative of the what is actually taking place. Wadge separated eruptions into a short waxing phase as the eruption rapidly builds up to peak QF, followed by a longer waning period as QF decayed exponentially. This is shown in Figure 1. The area under the curve is the total volume erupted. Calculation of QF: Selecting a suitable duration for the waxing phase (we use 5 days) and knowing the volume of lava erupted, for any period of waning activity a Wadge-like curve can be iteratively determined. Results are shown in Table 1. Maximum QF values range from 5.94 x 10 m s for a 52-day eruption (waning phase = 47 days) to 1.87 x 10 m s for a 167-day eruption (waning phase = 162 days). Max QF values can be compared with values of QE (Table 1). Rates of areal coverage: With direct measurment of Pillan flows yielding a thickness of ~10 m [2], the changing rate of areal coverage can be calculated. This ignores lava flow inflation, which may be reasonable for this relatively short-lived eruption. Values are shown in Table 1. It is likely that the Pillan flows, like the flows at Tvashtar and other Io outburst eruptions issued from fissures [9,10]. From fissures, flows can advance on a broad front. If 25 km wide, then the maximum flow front advance rate is only 0.25 m s (or ~1 km per hour).