Thermal remote sensing of the low‐intensity carbonatite volcanism of Oldoinyo Lengai, Tanzania

Abstract

Although Tanzania, Kenya and Ethiopia contain a number of active and potentially hazardous volcanoes, none of them are routinely or continuously monitored. Of these, Tanzania's Oldoinyo Lengai (OL) has been active almost continuously over the past two decades (since 1983). Recent activity has been confined to small-scale effusive and explosive eruptions of natrocarbonatite within the summit crater, with lava flows occasionally overflowing the crater rim and extending onto the volcano flanks. The automated MODVOLC algorithm falls short of detecting all thermal anomalies within OL's crater. The sensitivity of the algorithm is insufficient to detect anomalies of the size and magnitude presented by those at OL. We explore how Moderate Resolution Imaging Spectroradiometer (MODIS) infrared (IR) bands can still be used to monitor activity. We cross-verify our observations against field reports and higher resolution satellite images (ASTER, Landsat ETM + ). Despite the limited extent and low temperature (,585uC) of natrocarbonatite lavas, relative variations in eruption intensity and periods of increased activity alternating with periods of reduced or no detectable activity can be observed using the MODLEN algorithm. Although activity in the past two decades has been moderate at OL, a more intense explosive eruption is overdue and there is a need for routine monitoring in the future. Our work makes a case for low-cost thermal IR monitoring as an essential component of such a monitoring programme at several Tanzanian, Kenyan and Ethiopian volcanoes. The approach presented here is already available for routine use.

Voluminous lava flows at Oldoinyo Lengai in 2006: chronology of events and insights into the shallow magmatic system

“…MODLEN, an adaptation of the MODVOLC algorithm (Wright et al 2004), is designed to detect low intensity thermal anomalies at OL, based on threshold values applied to a Normalised Thermal Index (NTI, Kervyn et al 2008). MODIS data analysis for March-April 2006 helped constrain the onset, duration and end of the eruption and relative activity level changes although the volcano was often covered by clouds, as is common during the local rainy season, which lasts from February to May.…”

Assessing Performance of the RSTVOLC Multi-Temporal Algorithm in Detecting Subtle Hot Spots at Oldoinyo Lengai (Tanzania, Africa) for Comparison with MODLEN

“…In 1917In , 1940In -1941In , 1967, and more recently in 2007-2008, vulcanian to sub-plinian explosive eruptions occurred over weeks to months, generating ash plumes as high as 11 km spreading ash over the entire region [30,32,[37][38][39]. These eruptions are associated with nephenelite magma compositions [39].…”

“…Therefore, it is realistic to suppose that a number of thermal activities occurring during the period of interest were unreported by both validation sources [30]. In order to take this limitation into account, and also considering the lava cooling process, we considered that thermal anomalies flagged within ±2 days from documented eruptions were associated to real (although undocumented) thermal activities of OL.…”

“…This remote volcano is the only volcano on Earth emitting natrocarbonatite lava that shows a very low level of viscosity (10 −1 to 10 2 Pa s [29]), and a relatively low temperature (ranging from 495-590 °C), compared to other types of lavas [30,31]. The eruption rates at Oldoinyo Lengai (OL) are extremely low; therefore, lava covers only a few tens of m 2 compared to several hectares or km 2 at other active volcanoes.…”

“…Due to the limited area and temperature of active lava flows, the OL thermal activity is generally difficult to investigate by satellite. A specific adaption of MODVOLC, named MODLEN, has been designed, tailored and tested with good results for better identifying subtle hot spots at OL [30][31][32][33]. This is not routinely monitored by ground-based systems, as is similar with other African volcanoes [34].…”