08/08/2013
In visiting Erta Ale with colourful hot springs at Dallol Volcano.
The Erta Ale volcano, located in the similarly named range in the hostile Danakil depression in NE Ethiopia, is a basaltic shield volcano known for the persistent lava lake activity since early in the 20th century. Whilst only 613m high, the volcano has a base with a diameter of nearly 30km.the summit caldera contains two large steep-sided pit craters, the N and S (or “central”) pit craters ,and one smaller pit at the south-east side of the N pit. The caldera appears to have been formed by 3 overlapping circular collars structures and has approximate dimensions of 166x700m. The first detailed study of Erta Ale in 1971 revealed active lava lakes in both pit craters. Between 1972 and 1974, lava was regularly reported to be overflowing the S crater, eventually covering much of the southern flakes of the caldera. Overflows and active hornitos on the rim of the N crater were also noted, between 1975 and 1986, political instability prevented field studies, but satellite monitoring showed no significant activity outside of the summit crater. In 1987, both lava lakes were still preset, yet by 1992, the lava lake in the N pit crater had disappeared, Since the 1990s, observations of Erta Ale have become more regular, with film teams and increasing numbers of tourists visiting the area in recent years. An active convicting lava lake has been present in the S crater of much of this period. Solidification of the surface and formation of hornitos on the crater floor has been reported. In September 2005, local people reported unusual glowing above the crater. A subsequent survey of the area revealed morphological changes in both craters with a lava bulge in the N crater and a new cone with an active lake within the S crater. The exact sequence of the events is however no documented. In November 2010, following a period in which the lava level had been steady rising, several short lava flows were emitted from the crater and a cone was built up around the lake due to repeated overflow episodes. By the end of January, 2011 a small lake could be observed about 10m below the level of the crater floor. The cones southern flank had failed by the time.
Lava lakes as seen at Erta Ale or previously examples at Kilauea volcano represent the exposed upper surface of convecting magma columns. Magma circulation must be maintained between magma chamber and the entire magma columns so that heat loss does not lead to solidification of the surface of the lake. Indeed, any heat loss at the surface must Balanced by cooling and crystallization in the magma chamber. Carbon dioxide levels in the magma suggest that it on lake is partially accounted for by constant introduction of fresh gas-rich magma from the mantle 10k/sec which is far too low to counter balance the 100-400 Mega watt of energy lost from the system .Oppenheimer and Francis (J. Volc. Geotherm. Res. 80,p.101-110(1998)) considered this issue in detail and concluded that whilst only little lava is erupted at the surface, at least 10-fold more may be erupted into faults and cavities beneath the base of the volcano.
Several groups have studied temperature, surface movement (which is correlated to convection levels) and seismic activity associated with the lava lake. Two papers analyse data collected during the Feb. 2002 expedition, during which some of the photographs on this page were taken (J. Volc. Geothem.Res. 142, p.207-223 (2005); J. Volc. Geothem.Res. 153, p.64-79 (2006)).the following information is assembled from these papers and their sub references.
In 2002, the lava lake was observed to be switched between low and high convection phases. During low convection phases, the crust was observed to move less that 10cm/sec, whilst during high convection phases, movement of 10-40-cm/sec was recorded. Low convection phases typically lasted for 1-10 hours, with high convection phases were usually between 1 and 3 hours long. It has been proposed that during the low phases the surface of the lake cools and degasses, thus becoming increasing dense. This dense surface region traps gases in the conduct leading to slight pressurization. The build up of gases is thought to account for the low frequency tremor whose source was localized to an area of conduit 300+350m below the lake surface. At the end of each low convection phases the system becomes unstable and convective overturn occurs. During the high convection phases the crust cracks and the denser the magma at the lake surface which includes the crust sinks whilst hotter gas-rich magma rises to the surface. A higher frequency tremor becomes super imposed on the low frequency tremor during these bursting at the lava lake surface as increased degasses is able to take place.
The surface of the lake does not convect uniformity during high convection periods. Lava rises to the surface in certain parts of the lake, whilst it sinks in other parts. Fountain is often observed at zones where hot gas-rich magma reaches the surface, whilst smaller fountains may be observed where section crusts are drawn wards. The fountains can often be observed moving across the surface of the lake since the zones of upwelling or sinking are not at fixed locations within the lake. Observed fountains heights were approx. 2-4 meters in February 2002 with up to 3 fountains being seen simultaneously. Interestingly, violent fountains were observed several seconds after a large box a refuse was thrown into the lake. This can be attributed to gas released by the garbage as it was rapidly heated.
The surface of the lake thickens during phases of low convection. After 7 min the crust (defined by region having temperature below 1070’c) can be calculated as being 205cm thick, whilst after 70min. A thickness of about 8.5 cm is attained. The surface of the lake may have a temperature of as low as 200ºC during periods of low convection (compared to lake temperature exceeding 1100 ºC) which demonstrates the insulating properties of a lava crust. When the crust is thick, heat loss may not significantly exceed that of a hot (water) crater lake and small objects (such as oranges) thrown onto that lake surface are no longer able to pe*****te it.
In febr. 2008, a second visit to Erta Ale revealed a similar sized lake as in 2002, yet the morphology of the S crater had changed somewhat. Further, several hornitos could be observed on the floor of the N crater. At least one of which was visibly incandescent inside. The lake was found on the same part of S crater as before yet both the crater terrace and the lake were higher than in 2002. Further, the active part of the lake had what appeared to be a stable thick Collar@ surrounding it. Whist no scientific data was collected during their visit, the level of the whole lake could be observed to rise and fall slightly, the former resulting in periods during which significant amounts of lava flowed out of the lake onto the collar surrounding it. A particularly strong overflow episode eventually caused subsisidence of parts of the collar into the lake, demonstrating that it merely represented a somewhat thicker crust which formed peripherally to the more actively convecting central part of the lake. These episodes of rising and falling lake levels are documented and can be seen in your trip.
