Erta Ale Volcano

Erta Ale volcano, located in the similarly named range in the hostile Danakil depression in NE Ethiopia, is a basaltic shield volcano known for persistent lava lake activity since early in the 20th century. Whilst only 613m high, the volcano has a base with a diammeter 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 collapse structures and has approximate dimensions of 1600 x 700m.

Aerial view of Erta Ale summit caldera from S during February 2002 overflight. The active pit crater is in the center of the picture.	Aerial view of Erta Ale summit caldera from E during February 2002 overflight. The large inactive pit and adjacent fumarole field are clearly visible.

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 flank 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 craters. In 1987, both lava lakes were still present, 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 even tourists visiting the area in recent years. An active convecting lava lake has been present in the S crater for much of this period. Solidification of the surface and formation of hornitos on the crater floor has also 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 events is however not documented.

Active pit crater viewed from nearby large hornito. Crater diammeter is approx. 130m. Feb. 2002	Evening view of pit crater from hornito in 2008. Note: left hand corner has widened.

Erta Ale lava lake, 2002. Lake is in deep pit.	Erta Ale lava lake, 2008. Note: lake size is as in 2002 but crater floor/lake are higher.

N crater. Intensely degassing hornitos on left. (2008)	Partially collapsed E rim of N crater. 1970s hornito on horizon. (2008)

Lava lakes as seen at Erta Ale or previously at e.g. Kilauea volcano represent the exposed upper surfaces of convecting magma columns. Magma circulation must be maintained between magma chamber and the entire magma column so that heat loss does not lead to solidification of the surface of the lake. Indeed, any heat lost at the surface must be balanced by cooling and crystallization in the magma chamber. Carbon dioxide levels in the magma suggest that it on remains in the convecting system for about 10 years which means that the longevity of the lava lake is partially accounted for by constant introduction of fresh gas-rich magma from the mantle into the system. However, Erta Ale has been calculated to have a surface eruption rate of only 10kg/sec which is far too low to counterbalance the 100-400 Megawatts of energy lost from the lava lake and certainly does not correspond to the assumed rate of injection of fresh magma into 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.

Animation of lake surface during slow convection phase. Speeded up 40x.

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 Febr. 2002 expedition, during which some of the photographs on this page were taken (J. Volc. Geotherm. Res. 142, p.207-223 (2005); J. Volc. Geotherm. Res. 153, p.64-79 (2006)). The following information is assembled from these papers and their subreferences.

In 2002, the lava lake was observed to be switching between low and high convection phases. During low convection phases, the crust was observed to move less than 10cm/sec, whilst during high convection phases, movement of 10-40cm/sec was recorded. Low convection phases typically lated 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 increasingly dense. This dense surface region traps gases in the conduit leading to slight pressurization. The buildup of gases is thought to account for the low frequency tremor whose source was localized to an area in the conduit 300-350m below the lake surface. At the end of each low convection phase the system becomes unstable and convective overturn occurs. During the high convection phases the crust cracks and the denser magma at the lake surface which includes the crust sinks whilst hotter gas-rich magma rises to the surface. A higher frequency tremor becomes superimposed on the low frequency tremor during these phases. This has been explained as the result of cracking of the lava lake crust and bubble bursting at the lava lake surface as increased degassing is able to take place.

Broken crust sinking into lava lake.	Bubbles of gas released at the lake surface cause fountaining.

The surface of the lake does not convect uniformly during high convection periods. Lava rises to the surface in certain parts of the lake, whilst it sinks in other parts. Fountaining is often observed at zones where hot gas-rich magma reaches the surface, whilst smaller fountains may be observed where sections of crust are drawn downwards. 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 fountain heights were approx. 2-4 meters in February 2002 with up to 3 fountains being seen simultaneously. Interestingly, violent fountaining was observed several seconds after a large box of refuse was thrown into the lake. This can be attributed to gas released by the garbage as it was rapidly heated.

	Lava fountains

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 2.5cm thick, whilst after 70 min a thickness of about 8.5cm is attained. The surface of the lake may have a temperature of as low as 200'C during periods of low convection (compared to lake temperature exceeding 1100'C) 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 the lake surface are no longer able to penetrate it.

In Febr. 2008, a second visit to Erta Ale revealed a similarly 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 in the same part of the 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 "crust collar" surrounding it. Whilst no scientific data was collected during this 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 subsidence of parts of the collar into the lake, demonstrating that it merely represented a somewhat thicker crust which had formed peripherally to the more actively convecting central part of the lake. These episodes of rising and falling lake levels are documented in detail below.

RISING LAKE LEVELS:

Lake crust is lifted and lava flows over rim from underneath as lava levels rise.	Small pahoehoe flows form from lake overflows.

Intense lake overflow episode eventually covering whole "collar" around lake.	Lava flowing out from under thin crust.	Fresh lava covering whole of crater floor after major lake overflow.

Overflow animation (approx. 2x normal speed)	Crust folds as leaves lake and slows down.	Overflow animation (approx. 2x normal speed)

FALLING LAKE LEVELS:

Glowing sides of lake become visible when lava levels fall.	Lava draining back into the lake after a period of overflow was followed by a rapid fall in lake level.	Lava lake in turmoil as level rapidly falls.

Violent lava fountaining accompanying falling lake levels.	Violent lava fountaining accompanying falling lake levels.	Violent lava fountaining accompanying falling lake levels.

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Fountaining was generally much less frequent than in 2002, except for a single location at the W perimeter of the lake which showed almost constant fountaining although with varying intensity. Also, sudden crust overturning was regularly observed (on average about every 30min) at a site at the S border of the lake. Fountaining at other sites was usually associated with phases of lake level fluctuation. The most violent fountaining was actually observed in association with dropping lake levels.

Site of almost constant fountaining at W side of lake, Feb. 2008.	Crust overturning at S side of lake, Feb. 2008.

As in 2002, many active fumaroles could be found at the S end of the N crater aswell as along the N and NW rim. One of the two prominent hornitos on the N rim was also degassing and small sulphur deposits in stalagtite form could be observed.

Fumarole at S rim of N crater, Feb. 2008	Fumarolic deposit W of N crater.	Sulphur stalagtites in fumarole on N rim of caldera

Much of the caldera floor around the S crater is coated in Pelees hairs resulting from the frequent lava fountaining activity and the formation of Pelees hairs was able to be documented. During fountaining, as the lava fragments are thrust apart, sections of lava still connecting the pieces can be drawn out into long threads which are then carried out of the crater by the current of hot air rising above active parts of the lake. Small lapilli were also found near the crater rim with a max. length of about 2cm. The age of these is unknown yet they could result from the apparent heightened activity in September 2005.

Lava fountain producing Pelee's hair	Detail from picture on left. Formation of Pelee's hair can be seen.

Reddish Pelee's hair scattered around Erta Ale lava lake.	Reddish Pelee's hairs near Erta Ale lava lake.

Pelee's hairs near Erta Ale lava lake.	Sulphur crystals on Pelee's hair.	Pelee's hair coated in sulphur crystals by fumarolic activity.

Tourism to the area has increased significantly and visitors were arriving nearly daily during the Febr. 2008 visit. This is in stark contrast to the situation in 2002, when the area was effectively only accessible by helicopter. It is presently (2008) possible to drive to within 7km W of the summit of the Erta Ale. The summit can then be reached on foot in less than 3 hours. Local Afar tribesmen organize transport of material or even tourists using camels. Numerous tour operators can organize travel into the area, often combining visits to Erta Ale with visits to the colourful hot springs at Dallol Volcano. For example, the 2002 visit was organized by Geo-Decouverte and the 2008 visit by Origins Ethiopia.

Further Photos

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