ARDOUCÔBA – the Seven-Day-Wonder in the AFAR TRIANGLE

…and the youngest volcano on Earth

Ardoukôba within the Assal-Ghoubbet Rift. View from SE, Lake Ghoubbet in the foreground and Lake Assal behind it. 3D-rendering of Google Earth satellite view.

This is hardly a place the average tourist would choose to use his saved up money and spare time to spend an extended holiday, and yet, it’s the place where many a scientist of any calling would gladly give an arm and a leg to be able to go for an extended research project. The paleo-archaeologist and -anthropologist would marvel at an opportunity to visit the “cradle of mankind” where our species has evolved and diversified in the last millions of years – proven by many important discoveries of early human’s artifacts. The biologist and botanist would study life in the hottest (<50°C) and driest (100-200 ml/yr) climate on earth, including the last few hundred individuals of the African wild ass (Equus africanus). But every scientist of a subject starting with Geo- would be looking at the place where Earth’s history is written right now, where they can watch continents move apart and – unique and most important – where a new ocean is about to be born, with the new ocean floor already being created.

Location of Tadjoura spreading centres at the westernmost tip of propagating Gulf of Aden ridge. Rectangle indicates the location of the Asal-Ghoubbet rift.

THE AFAR TRIANGLE

includes all of Djibouti, the Afar Region in Ethiopia, as well as areas of Eritrea and Somalia. This is a very special place because here meet three great active rifts where three plates move apart – a triple junction.
① Extending southwards from here, we have the great East African Rift System breaking the African continent in two: one chunk is part of the future Somalian plate and moving east towards the Indian Ocean. The rest of Africa (or the Nubian Plate) is moving N towards Europe (and closing the Mediterranean Sea in the process).
② To the NW, the Red Sea fills the fissure left by the African plate (Nubian part) & Arabian plate moving apart.
③ The eastern rift is the oceanic Aden Ridge under the Gulf of Aden, opening between the Arabian plate & African plate (Somalian part).

Perspective view of the Afar Depression and environs, generated by draping a Landsat image over a Digital elevation model. (© Zyzzy, via Wikimedia)

This stretching has begun some 30 My ago. As a result, the earth’s crust became much thinner and sank. In the Afar Triangle large parts of the territory are now forming the Afar and Danakil Depressions, representing the lowest subaerial point of Africa at ~155 m below sea level. During the past 1 My, spreading has continued to propagate west into the Gulf of Tadjura and into Afar via the subaerially exposed Asal rift. The active plate boundary continues along the west side of the Danakil Block, and links to the Red Sea at the Gulf of Zula.

Ardoukôba Fissure, where the Somalian plate (right) is departing from the Nubian plate (left). (© Rolfcosar, via Wikimedia)

Since 2005 the geological activities in the region have increased considerably: If previously movements were measured in the few-millimeter range, lately new cracks of several meters appear and the average diversion is now 10-20mm/yr. Seismic and volcanic activity is increasing above and below ground. About 50 volcanic edifices in Ethiopia have documented activity during the Holocene. The majority of recent volcanoes are located within the Afar depression; from here a chain of hundreds of volcanoes lines the Rift Valley down to its southern end. Known volcanoes are getting more active and some as yet unknown pop up.

Although there is still disagreement as to what causes all the tectonic activity, the main hypothesis is that of a deep mantle plume below the Afar region feeding the rift openings. A means of testing this is the use of seismic data to assess the ductility and temperature structure of deep mantle through a form of tomography. The closer the spacing of seismic recording stations and the more sensitive the seismometers are the better the resolution of mantle structure. Afar now boasts one of the densest seismometer networks worldwide.

Map of East African rifts showing some of the historically active volcanoes (red triangles) and the Afar Triangle (shaded, center). (USGS)

Among the active volcanoes, Erta Ale is surely the most famous, being characterized by a basaltic lava lake on its summit caldera that has been active more or less continuously in the last 120 years. On the same volcanic alignment, the DallaFilla volcano gave rise – during 2008 – to the biggest eruption ever recorded in Ethiopian territory in historical times. Also currently active is the Dabbahu volcano whose activity started during September 2005 with a dyke emplacement, a strong seismic swarm and a small eruption, followed by other 14 dyke intrusions and 4 small eruptions, the last of which occurred during May 2010. In June 2011, a major eruption occurred at Nabro volcano, at Eritrea-Ethiopia border; many other volcanoes (e.g., Dallol, Ardoukôba, Manda-Inakir, Alayta, Dubbi) have documented activity during the 19th and 20th centuries. (Dr. Giacomo Corti).

Here, the upwelling magma is of a characteristic composition – low in silica content – normally found at mid-oceanic rifts. The region more and more resembles a sea floor where only the water is missing. We won’t live to see it, but in a distant future a vast ocean basin will be here and all the continents around it will have been pushed away in their various directions. When will it begin? When will the Read Sea ultimately flood the desert? No one knows – but scientists of the University of Leeds (GB) think that, in the area of the Afar Triangle, it could be fast: There are only low hills keeping the sea out – strong erosion or a seismic event could let it in within days and so flooding of the Danakil depression would begin.

ARDOUCÔBA VOLCANO

Ardoukôba volcano, in the background Lac Assal. (© Rolfcosar, via Wikimedia)

Whether or not Ardoukôba is the youngest volcano on Earth may be debated – perhaps I am missing a newer volcano, some say perhaps it is part of the nearby Fieala caldera, or Holuhraun is younger – it doesn’t really matter. Fact is, Ardoukôba has a lot in common with Holuhraun: it erupted from a crack in the ground, caused by rifting between two diverging plates, fed by a hotspot.
Although it is often mentioned that the area hadn’t erupted for at least 3000 years before 1978, the most recent lava flows are much younger than that and local people do have legends of eruptions. The volcano was named by the well-known French volcanologist Haroun Tazieff after the region where it had erupted. Tazieff was one of the pioneers in validating Wegener’s theory of plate tectonics, but it took his colleagues a long time to convince him that the Ardoukôba eruption was NOT a direct result of plate tectonics. Rather, a hotspot, a magma plume below and more than three times as big as the Afar Triangle, is supposed to be the driving force.

Location:

Location of the Ardoukôba volcano. Google Earth.

The sea-finger that pokes into Djibouti from the Gulf of Aden is the Gulf of Tadjoura followed west by Lake Ghoubbet and, next on land, by the Assal Rift, followed again by Lake Assal. This entire rifting area is the western extension of the Aden Ridge that is progressing on land to meet the other two rifts of this triple-junction. In its course, after the great flood basalts of the initial break-up, it has produced at least two earlier volcanoes on the new rift floor: The western part of Lake Ghoubbet consists mainly of volcanic materials from what is sometimes called the Ghoubbet volcano or caldera.

Geologic map of the Assal-Ghoubbet Rift. The dark purple colored lavas are from the Ardoukôba eruption.

Further northwest, between Lake Assal and Lake Ghoubbet, the Assal-Ghoubbet Rift is subaerially exposed over 12 km. This is a very tortured area, stretched and squeezed and cut and broken by numerous faults, up to 10 km long and aligned mainly in NW-SE direction. It contains the Fieala Caldera, which probably erupted sometime during the past 3,000 years, and numerous basaltic cinder and spatter cones. Now we have almost arrived: another few kilometers NW from the Fieala, towards Lake Assal, barely recognisable as a volcano, lies Ardoukôba, where a brand new fissure volcano has opened in 1978.

The cone Gira-le-Koma of Ardoukoba volcano in its surroundings with Lake Assal in the background. 11.2009 (© Carole and Frederick Hardy, via Earth of Fire)

Eruption:
This 7-day-wonder lasted from November 7 to 14, 1978. It began with two main earthquakes >M5 and ~800 smaller events on 7th and 8th Nov.. Many of the surrounding faults were reactivated and new surface breaks and fissures appeared – the rift progressed by opening abruptly ~2 m wide and 0.7 m vertically. Early on 8 Nov. the crew of a French observation aircraft noticed an eruption SE of Lake Assal. After a brief period of gas emission, lava fountaining began from this 500-m-long fissure, forming three spatter cones. Then the activity shifted half a kilometer to the SE where two basaltic lava flows were extruded: one flow traveled ~1 km to the SE, the other ~0.5 km to the NW. Lava effusion rates reached an estimated maximum of 1,000 m³/min (or 16.7 m³/s). For the first 2-4 hours as much as 138.9 m³ of lava was extruded per second, but the extrusion rate declined rapidly.

Ardoukôba volcano, Djibouti.

By 9 November, only the main vent was still active. A cone (named Gira-le-Koma) about 40m x 200m x 25m had formed around this vent, which contained a small lava lake. On 13 Nov. the level of the lava lake dropped rapidly, then the lake disappeared. Scoria and bombs were ejected briefly before the eruption ended on 14 November.

Ardoukôba volcano 10/1990 (© Pierre Rollini)

The eruption, caused by basaltic magmatic diking along the rift axis, had formed a ~30 m wide crater and yielded 0.012 km³ of basaltic lava, covering an area of more than 3 km². About 25 fissures, parallel to the NW-SE trend of the rift, had opened during the activity. Most fissures were from a few mm to about 1 m wide, but there was one short segment of 3 m width. As of 22 November, some vapor emission continued from subsidiary vents. No casualties or damage were reported.
A 2012 study on the chemical composition of the freshly erupted basalt estimated “that the Asal Rift basalts underwent degassing for at least 20-50 years prior to eruption” and concluded that: “the Asal Rift magmas rose toward the surface and ponded for 1-3 decades before eruption.”

THE AFTERMATH – or NEW BEGINNING?

The period 1979-1986 immediately after the Ardoukôba eruption was the seismically most active; during this time the rift opened at a fast rate, yet mainly aseismically. The opening was driven by intruding magma, causing more fissures to appear. Then followed a quieter period of nearly twenty years until 2006, in which time the rift opening speed had drastically decreased to 1cm/year. Nevertheless, the chronology of the main seismic events at Fiealé caldera between 1991-2001 showed again a general rise in seismic activity. This was interpreted as a rise of the magma chamber, the upper part of which had migrated from 3.5 to 2.0 km depth.

Aerial view of the Asal rift. The fault, volcano and lava accumulation. Ghoubbet, Djibouti. (© P. Tappennier)

As a consequence of the extensive rifting, cold sea water had been allowed to penetrate from the Ghoubbet towards the Assal lake 155 metres below, continuously filling the axial part of the rift floor at an accelerating rate. When new magma rose underneath the adjacent Fieale caldera, pronounced seismic activity occurred in a zone of 1 to 2 km depth (but no seismic activity was seen below 3 km). This was interpreted as a zone of interference of hot magma (1200°C) with a faulted layer intruded by sea water. In fact, numerous steam vents were observed along open fissures on the caldera bottom as well as along the faults bordering the caldera, where the typical “Fiale” grass grew (which gave the name to the caldera), due to the humidity resulting from the condensation of steam. A new phase of activity developed from 2006 on, again centered underneath the Fiale caldera (with the depth of the seismic events 2-4 km), but also on a location further northwest on the fault where events appear to be deeper (4-7km) and also uplift was observed from radar interferometry. Is that the “head” of the rift, where it is progressing towards the other two arms?

 

A short digression to Lake Assal of the superlatives

The altitude of the Ardoukôba area is less than 100m a.s.l., the spatter cone builds up to 40m, so, from the shore of adjacent Lake Assal (-155m) you look up to a “whopping” ~300m mountain…

Lac Assal: Salts and other minerals crystallizing out of one of the saltiest waters on Earth. (© écrin du ségala, via Panoramio)

Lac Assal: Salts and other minerals crystallizing out of one of the saltiest waters on Earth. (© écrin du ségala, via Panoramio)

Lake Assal (or Asal) not only has the saltiest water on earth outside Antarctica (its salt content of 35% is ten times higher than that of the oceans) where your body will float without your help, but also is the lowest point on the African continent. With -155 meters it is the third lowest in the world after the Dead Sea (-400 m) and Lake Tiberias (-208 m). When the climate in this area became hot and dry, the lake began to evaporate and to decline gradually, increasing the concentration of minerals in the water.

The floor of the depression is now encrusted with billions of crystals, wreaths and roses of translucent gypsum, sometimes crystallized as “spearheads”. These crystals, typically honey yellow color, can reach up to 15 centimeters long. They form on any medium (rock, plant debris, etc.) but also the bottom of the lake is dotted with gypsum flakes.

Stand with minerals from Lake Assal for sale. 06/2011 (© christian vigna, via Panoramio)

The salt is mined, used and sold by the locals, and the beautiful minerals are a welcome source of additional income as well as a nice souvenir for tourists.

 

This is a nice video by Team Jaipalatrouille “Journée au Lac Assal. Djibouti Septembre 2016”:

 

Aah… we also have a HAIR-RISING MYSTERY in Lake Ghoubbet!

Since at least 1967 an invincible rumour surfaced time and again (and is still repeated and embellished occasionally) about an unexplicable experience that the team of French explorer Jean-Jaques Cousteau allegedly has made in the waters of Lake Ghoubbet. According to the rumour, it would not have been disclosed because it would have had too great an impact on current human knowledge! Scene: In preparation for an underwater filming of sharks a STRONG steel cage is baited with the carcass of a camel, then let down in order to attract the sharks. Something seems to be amiss and the cage is hauled up again. Discovery: Oh HORROR! The cage is completely destroyed and the camel gone! Conclusion 1: This must have been either a Mosasaur?, a Kronosaure?, a giant squid? or even a MEGALODON?! The legend goes on to say that Cousteau himself dove down to film what there might be (now, even if I believed the first part, this is where I’d start laughing), but locked the resulting video away (or, in other versions, destroyed it) and came to Conclusion 2: this is too horrible to be revealed to mankind!

Jean-Jaques Cousteau

This rumour has been dismissed by the Cousteau Foundation several times, and the HOAX is dismantled here. It was indeed contradicted by Cousteau himself in his 1971 book “La vie et la mort des coraux” (The Life and death of corals), and very funny too: [in 1967] “Before leaving Djibouti this morning, one of our comrades asked for some information about the Goubet from a local diver. ‘Ah’, replied the old sportsman, ‘it’s an amazing place. There is no bottom. It’s populated by monsters. They are capable of drawing into the abyss lines attached to 200-liter cans… In 1963, Commander Cousteau went there with Frédéric Dumas and his best divers: they were terrified by what they saw and preferred to leave.‘ We were very anxious to get acquainted with this terrifying place which gives us such an unflattering reputation.”

~~~

Special Disclaimer for this post: Since the evolution of a new ocean from a triple junction of diverging tectonic plates directly before our eyes is THE incentive for scientists to research and cover every bit of the process, a whole library of books and papers on the issue has been published in the last almost 40 years. Only a small percentage of them are freely accessible on the Internet. Not intending to become an expert on the matter, I have only read a few of those, partly for lacking time and partly for lacking the knowledge to understand them. The ones I did read were dotted with open questions, and with contradictions (i.e. new insights) to earlier works. For all these reasons I cannot be sure whether what I filtered out for this post is the latest state of the art or water under the bridge already. I’d be grateful for any corrections if something I wrote should be out of date.

General Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more facts, please leave a comment.

Enjoy! – GRANYIA

SOURCES & FURTHER READING

– GVP, Ardoukôba
– The Ethiopian Rift Valley (Volcanism and seismicity)
– Transient deformation in the Asal-Ghoubbet […] since the 1978 […] (2016, PDF, paywalled)
– Asal-Fialé geothermal field (Djibouti republic): A new interpretation […] (2014, PDF)
– Seismicity of the central Afar rift and implications for Tendaho dam hazards (2015)
– Ardoukoba volcano, Djibouti
– Afar Triangle, Wikipedia
– Crustal structure and magmato-tectonic processes in an active rift […] (2007, PDF)
– Afar: the field lab for continental break-up
– Fault propagation and climatic control of sedimentation on the Ghoubbet Rift Floor (2001)
– The Red Sea and Gulf of Aden Basins (2005, PDF)
– Earth of Fire (Blog)
– Origins of […] disequilibria in basalts: New insights from the 1978 Asal Rift eruption (2012, PDF)
– BBC: The Hottest Place on Earth. Episode 1 Part 1 (of ?? more than 10, very interesting video series)

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