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A look at Nabro’s history 19 June 2011

Posted by admin in Africa, Dubbi, Eritrea, Erta Ale, eruptions, Ethiopia, Nabro, volcanoes.
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Nabro is a little-known volcano. Its remote and inhospitable location has meant that very limited fieldwork has been carried out, and much of what we do know comes from remote sensing. Pierre Wiart and Clive Oppenheimer have analysed much of what is known about Nabro in a very useful paper published in the Bulletin of Volcanology in 2005: ‘Large magnitude silicic volcanism in north Afar: the Nabro Volcanic Range and Ma’alalta volcano’. The article is based upon remote sensing data and very limited fieldwork: thus eruptive sequences are established with reasonable clarity, but there is little dating information.

The authors note that much research on north Afar has focused on Quaternary basaltic volcanism, but that silicic volcanism in the region has been little considered, although ‘comparable volumes of silicic magma [compared with basaltic] have been erupted in the region’ (99). The Nabro Volcanic Range (NVR), which the authors identify as a single volcanic massif covering approximately 110 km and trending SSW-NNE from the Afar Depression to the Red Sea, is one significant but little-studied focus of silicic activity. The NVR encompasses the Edd Islands off the Red Sea coast, Dubbi volcano and the Edd lava field, Mabda volcano and the Bidu volcanic complex (Nabro and Mallahle calderas, and Bara Ale and Sork Ale volcanoes). The paired calderas of Nabro/Mallahle are described as follows:

Nabro has an 8 km diameter horseshoe-shaped caldera breached to the SW, and facing the 6 km diameter caldera of Mallahle. Nabro’s caldera contains a young volcanic centre, topped by two additional collapse craters. The flanks of the volcano are strongly dissected by gullies, whereas the inner walls of the caldera form 400-m high cliffs. The older of the two craters inside Nabro’s main caldera is also horseshoe-shaped and similarly open towards the SW. The walls of the innermost crater are 200 m high. (102)

Nabro and Mallahle are described as very similar in geomorphology and lithology, both being predominantly composed of trachytic lava flows. ‘On Nabro, geochemical and spectral evidence highlight at least two main periods of eruption of these flows’ (103). Both basaltic and silicic products are evident: ‘Whereas the main periods of edifice growth, collapse, and post-collapse volcanism have been characterized by silicic products, later volcanics of the Bidu Volcanic Complex include NNW-SSE basaltic lava flows, which streamed from vents or fissures located between the two calderas’. The authors suggest that these flows probably tapped separate source regions to those that fed the silicic volcanism, and note that ‘their superimposition on the boundary between the two calderas and orientation (i.e., perpendicular to the NVR axis), are enigmatic, but clearly not coincidental’ (103).

A very large eruption or eruptions in the past is evidenced by the presence of extensive ignimbrites, which the authors believe are ‘associated with collapses of both Nabro and Mallahle that formed the present day calderas’ (103). The age of these ignimbrites is unknown, but their spectral characteristics and state of preservation suggest their creation in a single eruptive sequence. The present-day ignimbrites cover an area of ~600 square kilometres, with a bulk volume tentatively estimated at 20 cubic kilometres. Prior to erosion of course a much greater bulk must have been present: the combined ignimbrite outcrop today lies within a 30 km radius of Mallahle, and the authors propose that if there was originally a single ignimbrite sheet extending this distance with a mean thickness of 40 m, the eruption magnitude may have exceeded 100 cubic kilometres (bulk volume):

These speculative upper and lower bounds on the ignimbrite volume [20-100 cubic kilometres] suggest an eruption (or eruptions) comparable in magnitude to the largest known historic eruption, that of Tambora in Indonesia in 1815, which expelled around 50 km3 of (dense rock equivalent) magma. This inference is compatible with the comparable caldera dimensions of Tambora (6 km), Nabro (8 km) and Mallahle (6 km). (107)

The total volume of the NVR is estimated by Wiart and Oppenheimer to be ~550 cubic kilometres, broadly comparable to that of Erta Ale. Overall they conclude that ‘the Erta’Ale range and NVR are, therefore, of broadly comparable area, volume, and age (Quaternary)’ but differ markedly in their composition and eruptive nature, with the Erta Ale range being composed of 91.4% basalt, 8.1% dark trachyte and 0.5% rhyolite, while the NVR is estimated as being 50% composed of trachyte and rhyolite (109). The authors suggest that the nature of the basement is responsible for this difference in composition, with the presence of continental crust on the margin of the Danakil region promoting the evolution of volatile-rich and ultimately explosive magmas.

The overall picture of Nabro is of a complicated and fascinating volcanic complex with a violent but varied history involving both explosive and effusive activity, some of the latter being possibly relatively recent in date, and set apart from the more northerly Afar volcanoes by its greater involvement of silicic and explosive magmas. The current eruption involved some initial explosive activity, but appears to have subsequently settled down to an intermittent and relatively low-level effusive event. It is to be hoped that clear images from the satellites will soon show the extent of the lava flows, and their sources. Given the evidence of Nabro/Mallahle’s history, it could be that their sources lie between the two calderas, replicating the behaviour which seems to have characterized the most recent activity at this obscure and intriguing volcano.

N.B. Several people have been in touch or left comments here to say that the Wiart & Oppenheimer paper is freely available via ‘a well-known file hosting site’. However, the paper in question is copyright Springer-Verlag 2004, and this blog respects copyright. Accordingly I won’t be providing the link here, and I respectfully ask that no-one else does so via the comments.

Reference
Pierre Wiart and Clive Oppenheimer, ‘Large magnitude silicic volcanism in north Afar: the Nabro Volcanic Range and Ma’alalta volcano’, Bulletin of Volcanology, vol. 67, no. 2 (2005), pp. 99-115 [DOI: 10.1007/s00445-004-0362-x]

Information
Global Volcanism Program: Nabro – summary information for Nabro (0201-101)

The Volcanism Blog

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Comments

1. Bruno Ribeiro - 19 June 2011

Interesting the number of volcano-tectonic aspects this scenario can
provide for the understanding of ancient volcanic provinces, in particular
related to Gondwana breakup (e.g. Parana-Etendeka).
Based only on “spectral geology” as well as Wiart e Oppenheimer did the Bidu Volcanic Complex seems to be on a interference zone between two extensional systems (i.e. Ethiopia Rifte and Red Sea Rift). Curious the SO2 signature from this new fissural (?) vents.

2. El Abuelo - 20 June 2011

http://foro.meteored.com/dlattach.html;topic=128852.0;attach=178463;image

¿Black smoke on Nabro?
Temp indicate 3-4Km high black smoke and >10Km plume

3. El Abuelo - 20 June 2011

Can see on modis ( temp data is on raw images)

4. bjdeming - 20 June 2011

I wonder why there is so much sulfur dioxide from this one — according to the following post, the highest ever recorded from space: http://earthquake-report.com/2011/06/20/unusual-series-of-moderate-volcanic-earthquakes-in-eritrea-and-ethiopia/

It has contaminated the Afdera salt bed, according to this report:
http://addisfortune.com/Eritrean%20Volcano%20Erupts%20Economic%20Mayhem%20for%20Ethiopia.htm

Note also the article’s report of additional small eruptions in the Afambo and Sireru areas. I found nothing else in the news about that.

PS: Three cheers for the stand on copyright! Most online sources will sell access to single articles rather than require a subscription, and they really aren’t that expensive, considering the limited audience. Someone has to support those who directly facilitate the exchange of ideas. That’s freedom, too.

5. admin - 20 June 2011

El Abuelo: thanks for the alert: it looks like there was some kind of very dark emission earlier today. Only small emissions since, but the volcano is definitely still erupting. I’ll post an update tomorrow – no time tonight.

Hi bjdeming! The SO2 emissions are extraordinary (an ‘SO2 bomb’, another commenter called it. Local geochemistry must be the cause: other volcanoes in the region such as Manda Hararo and Dalaffilla also churned out a lot of SO2, although not quite on this scale. Well, I’m looking forward to Erik’s return, to see what he says about it.

(And I appreciate and agree with your comments on the copyright issue.)


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