The movement of deep magma beneath the Afar Depression.

The Afar Depression lies at at northernmost end of Africa's Great Rift Valley, where it meets the sea at the junction between the Red Sea and the Gulf of Aden. All three of these features are areas of rifting, with Africa slowly being split into two new continental plates (the Somali and Nubian) along the Great Rift Valley, and new ocean crust being formed on rifts beneath the Red Sea and Gulf of Aden. The Afar Depression is an area of noted volcanism, and it has long been assumed that it sits on top of magma plume originating from deep within the Earth's mantle.

The Afar Depression at the northern end of the Great Rift Valley. Scientific American.

In a paper published in the journal Geology on 18 April 2013, a team of scientists led by James Hammond of the Department of Earth Science and Engineering at Imperial College London and the School of Earth Sciences at the University of Bristol discuss the result of a study of the deep geology beneath the Afar Depression using seismic stations across Ethiopia, Eritrea, Djibouti, Yemen, and Kenya, which was used to build a model of the movements going on beneath the area.

Map showing earthquakes and seismic stations used in the tomographic inversion. Solid white lines show major faults bounding Afar Depression, gray filled regions show Quaternary volcanic segments, and black lines show political borders. MER—Main Ethiopian Rift, GOA Gulf of Aden, RSR—Red Sea Rift, TJ—triple junction, EAGLE— Ethiopia Afar Geoscientific Lithospheric Experiment, EKBSE— Ethiopia Kenya Broadband Seismic Experiment. Red symbols show data used in tomography for first time, white symbols show stations used in previous studies. Hammond et al. (2013).

Hammond et al. could find no evidence for the presence of an active volcanic plume beneath the Afar Depression. Rather the volcanoes of the area appeared to be fed by the passive movement of magma drawn from the south (i.e. beneath the Great African Rift) by expansion beneath the depression, which appeared to be driven by the three surrounding areas of rifting, rather than being a driving force of them. They did, however, also conclude that hotspot magmatism was likely to have occurred in the area in the past, as the current passive drawing of magma into the area could not account for the flood basalts of the Ethiopian Plateau. 

The proposed model where passive upwelling of asthenosphere in mantle beneath Afar, Ethiopia, gives rise to melt-filled mantle above 75 km (Rychert et al., 2012), with melt oriented at rift axis causing significant seismic anisotropy and large velocity anomalies. Superimposed on this are focused diapiric thermal upwellings. These focused anomalies cause enhanced melting at three locations: triple junction (TJ), Nabro volcano (NV), and western margin (WM). Hammond et al. (2013).

See also Magnitude 5.0 Earthquake beneath coastal Eritrea, Earthquakes beneath the Gulf of Aden, Earthquake beneath the Red Sea, The magma chamber beneath the Erta Ale Volcano and Eruption in the Zubair Archiapelago, in the southern Red Sea.

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Eruption in the Zubair Archiapelago, in the southern Red Sea.

On 19 December 2011 fishermen on the Red Sea reported seeing an eruption on Saba Island about 60 km to the southeast of Jebel Zubair Island, with lava fountains rising 20-30 m above the island. The same day the Ozone Monitoring Instrument (OMI) on the Aura satellite recorded a rise in Sulphur Dioxide (SO₂) levels above the southern Red Sea. The next day (20 December) the Moderate Resolution Spectroradiometer on the EOS Satellites detected a volcanic plume rising from a submarine eruption roughly 1.5 km southwest of Haycock Island, or 12 km northeast of Jebel Zubair. This is the first eruption in the Zubair Archipelago since at least 1846.

Satellite image of the plume near Jebel Zubair.

The Zubair Archipelago are a group of volcanic islands off the southwest coast of Yemen; they are essentially a shield volcano on the Red Sea Rift with a number of vents. A shield volcano is a broad, low profile volcano made up of successive layers of lava; it lacks the cone shape of a stratovolcano, but can grow far larger. The Hawaiian Islands are massive shield volcanoes, as are the vast volcanoes of Mars.

The Red Sea Rift is a spreading boundary between two tectonic plates, the African Plate and the Arabian, where new oceanic crust is being formed. Arabia was formerly part of the African Plate, but split away about 30 million years ago. The Great Rift Valley of Africa is a continuation of this rift, that is slowly splitting Africa in two from the north to the south.

Diagram showing a cross section through the rocks beneath the Red Sea.

The same rift system is also responsible for the nearby Jabal al-Tair volcanic island, which erupted unexpectedly in 2007, after 124 years of inactivity, killing 8 people, as well as the volcanoes of the Afar Triagle in Eritrea and Ethiopia and the volcanic fields of southern Yemen and Saudi Arabia.

See also Eruptions on Mount Nymuragira, Eruptions on Mount Ebro and Volcanoes on Sciency Thoughts YouTube.

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Eruptions on Mount Nabro, Eritrea.

Mount Nabro is a stratovolcano (a cone-shaped volcano of the kind seen in Hollywood movies, made up of successive layers of ash an lava) in the southeast Danakil Desert in Eritrea, close to the Ethiopian border. It forms part of the Bidu Volcanic Complex, along with the Dubbi and Sork Ale volcanoes in Eritrea and Mallahle and Bara Ale volcanoes in Ethiopia. Until this year there have been no recorded eruptions at Nabro, and it had been assumed to be extinct, though due to its remote location it had not been studied extensively.

Satellite image of Mount Nabro.

On 12 June 2011 the area around Mount Nabro was shaken be a series of earthquakes, which rose in intensity throughout the day. At approximately 9 pm local time on 13 June a large volcanic eruption took place in the region, creating a column of ash that rose 13.5 km into the air. At first this was thought to have been caused by an explosion on Mount Dubbi, the only volcano in the area to have erupted in recorded history (in 1861), causing concerns for the settlement of Afambo, which is close to Dubbi, but Nabro was later identified as the source of the eruption from satellite photographs. Within hours the ash column had started to cause disruption to flights in the region.

Satellite image of the eruption on Mount Nabro.

On 16 June the Eritrean Government announced an evacuation of the people of Afambo, Nebro and Sireru, the three settlements closest to the eruption.

By the end of June the ash cloud had cleared sufficiently for NASA to obtain pictures of the situation on the ground, revealing that a large amount of lava had issued from the caldera of the volcano, producing a flow of lava that extended westwards for over 12 km before producing a lava lake. It was also becoming apparent by this point that the evacuation by the Eritrean Government had not been entirely successful and that seven people in Eritrea had lost their lives as a result of the eruption, and a further three had been injured.

Satellite image of the lava flows from Mount Nabro.

A week later the neighboring Ethiopian state of Afar reveled that at least 31 people had died their, and that the local government was having extreme difficulty coping with deteriorating environmental conditions associated with the eruption, particularly water pollution and damage to crops and livestock. The Afar Government estimated that 48 000 people were in immediate danger from the ash-fall and that over 167 000 more were potentially at risk. Clouds of poisonous sulphur dioxide gas were also reported to be blowing across the border. It launched an appeal for aid, but this was overruled by the Federal Ethiopian Government in Addis Ababa.

On 9 September 2011 the area began to experience seismic activity again with small earthquakes persisting for the next three days. On 28 September NASA again reported sighting fresh lava flows on Mount Nabro.

The Bidu Volcanic Complex is located on the Afar Triple Junction, a complex area tectonically, where three spreading plate margins meet. To the northwest the Red Sea Rift runs between Africa and the Arabian Peninsula, a new ocean ridge where oceanic crust is being formed, with the potential to turn the Red Sea into an ocean over the next few million years. To the northeast the Aden Ridge forms a similar divergent margin between the Arabian Peninsula and the Indian Ocean, although in this case both plates are moving northward, the Arabian Peninsula is simply moving faster than the Indian Ocean for the time being. It is likely that as Arabia collides with Eurasia it will slow down and the Indian Ocean will overtake it, closing the Aden Rift. To the south the Great Rift Valley is slowly splitting Africa in two, creating a long chain of volcanoes heading south to Mozambique. Some geologists believe that a mantle plume beneath the Afar Triangle is fueling this, pushing the plates apart rather than their being drawn apart by seismic activity elsewhere.

The hypothetical Afar Plume.

See also Tourists evacuated due to volcanic activity on El Hierro, The Dangers of a modern Laki-style eruption in Iceland and Volcanoes on Sciency Thoughts YouTube.