Increased activity along the East African Rift, Ethiopia
jeudi janv. 28, 2016 20:36 | IS
The high level of Erta Ale´s lava lake 21 November 2015
Sattelite image from GoogleEarth, showing the upper part of the East African rift in Ethiopia and surrounding countries.
Screenshot from interactive map on earthquakes.volcanodiscovery.com
Our local expedition leader informed us about increased volcano-tectonic activity along the Ethiopian part of the East African rift: a new lava overflow at Erta Ale and a 4.2M earthquake near Corbetti caldera.
June 2011 Lava flows and ash plume at Nabro Volcano, Eritrea. After Robert Simmon (NASA Earth Observatory), via Wikimedia Commons
In the Danakil depression, northern Ethiopia, the continuously active lava lake at Erta Ale started to overflow on the 27th of January. This latest overflow created new pahoehoe lava flows that cover Erta Ale´s caldera floor both to the north and south of the lava lake´s eruptive vent. At the moment of writing this news item, the lava lake is thought to be still overflowing. It is however unclear for how long this phase of lava overflow will continue.
The currently increased activity at Erta Ale was first reported in November 2015 when travellers visiting the caldera observed that the level of the lava lake was less than 1 meter below the rim of the vent. The lake´s activity remained high throughout the next 9-10 weeks, with the lava lake falling and rising repeatedly, and eventually culminated in a first overflow of its vent around midnight of 15 to 16 January 2016. The previous phase of increased volcanic activity with lava overflows dates back to December 2010, making it a rather rare event. Not so much is known about the average activity at Erta Ale´s lava lake,
because this shield volcano is best visited from November to February (the coolest months in the hottest place on earth) and has only been observed and reported on a more regular base over the past 10 years.
Erta Ale is one of many volcanoes dotted along the East African rift, which can be compared to a large tear running from Erithrea through Ethiopia, along the great lakes of Uganda, Kenyia, DR Congo, Tanzania, Zambia and Malawi down into Mozambique, and which is actively pulling the African continent apart.
A 4.2 magnitude earthquake occured along this same rift system but in southern Ethiopia on 24 January at 18h34 UTC, with a hypocenter at about 10 km depth. Although not very strong, such earthquakes are rather unusual in this area near Lake Awassa, as shown by the ´´I felt it´´ reports from people living 20 km east of the epicenter and which vary from V (moderate shaking) to VI (strong shaking) on the Mercalli scale which ranges from I (not felt) to XII (extreme). Reports describe books falling from shelfs and cracks in many local houses, frightening the inhabitants which remained outside for the rest of that night. Since this first earthquake, more shaking is reported to have occurred afterwards on irregular intervals. The epicenter of Sunday´s M4.2 earthquake was only 22 km S-SE of Corbetti caldera volcano.
This 15 km wide caldera overlaps the NW margin of the older 30 by 40 km Awassa caldera where the present-day Awassa lake is situated. After formation of the Corbetti caldera, volcanic eruptions deposited lava flows in its centre (known as Urji volcano), and built a large obsidian dome on its SE rim (known as Chabbi volcano) and a pyroclastic cone on its west rim (unnamed). Although the ages of these eruptions are unknown, it has been shown that these volcanic deposits are too young to be dated with the K-Ar method (so less than 100 000 years old). Fumarolic activity is still on-going at these post-caldera cones. Could the increase in seismicity be a sign of an upcoming new volcanic eruption in Corbetti caldera?
A series of similarly sized earthquakes (magnitude 4.5 or greater) started on the night of 12 January 2011 near a sparsely populated area of the Erithrea-Ethiopia border in the northern part of the Danakil depression. The earthquakes were linked to the simultaneous eruption of nearby Nabro stratovolcano, which up to 12 January 2011 had no record of historic eruptions. This sudden volcanic activity at the northern end of the East African rift continued for about one month and formed a ca. 15 km long lava flow. The ash cloud of this eruption rose up to 14 km and drifted to W-NW, forming a minor disruption to air traffic. On the 19th of June, the Nabro eruption produced the up to then highest concentrations of sulphur dioxide emission in the earth´s atmosphere ever recorded from space. The ash and tephra fall from this eruption covered a large area, negatively affecting water, crops and cattle of the local nomadic Afar people. Many Afar villages near the volcano were evacuated and at least 38 people were reportedly killed, but the exact death toll is this eruption is unknown.
The recent earthquake activity along the East African rift in south Ethiopia should therefore be closely watched. Because although the chances of an eruption near Awassa are small, the 2011 eruption of up to then dormant Nabro volcano shows that they are not nihil.
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