Under titles like “Fledgling mantle plume may be cause of African volcano's unique lava”
a new paper on the Nyiragongo volcano has had a lot of attention in the media. I am not going to say too much on the paper itself. Ralph Harrington at the Volcanism Blog wrote a good post
on that. What I would like to do here is to place it in a larger regional context - and also to question how ‘fledning’
the plume is, as its geologic history may go some 45 (or even 50?) million years back, and here I am only talking about events visible on the Earth’s surface. I shall try to be brief and use many illustrations (many of which are reused older images I have used in my blog over the years).
Hot mantle rock that rises toward the earth's surface in a narrow column (possible from as far below as the core/mantle boundary) is referred to as a mantle plume. Plumes are thought to spread out laterally at the base of a continent, creating increased pressure that stretches the crust and results in uplift, fracturing, rifting, or flood basalts. Mantle plumes are thought to be strong enough to induce rifting and the formation of (new) plates. The pressure creates a domed region that eventually splits in a three-pronged pattern (triple junction or triple point). The three-pronged splitting is due to the fact that our earth is a sphere. If rifting continues, two of the three faults become active, forming the continental margins of two new continents. The two faults join to form an active divergent boundary that dissipates the tectonic forces. The third “arm” becomes a failed rift. This is what happened in the Afar region about 45 million years ago. One arm continued rifting to form the Red Sea, which in time may evolve into a real ocean (oceanic crust has been formed for at least 5 million years). Another arm is the East African Rift, where the Nyiragongo Volcano is located. A third arm up through the Gulf of Aden later became a failed rift.
The dome-triple junction-failed rift concept introduced by Burke and Dewey in 1973 is a fundamental idea behind the plate tectonic theory.
Further south the East African Rift today splits in two main branches called the Eastern Rift Valley and the Western Rift Valley. Nyiragonga is at the rim of the Western Rift Valley in the Virunga Mountains, a chain of volcanoes (also including Nyamuragira) in East Africa, along the northern border of Rwanda, the Democratic Republic of the Congo and Uganda.
The eastern branch of the East African Rift system extends over 2000 km from the Red Sea southward to Mozambique. It crosses two regions of topographic uplift, the Ethiopian dome to the north and the Kenyan dome to the south, both regarded as the surface manifestation of mantle plumes. Volcanism in the East African Rift system began in southern Ethiopia (Afar) near 45 million years ago, followed by flood basalt activity in northern Ethiopia and Yemen at around 30 million years ago. There is a general consensus that the modern Afar plume underlies the Ethiopian plateau and the triple junction region that encompasses the southern Red Sea and Gulf of Aden. It is also widely accepted that the onset of volcanism in northern Ethiopia corresponds to the impact of a mantle plume head beneath the modern Afar region, as reflected in flood basalts in Ethiopia.
In 2000 Rogers et al. suggested that magmatism in the Ethiopian (Afar) and Kenyan sectors of the East African Rift system involved more than one mantle plume. It has also been suggested that the modern Turkana Rift may be part of one of the largest plumes on Earth. The Turkana rift, which separates the Ethiopean and Kenyan domes, is about 150 km wide, roughly three times the normal width of most other portions of the East African Rift system.
Findings in a paper by Furman et al. from 2004 suggest that a a single mantle plume - or multiple plumes with a common composition - could contribute to Quaternary volcanic activity from the central Red Sea to northernmost Kenya. The Quaternary Period is the latest 1.805 million years. Previous workers had suggested the presence of two geochemically distinct plumes beneath the Ethiopian and Kenya domes. Fortunately the Furman paper is freely available from here
(One of the reasons that I am quoting it).
The existence has been suggested of a microplate (the Victoria plate) between the Kenyan and western rifts and at the core of this microplate is the 2.5-3.0 billion year old Tanzanian craton.
And now to the Virunga Mountains, in particular the two active volcanoes Nyiragongo and Nyamuragira that lye only 15 km apart. Nyiragongo and Nyamuragira are together responsible for 40% of Africa's historical volcanic eruptions, and Nyiragongo is one of the ten most dangerous volcanoes on Earth. These volcanoes are at the fringes of a topographic uplift within the East African Rift system. What the authors of Isotopic and geochemical evidence for a heterogeneous mantle plume origin of the Virunga volcanics, Western rift, East African Rift system
argue is that simultaneous volcanism in adjacent Nyiragongo and Nyamuragira, with magmas originating from different depths requires the presence of a heterogeneous mantle plume beneath the Tanzanian craton. This plume caused chemically distinctive volcanic provinces around the Tanzanian craton, in the Western and Kenya Rift.
The lavas of Nyiragongo are unique. It is the most fluid lava anyone has seen in the world. According to the new paper the lava derives directly from a mantle plume magma whereas the lava from Nyauragira is distinct from the Nyiragongo lavas and originating from lover depths, derived from a mixture of mantle plume magma and material from the crust and upper mantle. Flood basalt, which is thought to originate from mantle plumes, is also extremely fluid (hence the name). Nyiragongo's lava flows may race downhill at up to 100 km per hour.
The Nyiragongo volcano was the focus of global attention in January 2002 when its spectacular eruption caused a humanitarian crisis as the lava rapidly flowed through the city of Goma before draining into Lake Kivu to the south. The satellite image shows some lava flows from this eruption. much of the city was destroyed, including 4,500 buildings, 400,000 people were evacuated and 120,000 became homeless. Unfortunately this is also an area where a regional conflict is still going on. The humanitarian situation
(in the Nord-Kivu province) is still more or less hopeless. This, by the way, also means that study on the ground of the Virunga Mountains is practically impossible for the time being.
Ebinger & Sleep (1998) have suggested that magmatism throughout most of Africa could be related to a single deep mantle plume that initiated basalt volcanism in southern Ethiopia about 45 million years ago. George & Rogers (2002) have suggested that the southward migration of basalt activity from southern Ethiopia towards Tanzania reflects the northeastward movement of Africa over the Kenya plume for about 50 million years, whereas magmatism in northern Ethiopia reflects 30 million years of sustained activity of the Afar plume. Or do we have one, but discontinuous plume?
I would like to know where the “Nyiragongo” plume fits in?
We could also discuss tectonic plate names. Is the African plate one single plate, or should we distinguish between a western Nubian plate and an eastern Somali(an) plate, and is there a small Victoria plate in between (between the Western and Eastern Rift valleys)?Main references:Isotopic and geochemical evidence for a heterogeneous mantle plume origin of the Virunga volcanics, Western rift, East African Rift system
by Chakrabartia et al. in
Chemical Geology, Volume 259, Issues 3-4, 25 February 2009, Pages 273-289
doi:10.1016/j.chemgeo.2008.11.010 East African Rift System (EARS) Plume Structure: Insights from Quaternary Mafic Lavas of Turkana, Kenya
by Chakrabart et al. in
Journal of Petrology, volume 45, number 5 ,pp. 1069–1088 2004
Press release from University of Rochester: