olelog

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
doi: 10.1093/petrology/egh004

• http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V5Y-4V0TD5T-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=c52e686ed6f5c759fad89c3a8bacd347
• http://www.rochester.edu/news/show.php?id=3336
• http://petrology.oxfordjournals.org/cgi/content/full/45/5/1069

Press release from University of Rochester:
• http://www.rochester.edu/news/show.php?id=3336

Media coverage:
• http://www.geologytimes.com/research/Fledgling_mantle_plume_may_be_cause_of_African_volcanos_unique_lava.asp
• http://earthobservatory.nasa.gov/Newsroom/view.php?id=37551&src=eorss-manews
• http://www.terradaily.com/reports/Fledgling_Mantle_Plume_May_Be_Cause_Of_African_Volcano_Unique_Lava_999.html
• http://www.eurekalert.org/pub_releases/2009-03/uor-fmp031309.php

Other:
• http://www.scientificblogging.com/news_releases/why_nyiragongo_volcano_lava_different_others

In german:
• http://amphibol.blogspot.com/2009/03/junger-plume-unter-afrika.html
• http://www.wissenschaft-online.de/artikel/985340

From here it seems only a small hop from Madagascar to the Comoros Islands. Well I suppose that most of you know where Madagascar is. I wrote about it yesterday. The Comoros are probably well less known. They also lie in the Indian Ocean - somewhere between North Madagascar and the African coast (See map below).



The Comoros consist of volcanic effusive rocks, mainly basalt, and to a lesser extent trachyte and phonolite. Two thirds of the largest (and newest) of the islands, the Grande Comore Island, is formed by the Karthala volcano, a basaltic shield volcano with an active hydrothermal system. Mount Karthala is the highest point of the Comoros at 2,361 m above sea level. La Grande Comore is by the way also known as Ngazidja Island. Here are some of the eruption dates of Karthala over the last 1000 years or so: 2007, 2006, 2005, 1991, 1977, 1972, 1965, 1956, 1952, 1948, 1928, 1918, 1910, 1904, 1883-84, 1880, 1876, 1872, 1865, 1862?, 1860, 1859, 1858, 1857, 1855, 1850?, 1848, 1833, 1830, 1828, 1821, 1814, 1808, 1050. I think it is prudent to say that the volcano is extremely active, having erupted more than 20 times since the 19th century. Karthala contains a 3 x 4 km summit caldera generated by repeated collapse.

The volcanic chain of the Comoros apparently represents a “hot spot trace ”. The Comores islands and the tertiary volcanic province of Northern Madagascar form a sub-linear trend of alkalic shield volvanoes across the northern Mozambique Channel. Potassium-argon dating of shield building lavas confirms an eastward increase in age of volcanism along the chain, consistent with a hotspot origin for the lineament. The rate of migration of the Somali Plate over the mantle source is approximately 45 mm/yr.

Experts at a three-day conference on the slopes of the Karthala now say that the Comoros could tame the feared Karthala, which is indeed one of the world's largest active volcanoes, and use its geothermal and tourism potential to boost the archipelago's economy as a sort of goldmine for the Indian Ocean nation. One aim of the conference is to look at the possibility of exploiting this source of clean, renewable energy and transform it into a driving force for development. The volcano's ecotourism potential should also be examined during the conference.

The Comoros came to global attention in March 2008 when rebel leader Mohamed Bacar fled to France's Indian Ocean island of Reunion as African Union troops waded in to restore federal democratic institutions.


* http://www.terradaily.com/reports/Comoros_sees_future_in_volcano_economy_999.html
• http://www.volcano.si.edu/world/volcano.cfm?vnum=0303-01=
• http://adsabs.harvard.edu/abs/1985PhDT.........5E

This month’s Accretionary Wedge Carnival is now up at Harmonic Tremors. The theme is significant geological events. The timing is very appropriate as 22 May is the anniversary of the 1960 magnitude 9.5 Valdivia, Chile earthquake, the largest ever recorded. A month ago I happened to have a post about that event. A month too early I realise now. Sorry about that.

My post for the ninth Accretionary Wedge Carnival is about the Mount Toba eruption.

All the carnival posts are of course as usual superb. My own favourite is Cenozoic magmatism and the subduction of the Farallon slab at Magma Cum Laude. I am green with envy.

If you have been near a smoking volcano or solfatara you may have experienced the foul odour of rotten eggs, which comes from the hydrogen sulphide that occurs in volcanic gases.

Huaynaputina (Quechua: "New Volcano") is a stratovolcano located in a volcanic upland in southern Peru. The volcano has also been variously known as Omate, Quinistaquillas, Chiquimote, and Chequepuquina. The volcano does not have an identifiable mountain profile, but instead has the form of a large complex 2.5 km diameter explosion crater with a maximum elevation of 4,800 m above sea level and edifice height of no more than 500 m. On 19 February 1600 it exploded catastrophically, in the largest volcanic explosion in South America in historic times. The eruption caused substantial damage to the major cities of Arequipa and Moquengua. It blanketed nearby villages with glowing rock and ash, and killed some 1,500 people.

The eruption is known to have put a large amount of sulphur into the atmosphere, and tree ring studies show that 1601 was a cold year. Sulphur reacts with water in the air to form droplets of sulphuric acid, which cool the planet by reducing the amount of sunlight reaching the Earth's surface. But the droplets soon fall back to Earth, so the cooling effects last only a year or so. In the Northern Hemisphere 1601 was the coldest year in six centuries. In Greenland the sulphuric acid spike was larger than that from Krakatau (1883). Regional agricultural economies took 150 years to fully recover. According to a new study of contemporary records the eruption had a global impact on human society. In Russia 1601-1603 brought the worst famine in the country's history, leading to the overthrow of the reigning tsar. Records from Switzerland, Latvia and Estonia record exceptionally cold winters in 1600-1602. In France, the 1601 wine harvest was late, and wine production collapsed in Germany and colonial Peru. In China, peach trees bloomed late, and Lake Suwa in Japan had one of its earliest freezing dates in 500 years.

"The volcano that changed the world" - Ken Verosub and his coauthor, student Jake Lippman, explore the effects the 1600 Huaynaputina eruption had on the global agricultural economy. Their work appears in the April 11th issue of American Geophysical Union newsletter EOS.

* http://www.eurekalert.org/pub_releases/2008-04/uoc--1ec042308.php
* http://www.physorg.com/news128177951.html
* http://www.scientificblogging.com/news_releases/the_climate_disruption_from_the_huaynaputina_eruption
* http://en.wikipedia.org/wiki/Huaynaputina
* http://www.nature.com/news/2008/080411/full/news.2008.747.html
* http://www.nature.com/nature/journal/v393/n6684/abs/393455a0.html

Much has been written about the impact from volcanic eruptions on climate change (cooling) and sea level changes. Less has been said about the impact from global warming and sea level rise on volcanism.

Sea level rise can reactivate volcanoes situated near sea level. Alaska's Pavlof volcano erupts every winter when (local) sea levels are higher - well just about 30 centimetres. Thirteen of sixteen magmatic eruptions of Pavlof Volcano in nine of the years from 1973 to 1998 have occurred between 9 September and 29 December. A significant correlation exists between the eruptions and yearly nontidal variations in sea level and may result from ocean loading. (See this abstract). The melting of polar ice sheets from global warming and the resulting stress placed on the earth's crust from rising sea levels will cause more magma and increase volcanic eruptions on a global scale in the years to come.

Carolina Pagli of the University of Leeds, UK, and Freysteinn Sigmundsson of the University of Iceland in Reykjavik calculated how shrinkage of the Icelandic ice cap Vatnajökull affects what is happening below ground. Their findings will shortly be published in Geophysical Research Letters.

Pagli, C., and F. Sigmundsson (2008), Will present day glacier retreat increase volcanic activity? Stress induced by recent glacier retreat and its effect on magmatism at the Vatnajokull ice cap, Iceland, Geophys. Res. Lett., doi:10.1029/2008GL033510, in press.

When ice disappears, the added weight it forced upon the crust below it disappears as well. As a result this is increasing the rate at which the rocks under the ice sheet melt into magma. Iceland is home to several active volcanoes that exist underneath the ice, including Gjàlp, home of the last big eruption in 1996, and 58 years earlier in 1938. But according to Pagli and Sigmundsson the extra magma produced over the past century and more could reduce that time down to a gap of 30 years between each eruption. Volcanoes in Antarctica and Alaska will be at risk of similar increased volcanic activity. The shifting stresses could even cause eruptions in unexpected places.

And now that I am talking about Icelandic volcanoes - according to Iceland Review Online a giant volcano has recently been discovered off Reykjanes peninsula, Southwest Iceland, almost as large as the peninsula itself, and expected to erupt at any time. In the centre of the volcano there is a caldera measuring ten kilometres in diameter.

Since the volcano is at a depth of 1,500 metres eruptions would not have any effect on Iceland, except perhaps causing earthquakes (and tsunamis?). The volcano’s discovery is considered significant because it was believed it couldn’t exist in that area. Such large volcanoes are not supposed to be located on oceanic ridges. They are always drifting apart and that prevents a volcano from being created, so the volcano’s existence really came as a surprise.

• http://www.dailygalaxy.com/my_weblog/2008/04/will-a-warmer-w.html#more
• http://environment.newscientist.com/channel/earth/mg19826515.100-melting-ice-caps-may-trigger-more-volcanic-eruptions.html
• http://www.abc.net.au/news/stories/2007/09/14/2033161.htm?section=justin
• http://environment.newscientist.com/channel/earth/climate-change/mg19626324.600-volcanoes-give-sea-level-a-temporary-boost.html
• http://www.abc.net.au/science/news/stories/2005/1494475.htm