As part of our exploration of volcanoes in the Kenya Rift valley, perhaps a return to Menengai is in order. Granyia wrote an excellent post Dec 2017. This post revisits that location from another point of view.
Menengai is a massive shield volcano on the floor of the Rift Valley in Kenya. It is topped with an 8 x 12 km caldera that formed during an eruption some 8,000 years ago. This is thought to be one of the largest and best-preserved Krakatau-style calderas in the world. Post-eruption activity emplaced over 70 lava flows on the caldera floor. The youngest of these may be only a few hundred years old. The volcano has an active hydrothermal system being exploited for geothermal energy.
The volcano is located within the Kenya Lake System in the Great Rift Valley, a UNESCO World Heritage property. Lake Nakuru National Park is also nearby as is neighboring Nakuru Town, with its population over 570,000. Elevation of the rift valley floor is around 1,850 m. Caldera walls top out around 2,300 m.
Climate is generally temperate, with cool nights and a cold season May – Aug. Summer is described as Mediterranean and is Feb – Mar. Cloud cover is common throughout the year. Average temperatures through the year generally range between 28° – 8° C. The wettest months are Apr and Nov. Average annual precipitation (mostly rain) is 97 cm. Being nearly on the equator, there is little variation in length of day over the course of the year.
The UNESCO Kenya Lake System includes lakes on the floor of the Kenya Rift valley. These are Nakuru (S of Nakuru Town), Bogoria and Elmentita. The lakes are all shallow and alkaline, ‘soda’ lakes that support massive populations of lesser flamingos, over 4 million birds that move between the lakes and other destinations. The lakes also support a diverse array of large mammals including populations of endangered rhino, giraffe, lion, cheetah and leopard. The lakes have no outlet and are under increasing pressure from local development.
Lake Nakuru is S of Nakuru Town and surrounded by Lake Nakuru National Park established in 1961 to protect the wetlands and populations of birds and mammals. Over the years, the park has been expanded to protect endangered rhinos. Conditions at the lake have changed for flamingos since 2012 and most have moved on to other lakes in the rift valley.
Nakuru is the fourth largest city in Kenya and the capital and economic center of the Rift valley Province. It is an important agricultural and transportation center for the region. It is located some 153 km NW of Nairobi. It was a center of European activity during the colonial period and now has a population of some 570,000.
Volcano
Mengai is located on the floor of the Kenya Rift Valley. It is one of seven recent caldera volcanoes on the inner trough of the rift valley, all of which are associated with a high thermal gradient due to shallow magma intrusions. Neighboring volcanoes in the rift from N – S include Barrier, Emururangogolak, Silai, Paka, Korosi, Longonot and Suswa.
Menengai is located 24 km S of the Equator and 10 km N of Nakuru town. The caldera is a prominent feature on the rift floor, noted as early as 1894 by Western visitors, though not studied in detail until 1957. It is partly filled with young, rugged lava flows. Topography in the rift is rather flat, having been filled with volcanic rocks, ignimbrites and other pyroclastic deposits. These covered local faulting and differential uplift within the rift. Elevation in the vicinity is 1,560 – 2,260 m. The eastern shoulder of the inner trough is 600 m above the floor. It forms the Bahati platform. The western shoulder forms the Mau platform.
There is a neighboring caldera to the N that emplaced the Ol’ rongai ignimbrite and the Athinai trachyte. These are associated with the Ol’ rongai caldera and probably older than 0.3 Ma. An earlier major eruption took place 1.0 Ma to the S and emplaced Mburak and West Lake trachyte formations. Both of these major activities took place before the initial eruptions at Menengai.
Activity at Menengai started some 200,000 years ago growing a trachyte lava shield with an eventual volume of 30 km3. Exposed lavas on the caldera wall are up to 250 m thick. Chemistry of the lavas indicate periodic addition of new magma batches into the growing system. The chamber was zoned at this point of its evolution.
Growth of the shield was stopped around 29 ka, with a caldera collapse, eruption of an ash flow tuff, all preceded with pumice falls. The 20 km3 ash flow tuff was erupted as a single unit from a compositionally zoned magma chamber and emplaced as a pyroclastic flow. Its source was the caldera and its rim.
The second caldera collapse took place some 8 ka, with formation of the final 12 x 8 km caldera. A Krakatau-style collapse created a 77 km2 caldera within a ring fracture. This erupted two ash flow tuffs with a combined volume of some 50 km3. Eruption of both tuffs took place after an initial ashfall phase. Both were emplaced as single flow units. Total coverage by these is 1,350 km2, including outcrops higher than the eruptive vents.
Post-caldera eruptions put some 25 km3 of magma mostly as lava flows onto the caldera floor. While these 70 eruptions may have been preceded by explosive episodes, pumice and ash were not the dominant material erupted. Most of these took place on the central and western parts of the caldera floor. The pile reached 2,160 m. The lowest current point on the lava-covered caldera floor is about 1,700 m. There are also a few cinder cones on the floor of the caldera. There are lake sediments inside the caldera, indicating a lake 10 – 8.3 ka. The youngest post-caldera lava flow may be as recent as a few hundred years old, though the only dated flow is 1,400 years old. Most of these came via fissure eruptions.
The final stage of activity is hydrothermal with active fumaroles, hot springs, steaming/gas boreholes, hot/warm water in boreholes and altered rock/grounds. The fumaroles are mainly located on the caldera floor with a few weaker ones outside the caldera. Their location appears to be structurally controlled by local faulting.
There is a low velocity zone 1 – 4 km below the volcano. This may be related to a high temperature zone or hydrothermally altered rocks. There is a second zone 4 – 6 km below the volcano that may be related to a magma chamber, the source of heat for the geothermal system. There are shallow intrusions from it, as close as a couple kilometers from the surface demonstrated by very high temperatures discovered during drilling. Granyia’s 2017 post wrote of a geothermal exploration well finding magma at 3 km depth.
At shallow depths, 0.5 – 1.5 km below the caldera surface, there is a low to moderate density zone interpreted as a highly fractured zone with clay materials from hydrothermal action.
Eruptions
VOGRIPA lists Menengai with four Plinian-sized eruptions. The first and last produced extensive tuffs. The middle two created the two calderas. Oldest eruption took place some 180 ka, producing some 3 km3 of pre-caldera tuffs via a VEI 5.5 eruption. The first caldera forming eruption created the 26 km3 Menengai Tuff, via a VEI 6.4 eruption 36 ka. The Menengai Tuff was deposited over 115,000 km2. It is an important marker for analysis of human evolution in the Rift valley. The Caldera 2 eruption produced another 26 km3 of material via another VEI 6.4 eruption 21 ka. The most recent eruption listed in their database is listed at 12 ka, producing the 25 km3 Ruplax Tuff.
Dates on all the major eruptions have significant error bars, over 10 ka for the initial major eruption and for Caldera 2. Even the most recent eruption has an error bar of 3.3 ka, enough to bring it close to the 8 ka date for the most recent major eruption. Note that VOGRIPA describes a pair of relatively recent VEI 6.4 eruptions rather than a single caldera-forming eruption some 8 ka. I have not yet reconciled this discrepancy.
There are two ashflow outcrops in the caldera walls. The Lower Menengai Tuff is an outcrop visible some 20 km NW of the caldera. Here, about 20 m of reworked tuffs are capped by a 7 m ash flow. It dates 0.7 Ma, older than the 29 ka Menengai Tuff. The present Menengai Crater is too young to be the source of this flow. A possible source may lie to the NW of the present caldera.
The second caldera forming ash flow tuff is up to 80 m thick in places. Activity started with a massive plume and airfall tuffs, possibly from a central vent. This was followed by welded ash-flow tuffs and then unwelded flow materials. Part of the flow materials were emplaced via ring fracture. The tuffs were erupted from a zoned magma chamber with strong enrichment toward the roof. As the eruption proceeded, a range of compositions was tapped. The chamber was up to 75% fractionally crystallized of the least evolved trachytes with minor amounts of wall rocks in the deeper parts of the chamber.
Post-caldera eruptions over the last 8,000 years cover the floor of the caldera with lava flows.
Menengai Geothermal Field
Geothermal development at Menengai has been the biggest change since Granyia’s original post, with the first tranche of production coming online in 2021. The second change has been the Kenyan government’s decision to go with government-connected partners rather than private ones in future phases of the project.
Menengai is third geothermal field in Kenya following Olkaria and Eburru. Drilling began in 2011 with 24 wells completed by 2014. There is a related Baringo – Silali geothermal project to the N of the caldera which will attempt to tap the Baringo – Silali block geothermal potential of 3,000 MW. This project will proceed in four phases, first bringing 300 MW, followed by three phases of 100 MW apiece online. Drilling rigs are already being moved into place.
Drilling geothermal wells in the Menengai area found exploitable steam, but the complexity of the underlying geology made early well targets unproductive. Analysis clearly needed to improve and subsequently used surface faults, geological structures, and lineament structures. They found that there was a close relationship between faults and subsurface volcanic features like dikes near the surface. The underlying faults appear to be filled with feeder dikes, conduits for magma to shallow levels, which supply heat to the geothermal reservoir.
The 850 km2 Menengai geothermal area includes the Menengai volcano, the Ol‘ rongai volcano, Ol’ banita plains and parts of the Solai graben to the NE. Underlying faults and grabens are generally oriented NNE – SSW, with steaming ground and fumaroles common.
The Menengai caldera is an elliptical depression. The circular rim of the caldera ring fault is well preserved, with a cliff to some 400 m in places. The Solai graben cuts the rim on the NE. There is one fault on the SSW that extends further southward. The Molo Tectono-volcanic axis (TVA) is a zone of faults and fractures that had volcanoes erupt through them. This activity built a NNW – SSE – trending ridge called the Ol’ rongai volcanoes. These include explosive craters. This structure is adjacent to the Menengai caldera and possibly extends into the caldera itself. The numerous eruptive centers in the western part of the Menengai caldera are probably due to intersection of the caldera structures and the Molo TVA / Ol‘ rongai fracture system.
The Solai graben is a narrow graben the runs NNE – SSW through Lake Solai. These also appear to extend into the Menengai caldera system, and beyond, via the Makalia fault system. There are open fissures and fumaroles associated with these fault systems.
The deep flow of groundwater from highland and lowland areas surrounding the caldera passes through permeable structures. At 2 km below the surface, there is a soft, hot layer with superheated fluids 350 – 400° C. This is likely a magma body or intrusive (dike or sill) structure. Water source is either rain on the caldera floor or groundwater flows from the rift escarpments E and W of the caldera with the flow traveling along fractures.
There are two main zones for water. The shallow system is located 900 – 1,300 m below the surface. It is a liquid aquifer. The deeper feeder zone is located below 1,400 m. Hydrothermally altered minerals and clays indicate high temperatures at the bottom of wells – 350 – 400° C. Comparing recent results with older ones indicate the geothermal system around the caldera is generally heating up, perhaps via recent magma intrusions.
There was a minor amount of basalt lavas found in two of the test wells that may be from basaltic intrusions into the trachytic magma chamber below the caldera. Basaltic magmas were erupted in recent times within the caldera. There may be a fracture zone between these test wells and the remainder of the caldera, which may be an issue for reinjection of fluids. One paper recommended geothermal production wells be drilled near the caldera wall due to high vertical permeability associated with the wall.
Menengai Geothermal Development started in 2014. It originally targeted 50 wells to produce over 50 MW of electricity. By Nov 2019, 49 wells had been drilled with a capacity of producing 170 MW. It was originally developed in four phases of 100 MW apiece. Phase I was originally planned for 3 power stations in the caldera approximately 35 MW apiece. But the project has an embarrassment of thermal riches, as the production out of the wells is much higher than expected, so electric production in later phases may increase significantly. Kenya decided to change the later phases from private project partners to state-owned entities. The change took place in late 2020 and was driven in part by the inability of the private project partners to get approvals soon enough for the Kenyan government.
Estimated total production for the entire project is 1,600 MW. Phase I should bring 105 MW online in 2021. Phase II in the eastern sector of the caldera is estimated at 60 WM. Phases III – V are as yet not determined, though will be located in the N and W portions of the caldera and perhaps outside of the caldera proper.
Tectonics
The Kenya Rift valley runs the length of the nation N – S. It is a classic graben averaging 40 – 80 km wide. Deepest in the Naivasha area and shallowest at Lake Turukana where it disappears into the desert. It is part of the East African Rift system, an intercontinental divergence zone between the Somali and Nubian plates. These are rifting at 2 cm/yr. Rift tectonism has triggered intense volcanism from late Tertiary to Recent. Extension has thinned the overlying continental crust allowing upwelling mantle melts to produce magmas which make their way to the surface.
Menengai is located near a triple junction of the rift, where the failed rift arm of the Nyanza Rift joins the main Kenya rift. E-W tension has created block faulting, tilting blocks on the floor and scarps of the rift. Peralkaline calderas like Menengai are associated with rifting zones.
The seven recent caldera volcanoes in the Kenya Rift valley are all located in the inner trough of the rift. Their locations do not appear to be related to transverse lineaments or basement structures. Development and growth was complex. The volcanoes are broadly grouped, with the southern group of Suswa, Longonot and Menengai being trachytic, Krakatau-style formation of calderas approaching 100 km2, voluminous ash flow and airfall tuffs (20 – 50 km3). The northernly group has basalts and mugearites erupting with trachytes. Caldera formation in this group is generally more related to Hawaiian like subsidence following withdrawal of magma from depth.
Volcanics in the rift are associated with the development of the rift. The area N of Menengai has Miocene trachyitic and trachy-phonolitic lavas from N-S trending fissures. They are covered with ignimbrites and pyroclastics likely from Menengai eruptions to the S.
More recent eruptions from these fissures are associated with subsequent faulting episodes. The first of these spread the Mau – Kinangop tuff ash flows 3.7 – 3.4 Ma. Major faulting took place following the ignimbrite eruptions and converted a half-graben into a graben. This was followed by the eruption of the Limuru flood trachyite 2.0 – 1.8 Ma. These fissure eruptions were driven by upward migration of the fault zones and creation of the step-faulted Mau escarpments. The next two phases erupted basalts and basaltic trachy-andesites 1.65 – 0.9 Ma. These were thought to be driven by convecting mantle which opened fissures to the surface for the floods and construction of a number of large shield volcanoes along the axis of the rift.
Conclusions
Menengai is an example of an active, but currently dormant volcano located in an active spreading tectonic rift. It has nearly 600,000 Kenyans within 10 km of a caldera with multiple eruptions over the last 8 ka. Geothermal exploration found basalt lava at depth. Worse, temperatures from geothermal wells seem to be increasing over time. This volcano is not done yet, nor apparently not any time soon.
Additional information
Menengai Crater – Trip Advisor
Geochemical evolution of the Menengai caldera volcano, Kenya, Leat, et al, Sept 1984
Geological evolution of the trachytic caldera volcano Menengai, Kenya Rift Valley, PT Leat, Nov 1984
The Menengai caldera structure and its relevance to geothermal potential, LM Njue, Nov 2010
Late Quaternary caldera volcanoes of the Kenya Rift Valley, Williams, et al, Sept 1984
Significance of tectono-volcanic axes in Menengai Geothermal Field, K Kahiga, Oct 2014
agimarc
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Granyia
volcanohotspot.wordpress.com 
UAF Geophysical Institute in Fairbanks, AK issued a press release 10 days ago announcing results that magma is moving beneath Mount Edgcumbe in Alaska. Movement includes both earthquakes and deformation since 2018. Note that Edgcumbe is part of a volcanic field near Sitka, AK. We covered the field in 2016. Cheers –
https://www.gi.alaska.edu/news/research-reveals-magma-activity-beneath-mount-edgecumbe
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