Homa Mountain is a 1,751 m carbonatitic volcanic complex on the E shore of Lake Victoria in Kenya.  It is a relatively old volcanic system, active possibly over the last 13 Ma, with the most recent eruption likely a maar formation described in oral tradition by local natives.  The mountain forms a peninsula on the E shore of the lake.  There are multiple vents on the flanks and the top is relatively flat. 

The closest town is Homa Bay, some 20 km S.  The area is relatively densely populated, with 61,000 within 10 km, 630,000 within 30 km, and a whopping 10 million within 100 km.  The mountain name in the Luo language translates as The Famous Mountain. 

Homa Bay County is the administrative unit on the S side of the Nyanza Gulf, an arm of Lake Victoria flooding the floor of the Kavirondo Rift to the E.  Carbonatite volcanic activity is located along the S side of this rift, in Homa Bay County.  There are over 1.1 million in the county, mostly in towns of over 100,000.

The economy of Homa Bay County is mostly based on agriculture, with coffee, sugarcane, maize, beans and fish farming.  There is an effort to revive the cotton industry.  Dairy, cattle, sheep, goats, rabbit, pigs and poultry are also raised.  There is a small but growing tourist activity in the region. 

The village of Kanjira on the Homa Peninsula is famous as an archaeological site, excavated by archeologist Louis Leakey in the 1930s.  It is one of the oldest known Oldowan paleontological sites, with human artifacts, tools and remains dating back at least 2 Ma.  The Homa Peninsula contains East African environments during the nearly 6 Ma time span of human evolution.  The location between the two branches of the Great Rift Valley is significant in studies of East African landscapes. 

There is increasing interest in the area exploring for Rare Earth Elements (REE) and radioactive ores.  Carbonatite rock intrusions have been associated with high concentrations of radioactive Potassium, Thorium and Uranium.  Carbonatites in other parts of the world are commercially exploitable sources of niobium, thorium and a variety of light REEs. 

None of the volcanoes in this region are recently active and are not currently monitored for activity.  Webcams in this part of the world spend more time monitoring wildlife and active volcanoes than inactive ones. 

Region and Tectonics

Volcanic activity in Kenya is generally associated with rifting activity in both arms of the East Africa / Kenya / Gregory Rift.  The two arms of the greater system split around the craton Lake Victoria is located upon, the East African Plateau.  There is a short third arm, the Kavirondo Rift that splits SW from the Gregory Rift and enters the NE corner of Lake Victoria, starting to penetrate the East African Plateau.  This rifting has triggered volcanic activity that has produced various basalts and carbonatites.   

As volcanism in Kenya is primarily associated with rifting, there is no volcanic front / volcanic arc like we think of in subduction zones.  And distances in this part of the world are large, with hundreds of kilometers between rift segments.  For example, Homa Mountain, located on the E shore of Lake Victoria some 175 km W from volcanic activity along the East African Rift.  Kilombe Caldera is the closest rift-associated system, located 143 km E. 

Over the years, we have done multiple posts on volcanic activity associated with the rift system.  Menengai, which we discussed in a pair of posts 2022 and 2017 is located 175 km E from Homa.  Longonot, 225 ESE from Homa and Suswa, 230 km ESE from Homa in 2021 and 2020 respectively.   All of these are on the main East African Rift / Kenya Rift / Gregory Rift.  This arm and associated volcanic activity extends S into Tanzania with Ol Doinyo Lengai, Meru, 2021, Chyulu Hills, 2025, and Kilimanjaro farther to the S.  There are multiple other recently active volcanoes along the rift. 

To the W, the distances are even larger, with Nyiragongo (2018 post) located nearly 600 km W. and the Virunga Volcanic Province, 2025, 550 km W of Homa.

Kavirondo Rift

The Homa Bay District is located along the Kavirondo Rift, a W branch of the Gregory Rift, extending 160 km toward Lake Victoria on the S side of the Nyanza Gulf.  There are three main alkaline – carbonatite volcanic complexes associated with this rift.  These include Homa Mountain, North and South Ruri, and the Kisingiri stratovolcano and its Rangwa caldera.  There are also 5 nephelinite to phonolite volcanic centers, 6 ijolitic and 8 subvolcanic carbonatite volcanic centers.  Volcanism along the Kavirondo Rift forms a carbonatite belt. 

The Kavirondo Rift forms a depression W of the Tinderet volcano.  An arm of Lake Victoria, the Nyanza Gulf, extends into the depression as far as Kisumu.  The rift follows the Lambwe fault SW into Lake Victoria.  The floor of the rift valley is around 1,200 m.  The volcanoes rise abruptly to 1,600 – 1,700 m. 

Ruri Hills, Rangwa, Homa Mountain and Legetet were built by extensive carbonatite pyroclastics including tuffs, tuff breccias, agglomerate and lapillis.  Pyroclastics are associated with carbonatite dikes and sheets.  This same evolution of activity is repeated in all the carbonatite centers where more recent carbonatites and silica rocks are closely associated. 

The similar evolution of carbonatite centers over time suggests physical processes feeding magma chambers along the Kavirondo Rift are also similar.  Early carbonate magmas produced tuffs, agglomerates, breccias and/or blocky lavas.  These eruptions were explosive.  Later magmas intruded at the same time as silicate magmas creating phonolite – nephelinite (basalt) plugs cut by carbonatites common in these complexes.  Extensive lava flows erupted from Kisingiri and Tinderet. 

Tinderet

Tinderet is located at the junction of the Gregory Rift and Kavirondo Rift valleys, some 100 km ENE from Homa Mountain.  It is a stratovolcano similar to Kisingiri, initially erupting mainly nephelinite basalts with some other variants, though not as highly eroded.  Later eruptions for both systems produced both basalts and carbonatites.  Carbonatites produced intrusions, cone sheets and pyroclastics. 

It is located close to satellite pyroclastic carbonatite vents Legetet, Songhor and Buru.  While Tinderet does not have a central carbonatite complex, there are significant carbonatite intrusions.  Like the other volcanoes along the Kavirondo Rift, activity here began some 20 Ma.  The satellite carbonatite vents were formed after the initial stratovolcano was built. 

Legetet Hills, 12 km WSW of Tinderet may have been a parasitic cone on its W flank.  There are widespread pyroclastic carbonatitic rocks, lavas and sandy tuffs.  Buru Hill is another carbonatite eruptive center some 30 km W of Tinderet area. 

Kisingiri – Rangwa (Kaksingri)

Kisingiri is one of the largest stratovolcanoes in W Kenya, located 43 km WSW from Homa Mountain.  It extruded nephelinitic volcanic rocks over more than 2,000 km2.  The center collapsed forming a 10 km diameter caldera.  Carbonatities extruded to fill the breached caldera.  The Kisingiri volcano rises gradually to over 2,100 m.  The Lambwe fault displaces Kisingiri lavas more than 300 m. 

Volcanism at Kisingiri dates back to 19.6 Ma.  The complex includes five vertical units:  Rangwa, Gwasi and Gembe Hills, several islands in Lake Victoria, the Kainiamwia escarpment and the Uyoma peninsula N of the Nyanza Gulf.  The units are all dominated by extensive nepheline basalts, tuffs and agglomerates.  The pile is divided into Lower Kisinigiri lavas (mainly basalts), Middle Kisingiri pyroclastics (mainly carbonatites and basalts), and Upper Kisinigiri lavas (mainly carbonatites). 

Early alkaline intrusions likely domed the basement granite producing magmas and vents that erupted in the three groups.  Part of the activity took place before carbonatite eruptions.  Part of the activity took place at the same time as carbonatite eruptions.  Rwanga caldera formation took place toward the end of its volcanic history.  Pyroclastics and some carbonatites were deposited during this period.  Continued eruptions created current sediments and various erosion deposits in the area.

Erupted carbonatites in the Rangwa caldera are found within a 2.6 x 2 km center.  The caldera is 4 – 5 km in diameter.  Pyroclastics erupted first.  Carbonatites erupted shortly afterwards and are intrusive through them.  Rangwa is likely connected to Kisingiri by a shared magma chamber. 

Sagarume – Nyangurka appears to be carbonatite cones 6 km or so NNE from the center of the Rangwa caldera.  They are located in the NE foothills of the Kisingiri

Additional carbonatite rocks extruded on the Homa Peninsula formed the Waski Peninsula, Ruri Hills and Homa Mountain.  These eruptions differed from Kisingiri in that there were no large-scale silicate eruptions, and their carbonatites are often cone-shaped. 

One of the most striking differences between Kisingiri and neighboring volcanic complexes in the Kavirondo Rift is the vast coverage of nepheline basalts from Kisingiri and the lack of them from other volcanoes.  The neighboring centers are deeply eroded which would suggest they are older.  This would be the wrong conclusion, as the neighboring centers primarily erupted carbonatites, which are much more easily eroded. 

Ruri

North and South Ruri form a double volcano with the central peaks separated by 1 km.  The complex is located some 23 km SW from Homa Mountain.  North Ruri is a cone sheet of carbonatites 1.5 – 2.0 km in diameter.  South Ruri is 2.5 km in diameter.  Both are massive carbonatite bodies.  Outlying phonolite plugs dating 7 – 4 Ma are found up to 10 km from the central volcanoes.  Ruri initially erupted mostly pyroclastics followed by carbonatites.  N Ruri is thought to be the younger of the pair, though still 7.7 – 5.7 Ma.  Both are still younger than Kisingiri. 

Ruri Hills carbonatites form ring-shaped intrusions.  The shape of the overall complex is defined by carbonatite outcrops.  Phonolite plugs associated with carbonatites are common in the Ruri Hills.  They form numerous conical hills with rounded summits, and occasional minor dikes and lava flows.  Both carbonate and silica magmas erupted simultaneously.  

Both the Ruri and neighboring Okuge complexes were uplifted by magma injection below them, doming them a bit.  It is possible that all volcanic complexes in this end of the Kavirondo Rift Valley were derived from the major magma chamber beneath Kisingiri.  Solid, circular phonolite plugs are somewhat resistant to carbonatites intrusion unless they are brecciated.  The various centers erupted intermittently, contemporaneously.  There is no evidence that activity ended at one center before beginning at another.   Erosion removed most of the soft volcanic surface, exposing buried intrusions emplaced during uplift. 

There is evidence of a former crater lake at S Ruri.  Later carbonatite plugs obscure much of the vent area.  There are no traceable lava flows.  N and S Ruri are either two small, twin volcanoes or separate conduits / vent areas of the same volcano.  Numerous recent phonolite plugs are on the flanks of Ruri. 

Ruri magmas were rich in juvenile gasses, making eruptions here highly explosive without lava flows.  Magma appears to have quickly risen from the mantle.  Mantle source was similar to other carbonatite districts in the Great African Rift. 

Kuge Hill is a carbonatite center on the N flank of Ruri Hills.  It is of interest due to rare earth enriched carbonatites erupted here.  Got Kayonyo is a satellite vent located 1.5 km from N Ruri. 

There are a number of carbonatite intrusions exposed on the Wasaki Peninsula between Ruri Hills and Homa Mountain, 10 – 12 km NW from the town of Homa Bay.  These are called Ngou – Kuwor and the Ungongo – Uyi – Kiyaya – Sokolo areas.  The hills are perhaps 140 m above lake level.  The Sokolo cape is a carbonatite body sticking some 60 m into the lake surrounded by cliffs. 

Okuge

Okuge is located between Homa Mountain and Ruri Hills, 15 km SW from Homa, 5 km NE from Ruri Hills.  It is smaller and less complex than Ruri Hills, only 0.5 km in diameter.  Okuge is a horseshow shaped complex of carbonatite intrusions encircling a central block of basalts.  The Okuge complex has multiple dikes.  Outer dikes surround most of the hill, thinning out on the S side of the hill.  The large central core is basalt, similar to that of N Ruri. 

It probably started out as a small dome and was surrounded by Kisingiri lavas.  Carbonatites of the central vent of Okuge are identical with those of N Ruri and considered to be of similar age. 

Homa Mountain

Homa Mountain is a large carbonatite complex that formed a broad peninsula on the E shore of Lake Victoria.  It is the site of a 1,751 m active volcano dating back at least 13 Ma.  The Homa peninsula protrudes into the Nyanza Gulf, forming the E flank of Homa Bay.  Homa Mountain is located along the local Kendu fault.  The mountain is covered by a thick layer of sediments on its lower flanks.  These include recent gravels and soils.  Volcanism began at Homa Mountain some 13 Ma.  The main carbonatite edifice was built 11 – 5 Ma. 

Activity at Homa began with doming of the central portion toward the end of the Miocene and shifted to peripheral vents over the last 2 Ma.  Today, the heavily eroded edifice stands some 600 m above the lake.  The lower slopes are heavily eroded by a radial drainage system exposing layering from its initial activity to the present. 

Homa carbonatites were emplaced with at least five successive intrusions.  A large number of carbonatite cone sheets and dikes of various sizes occur in a 5 x 6 km area.  The main cone sheet is the core.  Some carbonatite dikes are on the S part of the area.  Most of the carbonatites occur as a circular dikes.  There are very few large carbonatite bodies other than those of Ndiru Hill.  Gold-bearing quartz veins are located some 3 km E of the summit.  Rocks were and continue to be altered by hydrothermal activity. 

Homa Mountain is the youngest carbonatite complex of the Nyanza Rift.  Its eruptive history is similar to that of Ruri Hills.  The main center is a complex of multiple cone sheets of carbonatite breccias invading earlier volcanic rocks. 

Homa is also surrounded by numerous small satellite vents.  These are dominantly pyroclastic with carbonate sheets and dikes.  These include Awaya, Chiewo, Ojawa, Oloo and Rongo.  Rapogi, Ndiru-Yusso, Ndiru, Mbili, Chiewo, Rongo and Awaya have cone shaped layers on top of shattered breccias, pyroclastics, and sediments. 

Chiewo, Got Ojawa and Got Oloo vents on the S and W sides of Home Mountain formed during the most recent carbonatitic activity.  The Lake Simbi maar on Homa Mountain’s lower E flank may have been formed in human memories.  There are local legends of its eruption.  Current activity is indicated by several hot springs found in most parts of Homa Mountain.  There are hot springs around Bala, E of Homa Mountain and a pair of hot springs in the N, in the Rawe beds, on the foot of Nyasanja peak. 

Homa is considered to be a geothermal prospect due to the hot springs and steaming ground of the hydrothermal system.  There is a relatively deep heat source with shallow carbonatite dikes 200 – 750 m below the surface in the S part of the volcano. 

Homa Mountain Satellite Peaks

Nyasanja Peak is a small carbonatite peak separated from the Homa summit by Nyasanja valley.  It is a pair of peaks with the tallest 1,600 m high.  The secondary peak, Ratieng is 200 m NE.  This suggests double cone formation.  The carbonatite intrusions are cut by dikes and veins of more recent volcanic activity.  There is a third peak to the E that may also be related.  It is separated from the pair by a depression either caused by faulting or pyroclastics.  There is some argument whether rocks in the depression were deposited as sediments or pyroclastics. 

Nyamatoto Peak is located 1 km SE of Nyasanja.  It is a steep-sided peak covered by vegetation.  The peak is mainly phonolites and thought to be emplaced relatively recently.

Kanam and Rawe Beds are NW of Nysanja.  The Kanam Beds are mainly soils created by erosive runoff.  The soils include clays and some tuffs.  The Rawe Beds are E of Kanam at the foot of Nyasanja.  They are layered clays, ash and gravel covered by erosional runoff.  Gravel beds occur through the sequence and often contain rounded pebbles. 

Streams associated with the beds flow toward Lake Victoria.  They originate from hot springs at Nyagot Pala and Abundu Pala.   Hot springs are about 2 km apart, separated by Nyanzian rock and erosion deposits.  The hot spring locations appear to be controlled by local faulting. 

There are travertine deposits 100 m NW of Abundu hot spring.  Hot water discharge from the springs altered the volcanic rock, creating quartz and calcite.  There is an intrusion of welded pyroclastics indicating volcanic eruptions took place during the lifetime of the hot springs. 

Odiawo is a small plug-shaped hill 150 m high E of Nyasanja.  It does not show a cone-sheet structure.  Rock formations are carbonatites similar to Nyasanja Peak.  These are veined.  The hill is defined by concentric faults.  Got Akom is a small 50 m hill just to the N of Odiawo.  It appears to be a dome of reworked brecciated carbonatites.

Awaya is a small hill E of Homa Mountain rising over 400 m above the surrounding plains.  Unlike other carbonatite peaks around Homa Mountain, it has a relatively gentle slope.  Most of the exposed rocks are Nyanzian volcanics.  Drainage around this hill is radial.  Concentric fractures along intrusive carbonatites are visible.  There is a low area E of the peak filled with erosive deposits.

Ndiru Hill is a massive carbonatite complex 300 x 500 m 2.5 km from the summit of Homa Mountain.  Carbonatite dikes cut all previously erupted carbonatites.  There is visible hydrothermal alteration. 

Chiewo – Ndiru Mbili – Yusoo have abundant bedded pyroclasts, carbonatite intrusions and breccias.  Got Chiewo is a satellite vent to Homa Mountain.  It is a well-preserved carbonatite cone with bedded lapilli tuffs and carbonatites.  Got Chiewo demonstrates the highly fluid nature of erupting carbonatite lavas with welding, agglutination and pyroclasts.  There are fall deposits from violent disruption of a carbonatite melt by rapidly expanding gasses.  Lava fountaining and spatter also took place.

Carbonatite Lavas

No discussion of Homa Mountain (or Ol Doinyo Lengai for that matter) is complete without at least mentioning carbonatite lavas and why they are odd.  Ol Doinyo Lengai is the only active volcano today erupting carbonatites, though they have been found at over 500 locations worldwide. 

Carbonatites are defined as igneous rocks with over 50% carbonates.  And just so you know that this isn’t a circular definition, a carbonate is defined as a salt of carbonic acid, a carbon atom bound to three oxygen atoms.  All carbonatites contain abundant CO2.   There are four major variations of carbonatites, including calcite carbonatite (main carbonate is calcite), dolomite carbonatite (dolomite is the main carbonatite), ferro carbonatite (main carbonite is iron), and natro carbonatite (main carbonates are sodium, potassium and calcium). 

They generally occur in close association with alkali silicate igneous rocks.  While there is no definitive explanation for their origin, there are multiple theories.  These include residual melts, immiscible melt fractions of CO2-saturated melts, and melts generated through unique crystal fractionation.  Combinations of these theories are also popular.  One example would be carbonatite liquids generated by deep melting of carbonate bearing rocks, with the magma reaching the surface in two batches – a basalt and a carbonatite – erupting separately. 

Carbonatites are one of the best sources of rare earth elements (REE).  The only REE mine in the US is a carbonatite deposit.  Something about the process of their formation allows them to concentrate REE. 

The carbonatites make the lavas among the coolest lavas erupting, on the order of 500 – 600° C, 250 – 450° C cooler than the typical silica magma.  These lavas are very liquid and flow easily.  Carbonatite lavas have been invoked as an explanation for remarkably long lava flows found on Venus. 

Carbonatites are easily weathered, making it difficult to study their history, though they have been found in rocks dating back billions of years.   Their distribution appears to have increased over time, with the production episodic and possibly linked to major tectonic events.  For example, today over a third of all new carbonatites worldwide are found in the vicinity of the East Africa Rift System. 

Eric Klemetti in Wired noted that currently active Ol Doinyo Lengai did not always erupt carbonatites.  Its earlier history was alkaline silicate volcanism forming tuffs.  Recent carbonatite volcanism there has only been active for the last few millennia.  He believes that this suggests that we are seeing separation of carbonatite liquids from whatever source is feeding them. 

Conclusions

Recent volcanic activity at Homa Mountain has mixed silica (basalts) with carbonatites.  The most recent activity built plugs cut by carbonatite sheets, followed by a maar.  Existing heat drives a vigorous hydrothermal system.  Given ongoing rifting in the region, I would not expect the magma supply to be curtailed any time soon.  Future volcanic activity may combine silicic (basalt) and carbonatite eruptions, and likely be explosive in nature.  The system is not currently monitored though there is abundant volcanism in the region. 

Additional information

Earliest archaeological evidence of persistent hominin carnivory, Ferraro, et al, Apr 2013

Exploration of apatite (U-Th)/He geochronological analysis of volcanic units in fossil-bearing strata of the Homa Peninsula, southwestern Kenya, Lit, et al, Oct 2021

Carbonated melilitites and calcitized alkalicarbonates from Homa Mountain, western Kenya:  a reinterpretation, Moore & Roberts, May 2009

An investigation of the radioecology of the carbonatite deposits in the Homa Mountain region in South Nyanza, Kenya, D Otwoma, Dec 2021

The Pleistocene locality of Kanjera, western Kenya:  stratigraphy, chronology and paleoenvironments, Behrensmeyer, et al, Sept 1995

Geology and geomorphology of Homa area and its resource potential, J Bach – Achieng, ED/03098/011

The history of the alkaline volcanoes and intrusive complexes of eastern Uganda and western Kenya, King, et al, Mar 1972

The geology of the carbonatitic rocks of the north of the Homa Mountain carbonatite complex, western Kenya, AM Flegg, 1970

Estimation of annual effective dose and radiation hazards due to natural radionuclides in Mount Homa, southwestern Kenya, Otwoma, et al, Mar 2013

Ijolitic rocks near Homa Bay, western Kenya, W Pulfrey, Nov 1949

Volcanism of the Kenya rift valley, King & Chapman, 1972

Geochronology and physical context of Oldowan site formation at Kanjera South, Kenya, Ditchfield, et al

2D-Euler deconvolution and forward modeling of gravity data of Homa – Hills geothermal prospect, Kenya, Odek, et al, JAGST Vol. 15(1) 2013

Isotope geochemistry and petrology of the African carbonatites, Suwa, et al

Exotic lava flows:  carbonatites, L Tierney, Apr 2002

The structure and petrology of the carbonatitic complexes of North Ruri and Okuge, Kenya, JA Dixon, Mar 1968

Texture and mineralogy of tuffs and tuffisites at Ruri volcano in western Kenya:  a carbonatite, melilitite, mantle-debris trio, Stoppa, et al, Jan 2003

Geochemistry and mineralization of Buru and Kuge volcanic carbonatite centres, western Kenya, IO Onuongo, 1997

1-3 Geology, stratigraphy and mineralisation

An assessment for phosphates in the carbonatitic and alkaline – silicate complexes in western Kenya, H Idman, Aug 1984