Eastern Rift Research Articles

Oxo-magnesio-hastingsite, NaCa2(Mg2Fe3+3 )(Al2Si6)O22O2, a new anhydrous amphibole from the Deeti volcanic cone, Gregory rift, northern Tanzania

AbstractOxo-magnesio-hastingsite, ideally NaCa2(Mg2Fe3+3)(Al2Si6)O22O2, is a new anhydrous amphibole from the Deeti volcanic cone in the Gregory rift (northern Tanzania). The mineral occurs as megacrysts up to 12 cm in size in crystal-rich tuff. Oxo-magnesio-hastingsite is brown with a vitreous lustre and has a perfect {110} cleavage. The measured density is 3.19(1) g/cm3. Ferri-kaersutite is biaxial (–), α = 1.706 (2), β = 1.715(2), γ = 1.720(2) (Na light, 589 nm). 2V (calc.) = 73°. Dispersion: r > v, weak; orientation: Y = b; Z ^ c = 8°; pleochroism: strong, Z: dark brown, Y: brown, X: light brown. The average chemical formula of the mineral derived from electron microprobe analyses, Mössbauer spectroscopy and direct water determination is (Na0.67K0.33)Σ1.00(Ca1.87Na0.14Mn0.01)Σ2.02(Mg3.27Fe3+1.25Ti0.44Al0.08)Σ5.04(Al1.80Si6.20O22)(O1.40OH0.60)Σ2.00. It has monoclinic symmetry, space group C2/m and unit-cell parameters a = 9.8837(3), b = 18.0662(6), c = 5.3107(2) Å, b = 105.278(1)o, V = 914.77(5) Å3, Z = 2. The five strongest powder-diffraction lines [d in Å, (I/Io), hkl] are: 3.383 (62) (131), 2.708 (97) (151), 2.555 (100) (), 2.349 (29) () and 2.162 (36) (261). The isotopic composition of H and O, as well as the concentration of trace elements in oxo-magnesio-hastingsite suggest its formation from a melt originated from a mantle source metasomatized by slab-derived fluids.

The uppermost mantle shear wave velocity structure of eastern Africa from Rayleigh wave tomography: constraints on rift evolution

An expanded model of the 3-D shear wave velocity structure of the uppermost mantle beneath eastern Africa has been developed using earthquakes recorded by the AfricaArray East African Seismic Experiment in conjunction with data from permanent stations and previously deployed temporary stations. The combined data set comprises 331 earthquakes recorded on a total of 95 seismic stations spanning Kenya, Uganda, Tanzania, Zambia and Malawi. In this study, data from 149 earthquakes were used to determine fundamental-mode Rayleigh wave phase velocities at periods ranging from 20 to 182 s using the two-plane wave method, and then combined with the similarly processed published measurements and inverted for a 3-D shear wave velocity model of the uppermost mantle. New features in the model include (1) a low-velocity region in western Zambia, (2) a high-velocity region in eastern Zambia, (3) a low-velocity region in eastern Tanzania and (4) low-velocity regions beneath the Lake Malawi rift. When considered in conjunction with mapped seismicity, these results support a secondary western rift branch striking southwestwards from Lake Tanganyika, likely exploiting the relatively weak lithosphere of the southern Kibaran Belt between the Bangweulu Block and the Congo Craton. We estimate a lithospheric thickness of ∼150–200 km for the substantial fast shear wave anomaly imaged in eastern Zambia, which may be a southward subsurface extension of the Bangweulu Block. The low-velocity region in eastern Tanzania suggests that the eastern rift branch trends southeastwards offshore eastern Tanzania coincident with the purported location of the northern margin of the proposed Ruvuma microplate. Pronounced velocity lows along the Lake Malawi rift are found beneath the northern and southern ends of the lake, but not beneath the central portion of the lake.

Divergent pattern of nuclear genetic diversity across the range of the Afromontane Prunus africana mirrors variable climate of African highlands

Afromontane forest ecosystems share a high similarity of plant and animal biodiversity, although they occur mainly on isolated mountain massifs throughout the continent. This resemblance has long provoked questions on former wider distribution of Afromontane forests. In this study Prunus africana (one of the character trees of Afromontane forests) is used as a model for understanding the biogeography of this vegetation zone. Thirty natural populations from nine African countries covering a large part of Afromontane regions were analysed using six nuclear microsatellites. Standard population genetic analysis as well as Bayesian and maximum likelihood models were used to infer genetic diversity, population differentiation, barriers to gene flow, and recent and all migration among populations. Prunus africana exhibits strong divergence among five main Afromontane regions: West Africa, East Africa west of the Eastern Rift Valley (ERV), East Africa east of the ERV, southern Africa and Madagascar. The strongest divergence was evident between Madagascar and continental Africa. Populations from West Africa showed high similarity with East African populations west of the ERV, whereas populations east of the ERV are closely related to populations of southern Africa, respectively. The observed patterns indicate divergent population history across the continent most likely associated to Pleistocene changes in climatic conditions. The high genetic similarity between populations of West Africa with population of East Africa west of the ERV is in agreement with faunistic and floristic patterns and provides further evidence for a historical migration route. Contrasting estimates of recent and historical gene flow indicate a shift of the main barrier to gene flow from the Lake Victoria basin to the ERV, highlighting the dynamic environmental and evolutionary history of the region.

Integration of outcrop and subsurface data during the development of a naturally fractured Eocene carbonate reservoir at the East Ras Budran concession, Gulf of Suez, Egypt

Abstract The East Ras Budran Concession is located in the eastern rift shoulder of the Gulf of Suez. Syn- and pre-rift rocks are exposed in the north and east of the concession, and the Markha alluvial plain covers the SW. The Markha plain occupies the hanging wall of a large extensional fault which preserves most of the pre-rift stratigraphic sequence and >3500 m of syn-rift strata. Vertical wells drilled in 1999 indicated the presence of a >200 m oil column in low-porosity naturally fractured limestone beds of the Eocene Darat and Thebes formations. Outcrop, borehole image and core data define NW, WNW, N, NE, and ENE steeply dipping fracture sets. Borehole breakouts and drilling-induced fractures show that the minimum horizontal stress is aligned NNE to NE, so the NW and WNW fractures should be open in the subsurface. Using this structural picture, a near-horizontal well of 300 m length was drilled into the Darat in a NE direction. During testing, the well flowed at a rate of 1900 barrels of oil per day with no water. Future development of the field includes drilling similarly oriented wells with longer horizontal sections.

Mineralogy, geochemistry and petrology of the phonolitic to nephelinitic Sadiman volcano, Crater Highlands, Tanzania

Sadiman volcano is located in the Crater Highlands area of northern Tanzania, which lies next to the western escarpment of the Gregory rift—a part of the eastern branch of the East African Rift system. It consists of interlayered phonolitic tuffs, tuff breccias (with blocks of nephelinites) and nephelinitic lava flows. Rare xenoliths of phonolite lava and ijolite were observed within the nephelinite lavas with ijolite blocks occurring in phonolitic tuffs. No evidence for the presence of melilite-bearing and/or carbonatitic rocks was found during this study. On the basis of petrography, mineralogy and geochemistry the nephelinites are divided into highly porphyritic nephelinite, wollastonite nephelinite and phonolitic nephelinite, the latter of which is the dominant variety at Sadiman. Nepheline+clinopyroxene+titanite±perovskite±andradite–schorlomite±wollastonite±sanidine±sodalite are the principle pheno- and microphenocryst phases. The nephelinites are highly evolved (Mg#=0.17–0.26) alkaline to peralkaline (AI=0.88–1.21) rocks enriched in incompatible elements such as Rb, Ba, Th, U, Nb, Pb, Ta, Sr and light REEs, and strongly depleted in P and Ti. This suggests derivation from an enriched mantle source and fractionation of apatite and Ti-rich mineral(s). Primary melt inclusions in nepheline phenocrysts (Thomogenization=860–1100°C) indicate enrichment of volatile components in the melts, particularly of fluorine (up to 1.8wt.% in silicate glass) resulting in the formation of daughter fluorite in partly and complete crystallized inclusions. The Sadiman nephelinites crystallized under relatively oxidizing conditions (above the FMQ buffer), which differ from the reducing conditions reported for trachytic and pantelleritic rocks from other parts of the Gregory rift. Similar rock types and relatively oxidizing conditions are known from Oldoinyo Lengai and other localities, all of which are closely associated with carbonatites. By analogy, we conclude that andradite–schorlomite-rich nephelinites may indicate a pre-stage on the evolutionary path towards carbonatitic magmatism.

Nephelinite–melilitite–carbonatite relationships: Evidence from Pleistocene–recent volcanism in northern Tanzania

The younger (<1.2Ma) nephelinite–melilitite–carbonatite-volcanoes in the Gregory Rift Valley of northern Tanzania (termed the Younger Extrusives) comprise one of the most concentrated areas of carbonatite magmatism on Earth. The larger strato-volcanoes, characterised by strongly evolved magmatism, are surrounded by volumetrically much-smaller features (scoria and tuff-cones, explosion craters, lava flows) of carbonate-rich olivine-bearing nephelinites or melilitites that, in their relative primitiveness, are potential parents to the much-largerstrato-volcanoes. The bimodal size distribution is mirrored in bulk magma chemistry, and in their Sr, Nd and Pb isotopes, with the more primitive magmas lying close to a HIMU source; the isotope data for the larger strato-volcanoes cover a wider, linear array interpreted as due to variable degrees of interaction between HIMU magmas and enriched lithosphere, evidence for which is found in Tanzanian mantle xenoliths.An integrated model involves carbonated olivine–nephelinite magma generated in the asthenosphere by melting of carbonate-bearing peridotite. Small amounts of the melt migrate rapidly and are extruded as the olivine–nephelinites and –melilitites. Larger amounts of magma pond in the lithosphere with which they react and, during variable periods of stagnation, varying degrees of olivine–pyroxene–spinel fractionation take place before upward migration of now-differing magma batches. Variable residence time within, and reaction with, variably enriched lithosphere accounts for the array between the HIMU and enriched mantle end-members in the isotope mixing model.

Fluorine-, yttrium- and lanthanide-rich cerianite-(Ce) from carbonatitic rocks of the Kerimasi volcano and surrounding explosion craters, Gregory Rift, northern Tanzania

AbstractCerianite-(Ce), ideally CeO2, occurs as rounded grains up to 5 μm across in a block of highly altered calcite carbonatite lava from the Kerimasi volcano, and as euhedral crystals up to 200 μm across in carbonatite-derived eluvial deposits in the Kisete and Loluni explosion craters in the Gregory Rift, northern Tanzania. X-ray powder diffraction data (a = 5.434(5) Å) and Raman spectroscopy (minor vibration modes at 184 and 571 cm—1 in addition to a strong signal at 449 cm—1) suggest the presence of essential amounts of large cations and oxygen vacancies in the Kisete material. Microprobe analyses reveal that the mineral contains both light and heavy trivalent rare earth elements (REE) (7.9-15.5 wt.% LREE2O3 and 4.9-9.7 wt.% HREE2O3), and that it is enriched in yttrium (7.1 — 14.5 wt.% Y2O3) and fluorine (2.2—3.5 wt.%). Single-crystal structure refinement of the mineral confirms a fluorite-type structure with a cation—anion distance of 2.3471(6) Å. The cerianite-(Ce) is considered to be a late-stage secondary mineral in the carbonatitic rocks.

Geochronology of the Manyara Beds, northern Tanzania: New tephrostratigraphy, magnetostratigraphy and 40Ar/ 39Ar ages

The ∼25 m-thick Manyara Beds section near Makuyuni, northern Tanzania, contain abundant Acheulean lithics and vertebrate fossils, including possible Homo erectus remains. However, the age of the unit (and its most productive fossil and archaeological localities) has been provisional. To address this we measured 20 new stratigraphic sections and collected approximately 30 tephra and volcanic rock samples for 40Ar/ 39Ar geochronology and 12 mudstone/siltstone samples for a trial paleomagnetic analysis. Most sampled tephra deposits (from the greenish-gray, lacustrine lower member) proved too fine-grained or lithologically heterogeneous for 40Ar/ 39Ar analysis, but crystals from a pumice-rich unit (the ‘Hollywood Tuff;’ at the base of the reddish-brown, fluvial upper member), analyzed in three batches, yielded a middle Pleistocene 40Ar/ 39Ar age of 0.633 ± 0.039 Ma. Two-step analysis of 10 feldspars from a nephelinite underlying the Manyara Beds produced a weighted mean 40Ar/ 39Ar age of 6.00 ± 0.03 Ma, linking it to the nearby stratovolcano Essimingor. Paleomagnetic analysis showed that the Manyara Beds carry a stable ancient remanence, and characteristic directions obtained by AF demagnetization define a preliminary magnetostratigraphy for the unit. Four magnetozones over 21 m of section include two intervals of normal polarity tentatively correlated with the Jaramillo Subchron and Brunhes Chron. The Matuyama-Brunhes boundary is located within the upper half of the lower member, at least 6 m below the lower-upper member contact. Using undecompacted sedimentation rates of 24–46 m/Ma (calculated using geomagnetic polarity time scale correlations and the stratigraphic position of the ∼0.633 Ma tephra) we estimate that the base of the Manyara Beds was deposited between 0.98 and 1.3 Ma and the top between 0.27 and 0.44 Ma. Beds III–IV and the Masek Beds at Olduvai were probably also deposited in the same early–middle Pleistocene interval, during which the Mid-Pleistocene Revolution climatic event and active rifting and volcanism in the southernmost Gregory Rift jointly contributed to landscape and ecosystem instability.

Relationships between Kuroko volcanogenic massive sulfide (VMS) deposits, felsic volcanism, and island arc development in the northeast Honshu arc, Japan

The northeast (NE) Honshu arc was formed by three major volcano-tectonic events resulting from Late Cenozoic orogenic movement: continental margin volcanism (before 21 Ma), seafloor basaltic lava flows and subsequent bimodal volcanism accompanied by back-arc rifting (21 to 14 Ma), and felsic volcanism related to island arc uplift (12 to 2 Ma). Eight petrotectonic domains, parallel to the NE Honshu arc, were formed as a result of the eastward migration of volcanic activity with time. Major Kuroko volcanogenic massive sulfide (VMS) deposits are located within the eastern marginal rift zone (Kuroko rift) that formed in the final period of back-arc rifting (16 to 14 Ma). Volcanic activity in the NE Honshu arc is divided into six volcanic stages. The eruption volumes of volcanic rocks have gradually decreased from 4,600 km3 (per 1 my for a 200-km-long section along the arc) of basaltic lava flows in the back-arc spreading stage to 1,000–2,000 km3 of bimodal hyaloclastites in the back-arc rift stage, and about 200 km3 of felsic pumice eruptions in the island arc stage. The Kuroko VMS deposits were formed at the time of abrupt decrease in the eruption volume and change in the mode of occurrence of the volcanic rocks during the final period of back-arc rifting. In the area of the Kuroko rift, felsic volcanism changed from aphyric or weakly plagioclase phyric (before 14 Ma), to quartz and plagioclase phyric with minor clinopyroxene (12 to 8 Ma), to hornblende phyric (after 8 Ma), and hornblende and biotite phyric (after 4 Ma). The Kuroko VMS deposits are closely related to the aphyric rhyolitic activity before 14 Ma. The rhyolite was generated at a relatively high temperature from a highly differentiated part of felsic magma seated at a relatively great depth and contains higher Nb, Ce, and Y contents than the post-Kuroko felsic volcanism. The Kuroko VMS deposits were formed within a specific tectonic setting, at a specific period, and associated with a particular volcanism of the arc evolution process. Therefore, detailed study of the evolutional process from rift opening to island arc tectonics is very important for the exploration of Kuroko-type VMS deposits.

The Cenozoic western Svalbard margin: sediment geometry and sedimentary processes in an area of ultraslow oceanic spreading

The northeastern high-latitude North Atlantic is characterised by the Bellsund and Isfjorden fans on the continental slope off west Svalbard, the asymmetrical ultraslow Knipovich spreading ridge and a 1,000 m deep rift valley. Recently collected multichannel seismic profiles and bathymetric records now provide a more complete picture of sedimentary processes and depositional environments within this region. Both downslope and alongslope sedimentary processes are identified in the study area. Turbidity currents and deposition of glacigenic debris flows are the dominating downslope processes, whereas mass failures, which are a common process on glaciated margins, appear to have been less significant. The slide debrite observed on the Bellsund Fan is most likely related to a 2.5–1.7 Ma old failure on the northwestern Barents Sea margin. The seismic records further reveal that alongslope current processes played a major role in shaping the sediment packages in the study area. Within the Knipovich rift valley and at the western rift flank accumulations as thick as 950–1,000 m are deposited. We note that oceanic basement is locally exposed within the rift valley, and that seismostratigraphic relationships indicate that fault activity along the eastern rift flank lasted until at least as recently as 1.5 Ma. A purely hemipelagic origin of the sediments in the rift valley and on the western rift flank is unlikely. We suggest that these sediments, partly, have been sourced from the western Svalbard—northwestern Barents Sea margin and into the Knipovich Ridge rift valley before continuous spreading and tectonic activity caused the sediments to be transported out of the valley and westward.

The structures, stratigraphy and evolution of the Gulf of Corinth rift, Greece

SUMMARY A multichannel seismic and bathymetry survey of the central and eastern Gulf of Corinth (GoC), Greece, reveals the offshore fault geometry, seismic stratigraphy and basin evolution of one of Earths most active continental rift systems. Active, right-stepping, en-echelon, north-dipping border faults trend ESE along the southern Gulf margin, significantly overlapping along strike. The basement offsets of three (Akrata-Derveni, Sithas and Xylocastro) are linked. The faults are biplanar to listric: typically intermediate angle (∼35° in the centre and 45–48° in the east) near the surface but decreasing in dip and/or intersecting a low- or shallow-angle (15–20° in the centre and 19–30° in the east) curvi-planar reflector in the basement. Major S-dipping border faults were active along the northern margin of the central Gulf early in the rift history, and remain active in the western Gulf and in the subsidiary Gulf of Lechaio, but unlike the southern border faults, are without major footwall uplift. Much of the eastern rift has a classic half-graben architecture whereas the central rift has a more symmetric w- or u-shape. The narrower and shallower western Gulf that transects the >40-km-thick crust of the Hellenides is associated with a wider distribution of overlapping high-angle normal faults that were formerly active on the Peloponnesus Peninsula. The easternmost sector includes the subsidiary Gulfs of Lechaio and Alkyonides, with major faults and basement structures trending NE, E–W and NW. The basement faults that control the rift architecture formed early in the rift history, with little evidence (other than the Vrachonisida fault along the northern margin) in the marine data for plan view evolution by subsequent fault linkage. Several have maximum offsets near one end. Crestal collapse graben formed where the hanging wall has pulled off the steeper onto the shallower downdip segment of the Derveni Fault. The dominant strikes of the Corinth rift faults gradually rotate from 090–120° in the basement and early rift to 090–100° in the latest rift, reflecting a ∼10° rotation of the opening direction to the 005° presently measured by GPS. The sediments include a (locally >1.5-km-) thick, early-rift section, and a late-rift section (also locally >1.5-km-thick) that we subdivide into three sequences and correlate with seven 100-ka glacio-eustatic cycles. The Gulf depocentre has deepened through time (currently >700 mbsl) as subsidence has outpaced sedimentation. We measure the minimum total horizontal extension across the central and eastern Gulf as varying along strike between 4 and 10 km, and estimate full values of 6–11 km. The rift evolution is strongly influenced by the inherited basement fabric. The regional NNW structural fabric of the Hellenic nappes changes orientation to ESE in the Parnassos terrane, facilitating the focused north-south extension observed offshore there. The basement-penetrating faults lose seismic reflectivity above the 4–14-km-deep seismogenic zone. Multiple generations and dips of normal faults, some cross-cutting, accommodate extension beneath the GoC, including low-angle (15–20°) interfaces in the basement nappes. The thermally cool forearc setting and cross-orogen structures unaccompanied by magmatism make this rift a poor analogue and unlikely precursor for metamorphic core complex formation.

Crustal Seismic Velocities of the Rwenzori Region, East African Rift, from Local Travel-Time Tomography: Evidence for Low-Velocity Anomalies beneath the Mountain Range

We investigate the upper crustal velocity structure beneath the Rwenzori Mountains in western Uganda. This mountain range of nonvolcanic origin is situated within the western branch of the East African rift and reaches altitudes of more than 5000 m. The cause for the extreme uplift within a rifting environment is currently being debated. The local tomographic inversion described here is based on 2053 earth­ quakes recorded by a network of up to 35 stations covering an area of 140 x 90 km2• The deployment was limited by the international border between Uganda and the Democratic Republic of the Congo, such that a number of recorded events lie outside the station perimeter. We perform synthetic tests to assess the effect of location uncertainty on the results. The tests show that the resolution is good between 3 and 15 km depth within a restricted area covered by the array. However, smearing can be significant in some parts. The inversion for P-and S-wave velocity anomalies is performed independently and agrees well. The interpretation of the results is based on a synthetic model that reproduces the same pattern of anomalies as that obtained after inversion of the real data. Our models exhibit a significant negative velocity anomaly (up to -8%) beneath the central Rwenzori Mountains. This could be an indication for active magmatic intrusions beneath the mountains in relation to the rifting. The pre­ sence of low velocities in the northwest of the range, within the rift, may be related to magmatic processes beneath the Buranga hot springs. Higher velocities are found elsewhere beneath the eastern rift shoulder and are thought to be related to old cratonic crust.

A Preliminary Evaluation of the DDT Contamination of Sediments in Lakes Natron and Bogoria (Eastern Rift Valley, Africa)

Dichlorodiphenyltrichloroethane (DDT) is still used in Africa for the indoor control of malaria and it may represent a potential hazard for wildlife. The littoral sediments of two alkaline-saline lakes, Natron (Tanzania) and Bogoria (Kenya), in the Eastern Rift Valley, supporting large populations of lesser flamingos (Phoeniconaias minor), were analysed for DDT residues. Physical-chemical analyses (temperature, conductivity, pH and dissolved oxygen) were also performed on the water of the two lakes and in the tributaries of Lake Natron, to evaluate the influence of the environmental variables on pollutant occurrence. At Lake Natron, around 1 km from the sediment collection sites, tree leaves of Acacia tortilis were also collected. The main metabolite found in all sediment samples was pp'DDE, whilst equal concentrations of pp'DDT and pp'DDE were measured in acacia leaves. The levels of DDTs measured in the sediments were within 5.9-30.9 ng g(-1) d.w., reaching the maximum value in a tributary of Lake Natron. On the whole, the contamination of Lake Natron and Lake Bogoria basins seems to be quite moderate. Nevertheless, the pp'DDE/pp'DDT ratio equals 1 in the Acacia tortilis leaves, which makes one suppose that the input of the parent compound was rather recent and could have been from aerial transport or dust from relatively close-by old pesticides storage sites.

About this title ‐ The Life and Work of Professor J. W. Gregory FRS (1864–1932): Geologist, Writer and Explorer

Gregory's remarkable career and his scientific work are detailed and critically assessed. Accounts of his heroic 1893 expedition to the Rift Valley (a term he coined) in Kenya (now the Gregory Rift), his first crossing of Spitzbergen, and his resignation as Leader of the first British Antarctic Expedition of 1901, when racing to the Pole under Scott became the priority, draw on unpublished letters. While in Melbourne he published on mining geology and a series of geography textbooks. His 1901 Lake Eyre expedition in Central Australia initiated the phrase ‘The Dead Heart of Australia’ and controversy over the source of artesian water. In the Chair of Geology in Glasgow from 1904, he built up the largest first-year geology class in the UK, over 400 students. He worked in every field of geology and every continent except Antarctica. He was also involved with the search for a ‘homeland’ for the Jews in Libya and Angola. He shrewdly realized that Wegener's Continental Drift Theory erroneously supposed that the Pacific Ocean was wider than now before the Atlantic opened. This led to his influential rejection of Continental Drift. He drowned in Peru traversing the Andes having published over 30 books and nearly 400 articles.

The Ngorongoro Crater as the biggest geotouristic attraction of the Gregory Rift (Northern Tanzania, Africa) – geological heritage

The Ngorongoro Crater is the largest unflooded and not destroyed collapse volcanic caldera of the shield volcano on Earth. It attracts many visitors each year not only because of the undoubted wealth of the wildlife and breathtaking views, but also due to the geotouristic attractiveness of this definite location. The Crater is in fact a specific example of geological processes, relevant to the development of planet Earth. In a relatively small area one can observe rocks of different types and ages: Precambrian igneous and metamorphic rocks, volcanic rocks formed in the Pliocene, Pleistocene, and even nowadays, as well as sedimentary rocks, up to those currently forming within the caldera floor. The origin and development of the Ngorongoro volcano, and lately caldera, is closely related to the activity of rifting processes occurring along the Gregory Rift, belonging to the East African Rift System. It represents one of the three arms of the Afar triple junction associated with the located here hotspot. Due to the geotouristic attractiveness, as well as a richness of living nature and archaeological sites with discoveries of our ancestors, which illustrate an important stage in the history of mankind, the area of the Ngorongoro Crater was designated a UNESCO World Cultural and Natural Heritage.

The Ngorongoro Crater as the biggest geotouristic attraction of the Gregory Rift (Northern Tanzania, Africa) – geographical setting

The caldera of an extinct Ngorongoro volcano is the largest unflooded and not destroyed type of this form on Earth. The depression itself occupies an area of nearly 300 km2, while the Crater walls tower a few hundred metres (400–610 m) above the floor of the caldera. Almost all typical for East Africa plants and animals, as well as rare, endemic and often endangered species can be observed in the crater. The unique richness and diversity of natural world of the Ngorongoro Crater is caused exceptionally by favourable weather and hydrological conditions. These factors depend on local conditions, associated with significant relief of this area. Probably, the most important is the richness of the Ngorongoro Crater in water. There occur springs, perennial and seasonal rivers, marshes, swamps, as well as reservoirs of fresh and salty water. Essential is also the presence of the local autochthonous population of the Maasai people, which raises the attractiveness of that localization adding so important cultural values. Due to its unique natural and touristic values, the Ngorongoro Conservation Area (NCA) has been established in 1959. The area was also included into the UNESCO World Cultural and Natural Heritage List. This paper presents only the geographical setting of the Ngorongoro Crater, which should be understood as its morphology, hydrological and climatic conditions, wildlife and indigenous local people.

The Ngorongoro Crater as the biggest geotouristic attraction of the Gregory Rift (Northern Tanzania, Africa) – geotouristic valorization, touristic development and hazard

The Ngorongoro Crater, as the largest unflooded and not destroyed volcanic caldera on Earth, is one of the major geotouristic attractions of East Africa. Unique on the global scale richness of wildlife, exotic cultures of indigenous people and specific position of the Crater within the East African Rift System, each year attracts thousands of tourists eager for an unforgetable experience. Their number continues to grow from year to year, reaching the value of nearly half a million visitors within the last few years. Constantly developing tourism industry, besides many advantages, also causes a number of risks, both for the world of living nature and inanimate nature objects, as well as for the local population. In 1959, recognizing the unique and special touristic and geoeducative values of this location, the Ngorongoro Conservation Area (NCA) was established. Almost twenty years later, in 1978, the area was included in the UNESCO World Cultural and Natural Heritage List. Furthermore, within the Ngorongoro Conservation Area as well as in its immediate neighborhood there are many objects that also deserve to be called geotouristic attractions, such as: Olduvai Gorge, Crater Olmoti, Crater Empakai and Oldoinyo Lengai volcano. In a relatively short distance from the Crater also the highest mountain in Africa – the Kilimanjaro volcano and the biggest active volcano of this continent, Meru, are located.

Nyerereite from calcite carbonatite at the Kerimasi Volcano, Northern Tanzania

The extinct Quaternary Kerimasi volcano located in the southern part of the Gregory Rift, northern Tanzania, contains both intrusive and extrusive calciocarbonatites. One carbonate mineral with a high content of Na and Ca has been found in a sample of volcanic carbonatite, which is probably a cumulate rock. On the basis of Raman spectroscopy and SEM/EDS, this mineral was identified as nyerereite, ideally Na2Ca(CO3)2. It occurs as solid inclusions up to 300 × 200 μm in size in magnetite and contains (wt. %) 25.4–27.4 Na2O, 26.0–26.8 CaO, 1.6–1.9 K2O, 0.6–1.8 FeO, 0.3–0.6 SrO, <0.4 BaO, 1.4–2.3 SO3, and 0.6–0.9 P2O5. The average mineral formula is (Na1.84K0.08)Σ1.92(Ca1.00Fe0.03Sr0.01)Σ1.04[(CO3)1.91(SO4)0.05(PO4)0.02]Σ1.98. A few inclusions in magnetite also contain calcite, which is considered here to be a late-stage, subsolidus mineral. The occurrence of nyerereite in carbonatite supports Hay’s (1983) idea that some of the extrusive carbonatites at the Kerimasi volcano were originally alkaline rich and contained both calcite and nyerereite as primary minerals.

Landscape genetics of the key African acacia species Senegalia mellifera (Vahl)– the importance of the Kenyan Rift Valley

Acacias across Africa have enormous ecological and economic importance, yet their population genetics are poorly studied. We used seven microsatellite loci to investigate spatial genetic structure and to identify potential ecological and geographic barriers to dispersal in the widespread acacia, Senegalia (Acacia) mellifera. We quantified variation among 791 individuals from 28 sampling locations, examining patterns at two spatial scales: (i) across Kenya including the Rift Valley, and (ii) for a local subset of 11 neighbouring locations on Mpala Ranch in the Laikipia plateau. Our analyses recognize that siblings can often be included in samples used to measure population genetic structure, violating fundamental assumptions made by these analyses. To address this potential problem, we maximized genetic independence of samples by creating a sibship-controlled data set that included only one member of each sibship and compared the results obtained with the full data set. Patterns of genetic structure and barriers to gene flow were essentially similar when the two data sets were analysed. Five well-defined geographic regions were identified across Kenya within which gene flow was localized, with the two strongest barriers to dispersal splitting the Laikipia Plateau of central Kenya from the Western and Eastern Rift Valley. At a smaller scale, in the absence of geographic features, regional habitat gradients appear to restrict gene flow significantly. We discuss the implications of our results for the management of this highly exploited species.

The Lava sequence of the East African Rift escarpment in the Oldoinyo Lengai – Lake Natron sector, Tanzania

A 500 m sequence of horizontal lava flows forms the Gregory rift escarpment of the western rift shoulder between Lake Natron and Oldoinyo Lengai. A detailed volcanic stratigraphy of this >1.2 Ma evolution of the EAR in Northern Tanzania is presented. The sequence is formed by several distinct rock suites, with increasing alkalinity from base to top. Alkali olivine basalts of the Waterfall Sequence at the base are followed by a basanite series, and by a range of evolved nephelinites forming the upper part of the escarpment. Numerous dykes and Strombolian scoria deposits indicate local fissure eruptions as opposed to or in addition to more distant sources. Primitive compositions within each of the series indicate variable candidates for primary magmas. The composition of the basanite suite ranges from primitive mantle melts (high Mg#, Cr, Ni) to more evolved rocks, in particular hawaiites, generated by fractionation of olivine, pyroxene and magnetite. Inter-bedded within the basanite suite, one single olivine melilitite flow with high Mg# and abundant olivine and pyroxene megacrysts is the only primitive candidate for the nephelinite suite. However, in view of the large compositional gap and marked differences in incompatible element ratios, a relation between this flow and the nephelinites remains hypothetical. The variation within the evolved nephelinite series can be partly explained by fractionation of pyroxene, apatite, perovskite (and some nepheline), while magma mixing is indicated by zonation patterns of pyroxene. The most evolved nephelinite, however, differs significantly from all other nephelinites in major and trace elements. Thus the entire sequence is petrologically not a coherent evolution, rather the result of different mantle melts fractionating under variable conditions. Carved into the rift scarp of the study area west of Engare Sero is a young explosion crater, the Sekenge Crater. Sekenge Tuffs are olivine melilitites, similar to other craters and maars of the “Younger Extrusives” on the rift valley floor surrounding Oldoinyo Lengai. Further, still younger alkaline tuffs are found on the top of the rift shoulder.