Geology of National Parks of Central/Southern Kenya and Northern Tanzania

244 Pages·2018·30.263 MB·English

by  Roger N. Scoon

Roger N. Scoon
Geology of National
Parks of Central/
Southern Kenya and
Northern Tanzania
Geotourism of the Gregory Rift Valley,
Active Volcanism and Regional Plateaus
Geology of National Parks of Central/Southern
Kenya and Northern Tanzania

Roger N. Scoon
Geology of National Parks
of Central/Southern Kenya
and Northern Tanzania
Geotourism of the Gregory Rift Valley,
Active Volcanism and Regional Plateaus
123
RogerN.Scoon
Department ofGeology
RhodesUniversity
Grahamstown
SouthAfrica
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Coverphotographs:Frontcover(andp.ii):TheAshConeinthegiantcalderaofMountMeru,ArushaNationalParkin
northernTanzania,whichlasteruptedin1910(photographbytheauthor).Backcover:Theauthorandhiswife.
Printedonacid-freepaper
ThisSpringerimprintispublishedbytheregisteredcompanySpringerInternationalPublishingAGpart
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TheWesternEscarpmentoftheGregoryRiftValleynearLakeNatron,northernTanzania,asviewedlookingnorthwardsfromthe
slopesofOldoinyoLengaiismadeupofmultiple,near-horizontallayersoflavaandtephra.Thebraideddrainagesysteminthe
valleyisindicativeofarecentlyformedlandsurface(byR.N.Scoon)
Asatelliteimageofcentral/southernKenyaandnorthernTanzaniarevealsthegreatdiversityofecosystemsintheregion.Thetwo
principal geological phenomena are the occurrence of ancient regional plateaus, e.g. the Serengeti and Tsavo Plains and the
GregoryRift.ThelatterconstitutesanarrowlinearriftvalleyboundedbyescarpmentswithinKenya,e.g.theMauEscarpment,but
divergesintoseveralbranchesinnorthernTanzania.Acharacteristicfeatureoftheriftvalleyisthechainofribbon-shaped,alkaline
lakes, e.g. Lakes Natron, Manyara and Eyasi. Lake Baringo with its rounded profile and freshwater is unusual. Many of the rift
platformsjuxtaposedtotheriftvalleyarecappedbygreatthicknessesofplateau-stylevolcanicoutpourings,e.g.thoseforming
theAberdareRange.Riftplatformsalsoincludegiantvolcanicedificeswithsuccessive(annular)botanicalzonesthatincludedense
montaneforestsonthelowermostslopes,e.g.MountsElgon,KenyaandKilimanjaro.Theexplosivestyleofvolcanismthattypifies
continental rifting has resulted in some cones being capped by giant calderas, e.g. Ngorongoro and Menengai. Active volcanic
conesoccurbothintheriftvalley,e.g.OldoinyoLengai,andonriftplatforms,e.g.MountMeru.Source:Cloud-freecompositeimage
basedonTerra-MODISsatelliteimagesofNASAcourtesyofPhilipEales(PlanetaryVisions),withdigitalterraneshadingcreated
usingArcGIS®softwarebyEsribyOliverBurdekin(birdGIS).
National Parks, Reserves and Conservation Areas
Central and Southern Kenya:
Aberdare;Amboseli;ChyuluHills;Hell’sGate;LakeBaringo;LakeBogoria;LakeNaivasha;
Lake Nakuru; Mount Longonot; Mount Elgon*; Mount Kenya; Tsavo West
Northern Tanzania:
Arusha (Mount Meru); Kilimanjaro; Lake Eyasi; Lake Manyara; Lake Natron and the
Oldoinyo Lengai Volcano; Ngorongoro; Oldupai Gorge and Laetoli; Serengeti
*Includes a park in Uganda.
vii
Preface
The national parks, reserves and conservation areas of central/southern Kenya and northern
Tanzania host some of the largest and most diverse concentrations of wildlife remaining on
Earth. Theyalso have remarkablepotential for geotourismandcreationofgeoparks.1Many
of the most spectacular landforms are associated with the dominant geological feature of the
region,theEastAfricanRiftSystem(EARS).KenyaandnorthernTanzaniaaretransectedby
theGregoryRift,theeasternbranchofasystemthatextendsinitsentiretyforover6,000km.
Themostprominentexpressionoftheriftisanarrow,down-faultedlinearvalleyborderedby
spectacular escarpments (Frontispiece 1). The EARS also constitutes one of the largest vol-
canic provinces on Earth. Sections of the rift valley resemble lunar landscapes in areas of
particularly intensive volcanism. An unusual feature of the Gregory Rift is the occurrence of
giantvolcanicedificesontheriftplatforms(Cover).Asatelliteimage(Frontispiece2)reveals
many of these features including the chain of ribbon-shaped lakes which characterise the rift
valley.
TheriftplatformswhichconstraintheGregoryRiftValleyhavebeenupliftedtoelevations
sufficiently high as to report a temperate climate with montane forests, despite the equatorial
setting. Montaneforestsalsooccuronthehigher volcanic cones,severalofwhichareamong
thelargestfree-standingmountainsonEarth.Thetwohighestpeaksintheregion,Kilimanjaro
(a dormant volcano) and Mount Kenya (an extinct volcano), are capped by the relicts of
fast-melting icefields and glaciers. The region includes two ‘Natural Wonders of the World’,
the Serengeti Plains, famed for the biannual migration of almost two million grazers, and the
NgorongoroCaldera,aself-containedreservecreatedbyacatastrophicvolcaniceruption.The
palaeoanthropological sites of Oldupai Gorge and Laetoli, where some of the most important
discoveries of hominin fossils and footprints have been made, are located between Ngoron-
goro and the Serengeti.
The region is underlain in some areas by crystalline metamorphic rocks of the basement
complexes.TheSerengetiPlains,forexample,revealsexposuresoftheLakeVictoriaTerrane
whichcontainssomeoftheoldestrocksonEarth(oldestagesreportedinthisareaare2.810Ga).
TheTsavoPlainsincludeexposuresoftheMozambiqueBelt,anextensivegroupofrockswith
an average age of 800–500 Ma. A characteristic feature of the East African landscape is the
occurrence of regional plateaus; they formed due to repeated episodes of uplift and erosion
associated with the breakup of the ancient supercontinent of Gondwana (commencing at
approximately 180 Ma). It is these cycles of uplift and erosion which enabled the basement
complexestobeexposedonsurface.TheEARSisarelativelyrecenteventwhichcommencedat
approximately30Ma.RiftingisstretchingandthinningtheAfricanPlate.Themainperiodsof
faulting in the Gregory Rift are even younger (3.7–1.4 Ma). Continental rifting is invariably
associated with the alkaline style of magmas, and volcanism of the EARS is further charac-
terisedbyhighlyexplosivePlinian-styleeruptions.Numerouscatastrophiceventsarepreserved
1Geopark: A Geopark is a unified area that advances the protection and use of geological heritage in a
sustainable way, and promotes the economic well-being of the people who live there. There are Global
GeoparksandNationalGeoparks(Wikipedia).
ix
x Preface
inthegeologicalrecord;theseincludegiantcalderasandsectorcollapseswhichhavegenerated
among the largest debris avalanches recorded. The high sodium content of these magmas,
together with restricted catchments and high rates of evaporation, resulted in the extreme
alkalinityofmanylakesintheGregoryRiftValley.
The interrelationship between wildlife and geology is emphasised. The relatively youthful
volcanic uplands, for example, provide a range of ecosystems and supply groundwater to the
much older regional plateaus that would otherwise be far too arid to support large concen-
trationsofwildlife.Theintensityofvolcanismisnotcompatiblewiththegeneralisedviewof
endlessplainsandvastherdsthatmigrateoverancient,unchangedlandscapes.Thegeological
record provides evidence of persistent changes. Recent eruptions of the active Oldoinyo
Lengai Volcano have resulted in the spread of ashes onto the Serengeti Plains which support
the nutrient-rich short grasses on which the migrating grazers congregate annually. The epi-
sodic nature of the rifting and volcanism produced smaller and smaller geological terranes.
This may have resulted in the remarkable diversity of species for which East Africa is so
renowned. Anomalously rapid speciation may have been triggered as a reaction to these new
terranes as well as to the unusually high concentrations of radioactive minerals. Initiation of
new species by Darwinian evolution during the Pliocene and Pleistocene epochs (5.3 Ma–
11,500 BP), including hominins, occurred in remarkably short time intervals during extreme
climatic cycles (e.g. the Ice Ages) and intense volcanism. The Holocene epoch represents a
relatively quiescent period in which Homo sapiens thrived during a time of modest climatic
cycles and less intense volcanism.
Grahamstown, South Africa Roger N. Scoon