As you drive up towards the Ngorongoro Crater from the flat plains of the Serengeti, the searing heat begins to lift and the air gets cooler. The road becomes rougher and the scenery surprisingly mountainous, and it is easy to realise that you are climbing the steep sides of what was once a 5,000m high volcano. Small encampments of Maasai appear, with their tiny huts and bright red and purple cloths, and cows mingle with giraffes on the hillside.
Then you turn the final corner and spread out before you is one of the most famous yet still awe-inspiring and spectacular views in the world. About 600m below is the flat floor of the caldera, 20 km across and dramatically encircled by the steep crater walls. This natural enclosure is home to about 25,000 wild animals, the most densely packed concentration of wildlife in Africa. Nearly half a million tourists visit the Ngorongoro Conservation Area annually, most to view the astonishing microcosm of East African wildlife within its boundaries, but few probably realise just what a geologically unique area they are looking at.
Extensive Rift System
The Ngorongoro Conservation Area lies in the eastern branch of the East Africa Rift, part of the extensive rift system which resulted from the east - west separation of the Nubian and Somalian tectonic plates. Rifting started in the Early Miocene and is still continuing today, making the East African Rift system the most extensive active continental extension zone in the world.
The two branches of the East African Rift system have very distinct features, the western one being characterised by high mountains and some of the deepest freshwater lakes in the world. The eastern branch, by contrast, is dominated by a number of dormant or extinct volcanoes, which include the famous Mounts Kenya, Elgon and Meru, not forgetting Kilamanjaro, at 5,895m the highest mountain in Africa and one of the most iconic sites on the continent.
This volcanic activity is also responsible for the basalts of the Crater Highlands. Lava erupted from a number of vents, including from a volcano which eventually exploded 2.5 million years ago and left behind a vast, 260 km2, steep-sided basin: Ngorongoro Crater.
The crater is predominantly composed of basaltic (low silica) and trachytic (high silica content) lavas, all extruded between 2.1 and 2.8 million years ago. Flows attributed to the volcano have been identified 80 km away on the Serengeti Plains.
Ngorongoro, one of the largest unbroken calderas in the world, is especially remarkable because, unlike the majority of such features, it is not flooded. Its flat floor contains a small saline lake, the main water source for the animals living in the crater. It is also the daylight home to thousands of flamingos, their vibrant rosy hues, the result of grazing on algae from the lake floor, making the distant lake shimmer pinkly in the sunlight.
The majority of the animals living in Ngorongoro are ruminants, grazing on the rich grasslands of the crater floor, and include zebra, gazelles, eland and warthog, although giraffe, impala and topi are not found. There is also a herd of elderly bull elephants. The swamplands host hippos and many species of birds, while baboons, monkeys, waterbucks and bushbucks are found in the rain forest on the southern and eastern slopes.
Ngorongoro is also one of the few places where it is possible to see wild black rhinos. Subject to intense hunting and poaching, these have declined from 100,000 in Africa in 1960 to a mere 2,400 in 1995, by which time Ngorongoro was one of the few pockets of rhino in Tanzania. With careful management and the use of armed guards to reduce poaching, numbers have slowly begun to rise, and there are now about 20 within the crater.
With all this tempting prey, there are many carnivores, including several of prides of lions. With plentiful food sources, they rarely leave the crater, and the dominant males, known for their long black manes, deter any would-be newcomers, so inbreeding is a potential threat. There are also leopards, cheetahs and serval cats, as well as many hyenas.
Unique Oldoinyo Lengai
Although many visitors are attracted to the scenery and wildlife of Ngorongoro and the Serengeti, not many make it to Oldoinyo Lengai, which is, in geological terms, an even greater wonder, as it is Earth’s only active carbonatite volcano. This alkaline lava contains 50% carbonate (CO2-bearing) minerals and less than 10% silica and are almost always found in continental rift-related tectonic settings.
Most carbonatite lava flows are unstable and react quickly in the atmosphere, which explains their relative rarity in the geological record. Uniquely in Africa, the Oldoinyo Lengai magmas also have an unusually high amount of sodium, up to about 35%, making the them solid rather than gaseous at the surface, so the lavas are very thin and flow like water. Carbonatite lava is also much cooler than other lavas, being only about 510°C (950°F), in comparison to basaltic lavas at over 1,100 °C (2,000°F). Contact with moisture rapidly turns it white, resulting in the apparent ‘snow cap’ of the volcano when seen from the air.
Recent studies based on gas emissions at Oldoinyo Lengai suggest that a very small amount of melting of the Earth’s mantle from beneath mid-ocean ridges can produce carbonatites, and that the CO2 probably comes directly from the upper mantle, just below the East African Rift.
Oldoinyo Lengai – ‘Mountain of God’ in Maasai – is the youngest volcano in this part of the Rift Valley and is less than 370,000 years old.
Cradle of Mankind
The erupting volcanoes that formed the Ngorongoro Highlands may well have been watched by some of the earliest prehistoric men. About 40 km north-west of the crater is the world-famous Oldupai Gorge (also known as Olduvai), which contains some of the oldest evidence of hominoids, preserved by the abundance of volcanic ash. The lava layers have been subdivided into seven formations, ranging in age from two million to 15,000 years ago. These are underlain by volcanic tuff, probably derived from Ngorongoro, and then by the Precambrian metamorphic gneisses and quartzites which make up most of the African continent.
The climate in this part of Africa in the mid-Pliocene was wetter than it is now and Oldupai had springs, abundant vegetation and a large freshwater lake, making it very attractive to grazing animals – and to the early upright hunters which preyed on them. Successive layers of ash rapidly buried and preserved the remains of both animals and hominoids, and also artefacts such as pebble choppers and scrapers, the oldest recognizable tools on earth. Pleistocene earth movements and faulting resulted in the diversion of a stream which eroded down through these sediments, creating a deep ravine and exposing a 90m cliff of lava layers filled with fossil and archaeological remains.
The most famous fossil found in Oldupai is probably the 1.8 million year old ape-like skull known as Australopithecus boisei, often referred to as ‘nutcracker man’, due to its large molars. However, it is thought that several hundred species of hominoids may have lived in this region two million years ago, including Homo erectus which eventually evolved into modern man.
About 45 km south of the Gorge is Laetoli, where in 1975 Mary Leakey found 3.75 million year old fossilized footprints, proving that hominids walked on two feet. Their preservation, one of the greatest palaeoanthropological discoveries ever made, is once again due to the abundant volcanic ash.
We have barely touched on the many wonders of the Ngorongoro Conservation Area, which spans mountains, plains, woodlands and forests. There are the wide expanses of the Serengeti National Park, where the astonishing annual migration of nearly two million wildebeest can be followed; or the emerald green crater lake at Empakaai; Lake Eyasi, forming the faulted south-eastern edge of the NCA; the ‘Shifting Sands’, composed of fine black ash from Oldoinyo Lengai; or Nasera Rock, a granite monolith rising 80m above the plains and a meeting place for centuries.
All once again demonstrating that geology is not only responsible for shaping the earth around us, but also lies behind many of the natural and anthropological wonders of the world.
Acknowledgement: Many thanks to Dr. Joe McCall and Prof J. B. Dawson for assistance with this article.