A video and photos showing deep chasms opening in Kenya’s Narok and Suswa County have quickly gone viral on the internet. (Read Trevor Nace’s story here.) One tear in the earth, stretching several miles, is reportedly fifty feet deep and sixty feet wide.
Local geologists have explained the chasms in part as fossil faults, covered previously by volcanic ash from the nearby Mount Longonot. Strong rainfalls have washed the material filling the faults away. However, some appear to be geologically speaking very recent, likely associated with tectonic stress or a magma intrusion in the underground. This area is part of the Great Rift Valley, where Africa is slowly pushed and pulled apart by uprising material in earth’s mantle. Eventually, the rift will become deep enough to host a new ocean, separating Eastern Africa from the rest of the continent. The Great Rift was discovered almost hundred years ago by an intrepid geologist.
John “Jack” Walter Gregory was born in London in 1864. His interest in the natural world and adventures emerged in his early years of study. During his later scientific career he would visit Europe, Africa, Australia, India, North- and South America, even the remote islands of Spitsbergen in the Arctic. His interest in geology came from the necessity to know where he was:
… my attention was first directed to geology in order to understand the geography of the districts through which I rambled, and the, often, apparently erratic course of the rivers … and to understand local topography.
After graduation Gregory found work at the British Museum for Natural History, where he worked on the collections of rocks and fossils.
In October 1892, Gregory was asked if he would join an expedition to East Africa to explore the area of modern Kenya, Ethiopia and Somalia. This area was at the time still poorly known to Europeans and the origins of the mountains and valleys of the African continent were an unexplained geological mystery. Austrian geologist Eduard Suess summarized in 1891 the results of an expedition to Lake Rudolf by Count Samuel Teleki and suggested that the valleys and mountains crossing Africa were the results of periodic tectonic movements.
In November 1892, the expedition unloaded 300 tons of equipment in the harbor of Lamu. Hundred and ten camels and forty donkeys were needed to transport the material and three hundred soldiers were hired to protect the caravan. Destination of the expedition was Lake Rudolf.
Despite all the equipment soon it became clear that there were not enough supplies to feed all the men. Fever and various diseases spread among the members of the expedition. Gregory was first plagued by ulcers on the legs that immobilized him for weeks and in January 1893 he fell sick with malaria. In Mombasa, the expedition was officially canceled.
Gregory, still interested to see for himself the great valley of Eastern Africa, decided to take advantage of this situation. He was already in Africa and the equipment of the abandoned expedition could still be useful. With the financial help of his family and the British Museum he organized a new expedition. March 1893 the new expedition, comprising this time just forty-one men, left Mombasa. Gregory loved to walk alone for miles, far ahead of the caravan, collecting specimens of plants, animals and rocks.
… the geology was so tempting that I went off alone. By this time the men were accustomed to my going by myself, for I did so whenever the country was safe and the next camping-place easy to find. These solitary rambles were to me the most delightful incidents in the expedition. Free from the bother of the caravan, I could climb a mountain, track a river, visit a neighbouring lake, chase butterflies, and collect plants as careless as a schoolboy.
The main geological work was carried out from the village of Njemps on the shores of Lake Baringo, where they mapped the geology of the western wall of the Kamasin Scarp. Gregory confirmed Suess interpretation of the tectonic origin of this valley and deduced from the weak erosion seen on the mapped faults that this process must have been very recent. But Gregory recognized also similarities between the geology of the Red Sea and the African Rift Valley and proposed that these landscapes are connected. The dimensions of this feature, splitting apart an entire continent, surprised him. This was for sure a major element in shaping earth’s crust. Gregory described this discovery in an article published in the journal of the Royal Geographical Society in 1920 and in later books, where he coined also the modern term of Rift Valley and connected it to tectonic movements of the earth. This was quite a revolutionary explanation at the time when most valleys were regarded as a result of erosion.
For this type of valley I suggested the term Rift Valley, not implying that the whole valley was formed by the two sides being simply pulled apart, but as a breach due to a subsidence between two series of rents.
As both sides of the Great Rift are pulled apart, blocks slide down along vertical faults, forming the steep cliffs delimitating the rift valley. The tectonic movements and faults are also associated with volcanic activity. Many active, dormant and extinct volcanoes are like the pearls of a necklace located along the borders of the rift valley, following major fault systems. Near lake Kivu, a killer lake formed by tectonic movements, the two most active and dangerous African volcanoes can be found, the Nyiragongo and the Nyamuragira.
The Nyamuragira erupted between 1894 and 1979 fifteen times, fortunately without victims. The last eruption of the Nyiragongo in the Democratic Republic of Congo in January 2002 killed more than one hundred people and caused great havoc in the nearby city of Goma. The Great Rift also hosts one of the strangest volcanoes on planet earth. A rock made up almost completely of calcite may be a sedimentary limestone or marble rock, just not in the case of the Ol Doinyo Lengai volcano located in the Gregory Rift (named so after geologist Gregory), within the Arusha Region of Tanzania. Carbonatite is an almost unique type of lava, composed of the minerals calcite and feldspar, a very unusual combination, as in most lava quartz is the dominant mineral.
Only a few localities are known with fossil carbonatite lava and the Ol Doinyo Lengai is the only active volcano erupting this type of lava. It is believed that common magma stuck in the underground of the rift is slowly melting sedimentary rocks, forming the source of the carbonatite lava.
The Great Rift maybe played also an important role in our evolution. Many sites where fossils of early hominids were found are located in or near the rift valley. Maybe the rift formed an ideal landscape for our ancestors or, in contrast, as the rift slowly widened, forming a barrier for moisture from the Indian ocean, it forced our early ancestors to leave the dying pluvial forests. Venturing out into the open and dry savannah they adopted a bipedal locomotion and as their hands became free for the use of tools, slowly the brain evolved becoming larger and larger over time. Research published last year suggests even that a surge in volcanic activity along the East African Rift System might have forced early humans out of Africa, altering the course of our evolution forever.
