East African Fault (rift valley)

Plate tectonics and fault formation

The separation of the African tectonic plate from the Arabian plate is a continuation of the decay of the supercontinent Gondwana and began about 30 million years ago. The space of the rift began to fill the water and the Red Sea was formed.

The fault in the north of the African plate began to form 22-25 million years ago.

There was a long-term development of the rise of East Africa, associated with the Kenyan rift, which reveals the volcanoes Karisimbi and Kenya located slightly north of Kilimanjaro. Initially, 20 million years ago, this area was covered by fractured volcanism: lava poured out through cracks and channels in the earth's crust, forming basalt shields and plateaus. About 6 million years ago, the first volcanoes grew on the site of erosive shields. Then the rift first opened, and early volcanoes were located on the bottom and sides of its valley.

As the rift grew, generations of stratovolcanoes (layered) changed. During the formation of intra-rift vaults, giant shield volcanoes arose within their limits, including the well-known Ngorongoro. These processes occur between 3 and 1 million years ago. And only at one of the final stages of the formation of the rift valley appeared Kilimanjaro^[1].

The period of violent volcanism south of Lake Natron falls on the time interval from 5 to 2 million years ago. At this time, the highlands formed with the Ngorongoro Crater and all the main cones that have survived to this day.

Apparently, the local eruptions took place according to the Hawaiian type, which means they were accompanied by slow outpourings of liquid lavas. Such lava, which contains little silica, spreads quickly. As they set, they turned the upward-looking peaks of volcanoes into flat, rounded platforms.

The formation of the highlands ended shortly before the beginning of the ice age, about 1 million years ago. Due to frequent eruptions, the local volcanic mountains were formed very actively and today are located too close to each other. The space between them is mostly flooded with volcanic lava^[1].

Today, the African tectonic plate continues to be in the process of splitting into two protoplytes - Nubian and Somali - at a speed of 6-7 mm per year^[2].

In 3-4 million years, geologists expect the Red Sea to break through the ridges surrounding the Afar basin. In 10 Ma, across the entire fault for 6,000 km, East Africa (Somali Plate) will separate from the main part of the continent, turning into an island that will move towards the Arabian Peninsula. When they collide, mountains form, and the Red Sea will lengthen 3 times and begin to turn into an ocean.

Geography

Historically active volcanoes (red triangles) and the Afar Triangle (shaded area in the center), where the Arabian Plate and two parts of the African Plate - Nubian and Somali - stretch the East African Rift Zone

The East African fault, connecting with the Red Sea rift, passes through the Afar Triangle in Ethiopia and then dissects all of East Africa through, Kenya,, Uganda,, Tanzania, and Rwanda. Burundi Zambia Malawi Mozambique

The width of the valley is from thirty to one hundred kilometers, the depth is from several hundred to thousands of meters.

Two chains

The East African Fault (rift valley) is divided into two chains.

The western rift includes:

• Virunga Mountains,
• Mitumba Mountains,
• Ruwenzori Mountains.

The eastern chain consists of the highest peaks in Africa:

• Kilimanjaro and
• Kenya Mountains (5,199 metres) as well as
• Elgon Mountains in Kenya and Uganda.

All of the mountains listed except Ruwenzori are^[3] volcanic origin].

Active volcanoes

The East African Fault system also has active volcanoes:

• Erta Ale,
• DallaPhylla and
• Ol-Doinho-Lengai^[2].

The main hotspots of the Earth. Red indicates the most active points and one of them Afar

The East African fault is today the most seismically active rift system on the planet.

Although most of these mountains lie outside the rift valley itself, they owe their appearance to the East African Fault.

Пейзаж внутри Восточно-Африканской рифтовой долины (Северная Tanzania). On the left is the southern part of Lake Natron, further is the volcano Ol Doinho Lengai, on the right is the inner edge of the fault, in the background is the Ngorongoro Highlands

Effect of changes in the valley on human evolution

7 million hp: The ridges of the valley block the fault zone from the rainforest and force human ancestors to go straight

Tectonic movements led to the rise of the valley ridges, and this area was fenced off from another part of Africa, where moist rainforests remained. Savannas and semisavannas formed in the fault zone due to a drier climate, so distant human ancestors were forced to go straight to survive. In addition, active volcanism in these territories provided the opportunity to find material for the manufacture of stone tools. Volcanic ash fertilized the soil and attracted herds of animals - the feed base of early human forms.

320 thousand hp: Tectonic activity in the valley led to the transition of the ancestors of homo sapiens to the Mesolithic

In 2020, archaeologists analyzed a new sedimentary drilling core and published the results in the October issue of Science Advances. From it, scientists were able to study about 1 million years from the history of the environment in the East African Rift Valley^[4].

The analysis found that while early humans were abandoning older tools in favor of more sophisticated technologies, the landscape of their habitat was undergoing major changes. For example, vegetation changed, interruptions in water supply appeared, that is, the reliability of the usual resources was shaken. It is the instability of the surrounding climate and ecosystem that has been a decisive factor in the development of new traits and behaviors underlying human adaptation.

This behavioural and cultural shift was documented back in 2018 based on artifacts found at the Olorgesaili archaeological site. Then, according to the findings, scientists determined that the first people in Olorgesaili used the same tools and stone axes for 700 thousand years! Their lifestyle during this period was surprisingly stable - there were no major changes in their behaviour and survival strategies. Then, about 320 thousand years ago, these people began to create more complex weapons, including shells, which today is defined as a transition to the Mesolithic. At the same time, they began to trade their resources with other groups of people and even use coloring materials in everyday life.

Then scientists suggested that climate fluctuations could encourage people to develop such a quality as adaptability. But new research suggests the picture is much more complicated. Climate fluctuations are just one of several factors that led to the cultural shift described in 2018. Analysis of rocks shows that climate change, together with new land faults caused by tectonic activity and ecological disturbances in flora and fauna, have caused changes in the evolution of early humans in the region. It was they who provoked technological innovations, the start of trade and communication using symbols.

The authors of the work say that the entire history of human evolution is a story of increasing adaptability.

The Fault Movies

• Great Rift (Der grosse Graben), Austria, 2009
• Disappeared people, 2016

See also

• Paleolithic in Russia
• Neanderthal men

Notes