The Great Rift Valley of Eastern Africa


Above Image (Hormann 2007).

by Susie Chase and Jessica Petree

University of Washington, Tacoma, Interdisciplinary Arts and Sciences

Date Submitted: May 25, 2007


TESC 243-- Geography of the Physical Environment, Spring Quarter 2007


The Great Rift Valley is a large complex divergent plate boundary on the Eastern side of Africa, which extends a total distance of 3,000 kilometers. The Great Rift Begins in the Southwest portion of Asia and stretches all the way down to Mozambique. Along the axis of the Great Rift Valley, the earths crust is being lifted and spread apart in a long, ridge like fashion. The Rift Valley system consists of a large number of finger-like-troughs, each ranging in width from about 30 to 60 km, making up the complex system of plat tectonic activity, diverse climatic regions, and the site of anthropological wonders. (Strahler and Strahler 2006).

Near the Red Sea lies an area of the Great Rift Valley referred to as the Afar Depression, or the Afar Triangle, which is a geological depression in the Horn of Africa, overlapping Eritrea, the Afar Region, and Djibouti. This region is where the Arabian, Nubian, and Somalian plates meet, and is home to much plate tectonic action. Earthquakes are a regular occurrence in this area, due to the rifting of plates and geothermal activity.

Plate Tectonics

The Afar depression, which is located in the Eastern region of Africa in the country of Ethiopia and is part of the Great Rift Valley, is considered to be an active plate tectonic phenomenon. According to Strahler and Strahler (2006), Plate tectonics is a theory of tectonic activity dealing with lithosphere plates and their activity. The Afar Depression was created twenty million years ago because of faults in the region opening and the interior landmass lowering between the three plates. Since then this area has been the site of geological, geographical and anthropological wonders. 

Image from Hormann (2007).

 The Great Rift Valley lies on a continental divergent plate boundary. Divergent plate boundaries are where the earth's rigid lithosphere is being separated due to activity in the flexible asthenoshere and is creating new lithosphere. Because of this rifting process, the Great Rift Valley is constantly changing and "pulling from East and West to cause the North-South divide" (Smith 1988). In concurrence with the North South divide, the Red Sea is now said to be widening by about 2.5 centimeters a year, which would mean an increase of 25 kilometers in only a million years (Smith 1988). The occurrence of both these phenomenon's in junction with increasing accumulative displacement is a definite threat to this region. This occurrence could possibly cause the separation of the African Horn someday, which would change the continent of Africa forever.


The Afar depression is the second most active area of volcanic activity in the Great Rift Valley and one of the most hazardous areas in the world (Smith 1988). Volcanism is one of the major outcomes of the divergent plate activity in this region and a major player in the topography in the surrounding region. This region is littered with volcanoes and volcanic activity, yet most of the volcanoes associated with this region are currently extinct.

Erta Ale, one of the more well known volcanoes of the Great Rift Valley, resides in the Afar Depression. It lies in the northern region if the Afar Depression and has an elevation of 2,011 feet.  Erta Ale is classified as a shield volcano, which according to Strahler and Strahler (2006) means it is a volcano with, "low, often large, dome-like accumulation[s] of basalt lava flows emerging from long radial fissures on flanks". Erta Ale is an active volcano and has had several eruptions in the last hundred years or so. The most recent eruption happened in 2006.It is more recently more well know due to its active lava lake in its summit crater (Department of Geosciences at Oregon State University 2003). Because the conditions of the area around the volcano are so dangerous, it has been extremely difficult for scientist to study this area.


Image on the left, from Gunn (2006).

Image on the right, from Department of Geosciences at Oregon State University (2003).


Because of the diverse topography of the landscape and location in the Tropical zone, rainfall and temperature patterns are diverse in Ethiopia. According to the Koppen-Geiger system of climate classification, Ethiopia is positioned in the low-latitude climate zone and most of the areas would be classified as dry tropical or highland. In the dry tropical areas of Ethiopia, climate is marked by strong temperature cycles accompanied by extremely hot temperatures during the season when the sun is high. Rainfall is not abundant in these areas but still occurs when the Intertropical Convergence Zone is near. Temperature range varies due to the dry air and overall lack of cloud cover (Strahler & Strahler 2006). Due to the rifting the majority of Ethiopia would be classified as a highland climate because the rifting is causing the majority of the land to experience increased elevation. Examples of this would include plateaus, mountains, and volcanoes. The highland areas of Ethiopia are "cool to cold, moist climates that occupy mountain and areas of high plateaus" (Strahler & Strahler 2006). As a general rule, the higher the elevation, the lower the temperature. Precipitation is abundant in these areas.

Image from FAO-SDRN-Agrometeorology Group (1997).

There is a wide variation in precipitation throughout the country because of the differing elevations and seasonal changes in surrounding atmospheric pressure. Because of these two factors, several regions receive rainfall throughout much of the year, while in other areas precipitation is only seasonal. There are three different environmental zones, including cool, temperate, and hot. The cool zone consists of the Western, Eastern, and central portions of the Northwestern plateau. The elevation in these areas are usually above 2,400 meters, the average daily highs range from freezing to16 degrees Celsius, nights are usually cold, and warmth quickly decreases by afternoon. The temperate areas of Ethiopia lie between 1,5000 and 2,4000 meters in elevation and daily temperatures in this region range anywhere from 16 degrees Celsius to 30 degrees Celsius. The hot zones encompass areas, which are lower than 1,500 meters; these include such areas as the Denakil Depression, the Eritrean lowlands, the Eastern Ogaden, the tropical valleys of the Blue Nile, the tekeze rivers, and the areas along the Sudanses and Kenyan borders. During the day, conditions are extremely hot and arid and average temperatures range from 27 degrees Celsius to 50 degrees Celsius. There is a wide variation in precipitation throughout the country due in large part to the differing elevations and seasonal changes in surrounding atmospheric pressure. Because of these two factors, several regions receive rainfall throughout much of the year, while in other areas precipitation is only seasonal (LaVerle, B. and Ofcansky, T. 1991 ).

This Graph shows the monthly average of precipitation and temperature of Dire Dawa, Ethiopia, which is located in the Afar Depression.

The change in the earth's climate has recently been of great concern. Global warming, due to increased green house gas emissions, is causing an increase of the earth's average temperature as well as influencing other aspects of climate. Sub-saharan Africa, the location of the Great Rift Valley, has suffered greatly because of this change in climate (The Economist Print Edition 2007). The temperatures are getting more extreme and the desert area is expanding. Water scarcity is becoming a major issue in this region of the world. Rainfall is becoming less consistent and because of this all forms of life are suffering. Vegetation in the area is at great risk as the land becomes drier and the animals will also suffer as well. Lack of water is of great concern for people as well. Women and children are having to travel longer distances to gather water for their families. Africa has some of the highest rates of poverty in the world. With lack of resources and knowledge to help fight for environmental equity, the people of Africa are being placed in a very dangerous situation.

Image from the Economist Print Edition (2007). 

From Apekind to Mankind: Human Evolution in the Great Rift Valley

Over time, the remarkable evolution and superb ability of apes to adapt to a variety of complex and diverse environments has spurred the evolution of mankind, making the Great Rift Valley one of the settings to the rise of Australopithecus and Homo, as we know them today (Smith, 1988). The Great Rift Valley is most famously known for its geological, paleontological and archeological wonders, being home to many fossil records, which changed the world as we know it and our knowledge base forever.

The abundant quantities of fossil records in this region can be attributed to "the slow sedimentation of Eastern Africa's rivers, the constant changes in drainage and faulting that accompanied the tectonic shifts," the unencumbered nature of the fossil beds, and the constant renewal and exposure of unseen land by erosion creating new layers of earth to be examined (Smith, 1988). One of the most famous fossil records discovered in this area is Lucy, a Hominid species known formally as Australopithecus Aferensis, which lived in northeast Africa, in the Hadar region. The Leaky family in Ethiopia discovered Lucy, in the year 1974. Until 1995, this species was the earliest known member of the Hominid family. Australopithicus Afarensis lived from approximately to 2.7 million years ago along the northern Rift valley of east Africa. Lucy was the first complete adult skeleton found of this particular species. Fortunately for us, Lucy, and other creatures of the past, managed to die in some of the best conditions for fossilization anywhere in the world. Thus, creating some of our best information from the past, to enrich the knowledge and understanding of today (Smith, 1988). More impressive then the sheer extent of the fossil records found in this region, is the driving forces behind the evolution and creation of man.

Image of Lucy from Veneziano (2006).

According to Behrensmeyer, the author of Climate Change and Human Evolution, both climatic and biological evolution has interacted throughout Earths history to spur the beginnings of man. The Great Rift Valley provides an ideal backdrop for the beginning of the evolutionary process. Evolutionary changes within the last 7 million years are hypothesized to be responsible for hominine speciation, the shift to bipedalism, enlarged cranial capacity, behavioral adaptation, cultural innovations, and the intercontinental immigration of man (Behrensmeyer, 2006). Thus, climatic change, along with biological innovations, are thought to be huge driving forces in the evolutionary process. Although, it is not exactly clear whether the Great Rift Valley is home to human evolution, data does support that the Rift Valley is the site to the majority of our fossil record. Did climatic change cause the evolution of man? This is not certain, we can say for sure, regardless of where the evolution of man started in The Great Rift Valley or not, that this location is an excellent specimen of global changes and the evolutionary process (Behrensmeyer, 2006).


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