The East African Rift Valley

By Dale Easley

June 13th, 2003

Map of Africa
On Mt Kenya.
When I finished Guilford College with an undergraduate degree in mathematics, I had no idea what I wanted to do except travel. So I applied for a two-year volunteer position teaching math in the highlands of Kenya at a boarding school. The highlands are heavily farmed because of the climate---not too hot because of the altitude but not too cold because of the latitude. Within sight of the school is Mt Kenya, the second highest peak in Africa. It is snowcapped despite being almost directly on the equator. A bit further west is the East African Rift Valley, an arm of the best example in the today's world of a triple junction.

Map of Africa
With my students.
Classes met for three months at a time with a month's vacation between terms. During my school breaks, I travelled around Kenya, going on safaris, visiting lakes and beaches, and seeing the cities. Where I lived at an elevation of 6000 feet, malaria was uncommon, but I caught it during my first vacation when I went on a safari that took me to lower elevations. Among other sites, I got to see the Rift Valley. The highlands of Kenya are cut down the middle by a rift that drops the elevation by thousands of feet. From lush damp forests, one descends rapidly to the dry hot valley floor. A string of lakes along the valley represents the terminous of streams off the highlands----water flows in but doesn't flow out. Instead it evaporates. Some of the lakes have, over thousands of years, become very high in dissolved minerals, high enough that evaporites are deposited in some of the lakes in sufficient quantities to be mined.

Rift Valley map from the USGS
From the USGS
The formation of the Rift Valley, along with the Red Sea and the Gulf of Aden, is an example of the process of continental rifting. In this case, the Middle East Peninsula is being separated from Africa. When a continent rifts, the crust first bulges, explaining the elevations in highlands and north through Ethiopia. We geologists believe the bulge is due to the rise of hot magma from the mantle. The uplift leads to the formation of three-way split, with the central part of each arm of the split collapsing downward to form a graben. Two of the arms of that split continue to grow, in this case the Red Sea and the Gulf of Aden. As they continue to separate, mantle material flows out, forming new oceanic crust and the beginning of a new ocean. The third part of the split, the East African Rift, if it continues to grow, will turn East Africa into an island in the Indian Ocean.

Along the floor of the Rift Valley, I saw the evidence of hot rock still near the surface. As the rift forms, fractures develop through which magma can move upwards. Mt Longonot (photo below) is a recent volcano on the valley floor. Along the flanks of the valley can be seen evidence of old lava flows. At Lake Bogoria are natural hot springs [photo]. Ground water is heated by the hot rock near the surface and comes steaming to the surface, similar to (though less dramatic than) Old Faithful at Yellowstone, a continental hot spot.

Mt Longognot, Kenya
Mt Longognot, Kenya
One of the reasons for us in New Orleans to study the East African Rift is to better understand the way the Gulf of Mexico formed. Millions of years ago, Africa, Eurasian and the Americas were joined together. Uplift began to occur near where today's GoM is located. Multiple three-way splits formed and grew to form the Gulf. The failed third arm of one passes beneath what is now the Mississippi River. Though earthquakes are rare in the interior of continents, this weak spot accounts for the presence of the New Madrid earthquake zone 2. In the early 1800s, a series of earthquakes occurred at New Madrid so powerful that they rang the churchbells in Boston, created new lakes and land, and caused the Mississippi River to briefly flow backwards in places. Back then, few people were living the area, but such an earthquake now could be devastating. It's just a matter of time till it happens.

1Multiple links on the East African Rift Valley
2Earthquakes in the Mississippi River Valley