9 The East African Rift Valley

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.

Classes met for three months at a time with a month’s vacation between terms. During my school breaks, I traveled 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 terminus 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.

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.

One of the reasons for us 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 church bells 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.

Notes on Plate Tectonics:

Every field of science has its own revolution—relativity and quan-tum mechanics in physics, atomic theory in chemistry, evolution in biology. We geologists were a bit late to the barricades, but the Earth is a lot bigger than a test tube (though smaller than the universe). Plate tectonics was our revolution, pushed forward by the technological advances coming out of World War II, particularly with respect to understanding the ocean bottom. This theory pulled together lots of observations into a single coherent theory that made sense of them all.

In brief, the theory of plate tectonics states that the surface of the Earth is consists of large slabs of brittle rock that come together, pull apart, and slide by each other. In addition, the slabs that form continents are thicker and lower in density, while those beneath oceans are thinner and denser. The denser oceanic plates get shoved back down into the mantle below, melting, forming volcanoes sometimes, and recycling.

1. We will draw together in class the six types of plate interactions. Take good note—this is one of the most important exercises in preparation for the next test.

2. We will watch the Earth Revealed video, Birth of a Theory. The link is on the course website. Be ready to answer the following questions about the video:

• How is the development of plate tectonics an example of the use of the scientific method?

• Who was Alfred Wegener? What was his contribution to the early stages of what led to plate tectonics? Why were his ideas rejected?

• Who was Harry Hess? What were his two key contribu-tions to plate tectonics? What led to his discoveries?

• What are examples of additional support for plate tectonics?