East African Rift (): Continental Rifts
1. Zoom from Death Valley, CA, to the Serengeti.
2. Tanzania is a fascinating place. With many different environments, it has a
remarkable diversity of life. The Serengeti plains alone have 70 different
kinds of mammals and over 500 species of birds. It also hosts the largest
annual migration of mammals in the world – 2 million mammals migrate
each year with the change in seasons, crossing the large Mara river (quarter
million wildebeest die each year on the journey). This migration is certainly
one of the top Natural Wonders of the world.
a. But not far from here, to the east, is one of the world’s top Geologic
Wonders – Mount Kilimanjaro.
b. Kilimanjaro: tallest mountain in all of Africa – almost 6 km high
(19,341). More people have died climbing it than Everest, even though
it is a much easier climb (still very high!!! Hypothermia, hypoxia)
c. It is a huge, massive stratovolcano, with alternating layers of lava and
ash. Currently dormant, but could be active any time. The last big
eruption was 360,000 yrs ago, but volcanic activity as recent as 200
yr.
d. Molten magma is just 400 m below the summit.
e. It is so tall, that it acts as a Sky Island – it has climates
(vegetation/animals) that are not seen in any surrounding regions, so
they are cut off, and many endemic species.
f. And it is so tall, that even though it is not far from the equator, there is
snow and glaciers at the top year-round!
i. Actually, snow is rapidly disappearing. 80% gone since 1910.
ii. Due to a combination of warming climates and a drop in
humidity (and precipitation).
g. But there is a question here. We have seen stratovolcanoes before (Mt.
Fuji), but they have always been at established subduction zones.
h. No subduction zone here, so how did Kilimanjaro form?
i. The answer to this question gets at the very heart of geology and our
understanding of the whole idea of plate tectonics.
j. There is a clue to this in the fact that though there is not a subduction
zone here, there are many other volcanoes.
i. Mt. Kenya, just 200 mi away, across the border – 2nd tallest
mountain in Africa (5.2 km high; also a stratovolcano)
ii. Ngorongoro crater – huge collapsed caldera from a massive
volcanic explosion 3 million years ago
1. The crater floor is flat, surrounded by the caldera walls
2. Covers 100 mi2! Home to populations of many animals,
like lion tribes, that rarely leave or have interactions
with animals outside the crater.
iii. In fact, there are volcanoes all over the place. So what is going
on?
3. In Tanzania, Kenya, and the surrounding countries, we are witnessing
something that we don’t see actively happening anywhere else (at least, no
where to this degree): a continent is splitting into pieces.
a. We know this happens. Pangaea started to break up 200 Ma.
b. In a sense, Pangaea is still breaking up!
c. Africa into 3 pieces! [Explain]
i. Arabian plate first to break off, moving north [30 Ma; Red Sea,
Gulf of Aden, Dead Sea transform]
ii. Next step is breaking current Africa into 2 pieces: Nubian and
Somalian plates.
d. Rifting extend from Afar region in Ethopia 1000s of km way
southward into Mozambique and Zimbabwe.
e. But the rifting isn’t easy, apparently. Like trying to tear a dictionary in
half. The rifts don’t run straight, and are in several pieces: [show]
i. Ethiopia rift
ii. Kenya Rift
iii. East African Rift
f. In the middle of the rifting is the Tanzania Craton (ancient block of
crust) – the Kenya Rift and EAR run around it.
i. Lake Victoria is in the middle (one of the few regions without
volcanoes!)
ii. Incidentally, Lake Victoria is the source of the Nile River, that
ends up in the Mediterranean Sea, over 4000 miles later. There
is some debate about where the Nile River actually starts, as
there are thousands of streams that flow into it, and
determining the longest is difficult.
g. The reason for the rifting is not fully understood. The plates don’t
seem to be pulled apart (no subduction zones on the other sides of the
plates!), but it seems likely that they are being pushed apart!
i. Two large domes – Afar region and Kenya Dome.
ii. From looking at topography maps, you can see that these
regions are being uplifted from below.
iii. Best guess is mantle plume hotspots, like Iceland, Yellowstone,
and Hawaii. [explain]
iv. Not sure if there are two hotspots here, or just one, that has
split right beneath the plate.
4. The process of having a set of active volcanic rifts run right through a
continent is very dramatic. Devastating at times. But giving rise to a variety of
geologic features and environments that you wouldn’t otherwise have.
a. Volcanoes!
i. Two of the world’s 5 lava lakes are found in the African Rift
System: Erta Ale in Ethiopia; Nyiragongo in the Congo. Not the
usual kind of volcanoes you find in a continent (think of the
giant explosive eruptions at Yosemite!)
1. A sign that the rift is on its way to becoming an ocean!
Ocean-crust-type basaltic volcanism is coming out!
ii. Basaltic lava can have long lava tubes (we saw that at Hawaii!)
– one of the longest lava tubes in the world is in Kenya
(Leviathan): considered to have the longest continuous
segment of lava tube (9+ km).
iii. The volcanic areas can have fascinating hot springs, like the
beautiful Dallol hot springs in Ethiopia.
b. When you pull the crust apart, you also develop valleys and
depressions!
i. Danakil depression (where Erta Ale lava lake is) in Ethiopia:
100+ m below sea level. [Explain the process]
ii. More unusual – continuous graben structures: alternating
blocks that have dropped down, make multiple long, narrow
valleys (like Kenya grabens).
iii. Also develop deep, long valleys down the middle of the rift.
1. Rift valleys sometimes are mile below the surrounding
land!
iv. Often filled with water to make long lakes. Many dozens of
lakes in the rifts, in the different arms.
1. [Show on a map: Ethiopian rift vally lakes (like Lake
Abaya); West Rift Valley Lakes (Tanganyika, Albert,
Edward, Kivu); East Rift Valley Lakes (like Turkana);
Southern Rift Valley Lakes (like Malawi, Rukwa)]
2. Lake Tanganyika (Burundi, Democratic Republic of the
Congo, Tanzania, Zambia) – longest fresh water lake; 2nd
longest and deepest; fills in the EAR for almost 700
miles! Famous for many endemic species of fish (at least
250 species of cichlids alone!)
3. Lake Turkana (Kenya/Ehiopia) – world’s largest desert
lake, shores are abundant in creatures like crocodiles,
scorpions, and vipers. Rough life there.
4. Lake Malawi (Malawi/Mozambique/Tanzania) – likely
holds more species of fish than any lake on Earth (more
than 1000 species of cichlids!) – 75 km wide and almost
600 km long!
5. Some lakes can be extremely alkaline, like Lake Natron
(Tanzania) (in Lecture 26 – world’s most alkaline lake)
6. Some lakes are actually deadly, like Lake Kivu
(Congo/Rwanda)! Kivu is one of 3 three lakes in the
world that undergo catastrophic overturns due to the
release of gases (the other two, Nyos and Monoun, are
both in Cameroun).
a. Because of volcanic activity under the lake, more
than 250 km3 of dissolved CO2 is in the lake.
b. Microbial activity consuming the CO2 has also
produced 65 km3 of methane in the lake!
c. (In 1986, Lake Nyos suddenly released a giant
cloud of CO2 – 1700 people along the shores
suffocated to death – no oxygen!)
d. Lake Kivu could be much worse! Many more
people live there, and if the methane were also
released, it could explode!
e. Currenly, the methane is being extracted
(bubbles naturally out of the water when
brought to the surface) at a small level (like
running the boilers at a brewery), but there are
plans to remove more – could make Rwanda a
net exporter of energy.
v. The rifting can uplift tall mountains on either side of the rift:
Like the Mountains of the Moon (Rwenzori Mtns), in Uganda.
Rift Valley lakes are on either side (Lake Albert, north; Lake
Edward, south)!
1. They are 5 km high, also with snow year round (near
the equator!)). Formed recently (3 Ma!), uplifted from
below.
5. There may be deeper story to the African Rift System. (Yes, literally) The
Earth’s tectonic plates move about because of large-scale convection motions
within Earth’s mantle. It turns out that there are some very large variations
(in composition and temperature) deep inside the Earth.
a. By far, the largest variation is called the African Megapile. It is a
massive cone-shaped region of hot and heavy rock that rises up from
the top of the core more than 1500 km.
b. It sits beneath Africa, and may be the underlying reason for the Afar
hotspot and all the volcanoes in Earth Africa.
c. A lot of research is going on to see if indeed the volcanoes like
Kilimanjaro actually originate from the very base of the mantle.
6. There is one more reason why scientists are fascinated with the East African
Rift Valley system. It is where the first humans came from. Modern day
humans evolved slowly across a long sequence of hominid species that began
in Africa. There are many significant fossil finds in and around the Rift
Valleys.
a. The Rift Valleys provided not only a sheltered area to live, with very
fertile lands, but the high sedimentation rates are great conditions for
preserving the bones of our ancestors.
b. 10 Ma, Chororapithecus abyssinicus, found in the Afar rift, in eastern
Ethiopia
c. 10 Ma, Nakalipithecus nakayamai, Kenya rift
d. 5.6-4.4 Ma, Ardipithecus, Afar Rift
e. 4-1.8 Ma, Australopithecus: "Lucy", a partial skeleton discovered by
anthropologist Donald Johanson in Afar Rift. Also found in the Olduvai
Gorge in Ngorongoro crater in Tanzania by the Leakeys
f. 3.5-3.2 Ma, Kenyanthropus, Lake Turkana, Kenya
g. 3-1.4 Ma, Paranthropus, Olduvai Gorge, Tanzania
h. 2.5 – 1.7 Ma, Homo ergaster, Lake Turkana
i. 2.3-1.4 Ma, Homo habilis, Olduvai Gorge, Tanzania, and Lake Turkana,
Kenya
j. 1.8 – 0.2 Ma, Homo erectus, Olduvai Gorge and Lake Turkana
k. 200,000 years to present – homo sapiens sapiens, Ethiopia
l. During massive volcanic eruptions, like the Toba Eruption 72,000
years ago, the fertile African rift valleys made have provided the
shelter that allows us to survive and repopulate the planet.
7. Top 5:
a. Lake Baikal, Russia: The deepest and oldest lake in the world. Deep
because ancient rift valley, no longer rifting.
b. Rio Grande Rift, New Mexico
c. Rhine Graben, Germany – rifting due to uplift of Alps
d. Midcontinent Rift Valley – giant ancient rift (1.2 Ga!) that runs from
Oklahoma up through Kansas, Iowa, Minnesota and into Canada. Hard
to imagine, but 1.2 billion years ago, the kinds of volcanic activity that
are occurring in East Africa were going on right here, in the middle of
North America.
Questions:
1) What will happen to the African Rift System when spreading occurs to the
point that it is a new ocean?
2) Why do you think that Kilimanjaro, not in the rift valley, is a stratovolcano,
while volcanoes in the rift valleys are more basaltic (more like ocean crust)?