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Transcript
GEOTHERM supports East Africa regional boundary conditions for geothermal energy use
The East African Rift Valley
Federal Institute for Geosciences and Natural Ressources (BGR), Stilleweg 2, 30655 Hannover, Germany (GEOTHERM working group)
The 4000 km long and in average 60 km wide East African Rift
System is visible from the moon. It is the largest crack in the
continental crust on Earth and consists of two branches (Figure 1).
35°
45°
Re
30°E
Eastern branch of East African Rift System
Afar triangle
f
o
lf n
u
G de
A
Afa
tria r
ng
le
10°
Kenya
Tanzania
ea
dS
15°N
Eritrea
Ethiopia
Djibouti
S
Erta Ale
Lengai
t
s
u
r
c
l
a
t
n
e
n
i
t
n
o
c
N
5°
Lake Turkana
Figure 3 Schematic sketch of eastern rift with central volcanoes
and changing thickness of the continental crust
0°
5°
Indian
Ocean
Lake
Victoria
Lake Tanganiyka
10°
Lake
Malawi
15°
East
African
Rift
System
Rifting started 30 million years ago in the Afar region (dilatation of
continental crust up to 60 km), xx Ma ago in northern Kenya
(dilatation 35-40 km), 12 Ma ago in central Kenya (dilatation 5-10
km) and less than 5 Ma ago in northern Tanzania (dilatation less
than 5 km). The continental crust is composed of units different in
age, composition, thickness, rigidity etc. and the rift mainly follows
pre-existing zones of weakness along contacts between these
units. The rift is not a symmetrical structure because it consists
mainly of linked half-grabens (Figure 5).
35°
45°E
Figure 1 Simplified map of the East African Rift System
The eastern branch is extending from the southern Red Sea (Afar
triangle) to Tanzania and the western branch is extending from
Uganda to Mozambique. A much bigger volume of volcanic rocks
were erupted in the eastern branch of the rift compared to the
western branch (Figure 2). Most of the eastern rift belongs to
Ethiopia and Kenya and therefore the highest numbers of active
volcanoes and geothermal resources are found in these two
countries.
15°N
45°E
f
o
f
Gul
n
e
d
A
30 my
45°
35°
Afa
tria r
ng
le
5°
0°
10°
Half-Graben Model
xx my
12 my
0°
5 my
5°S
5°
Indian
Ocean
Lake
Victoria
Lake Tanganiyka
Rifting
5°
10°
40 km
10 km
5 km
Lake
Malawi
Indian
Ocean
45°E
Figure 4 Maximum extension of continental crust, age of initial
rifting in million years before present and triple junction formed by
Red Sea, Gulf of Aden and East African Rift System
GEOTHERM programme supported by:
15° 30°E
Lake Turkana
60 km
Figure 5 Schematic sketch of linked half-grabens
Rifting of the eastern branch with a rate of millimetre per year is
caused by uprising mantle plumes. These plumes are transporting
melts and heat to the earth surface and bulge the continental crust.
If the tensional forces exceed the breaking strength of the upwarped crustal rocks cracks occur (i.e. the tension is released by
earthquakes) leading to a Y-shaped triple junction at the spherical
earth surface (Figure 4).
10°
d
Re a
Se
20°S
The continental crust of the eastern rift area (Figure 3) thickens
from North (Afar triangle) to South (Tanzania) and the amount of
dilatation and age of rifting decreases in the same direction
(Figure 4).
15°S
Volcanic
Products
45°E
Figure 2 Difference in volcanic activity in the two branches of the
East African Rift System
Two arms of the triple junction will open to an ocean by forming
new oceanic crust and the third arm will be a “failed rift”. The Red
Sea and the Gulf of Aden are the two new oceanic arms spreading
with a rate of centimetre per year and the rift valley is the remaining
third arm which will never open to an ocean. Therefore it is most
unlikely that East Africa will be split off from the rest of the African
continent like it happened to Madagascar 165 million years ago..