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continent. Davies (1998) disagreed with this idea and wondered how so many
scattered volcanism could be accounted for by only one or a few plumes. Ebinger
and Sleep (1998) proposed a method by which most of the volcanism could be
explained by a single large plume. According to them, the topography at the base of
the lithosphere lying 100
150 km below the surface, might channel the plume
material into streams and pools. The model of Ebinger and Sleep is shown in
Fig.
14.6
. In that model a large plume head followed by a thin plume tail rise from
the base of the mantle. The head
-
flattens and spreads as it approaches the base of the
lithosphere, and it tends to
flow up and under the thinnest parts of the lithosphere.
Melting of the hot plume material only begins above a critical depth, and it only
occurs as the plume material rises and decompresses. Thus, melting tends to be
concentrated, where the plume material
flows up a slope towards thinner litho-
sphere, as well as over the plume tail. According to them, when a new plume starts,
it is led by a large spherical `head
'
1,000 km in diameter. As the head comes near
the top of the mantle it `pancakes
under the lithosphere, which is the cool, strong
outer part of the earth comprising the crust and the upper part of the mantle down to
a depth of 100 or 200 km. The plume head is then spread to a diameter of about
2,000 km and thins out to about 200 km vertically. According to Davies during
most part of its ascent, the plume material is in the solid state, but because the
temperature is high it gets deformed gradually and behaves like a
'
fluid on a geo-
logical time scale. As the hotter material reaches shallow depths, melting is initiated
because of the reduction in pressure, resulting in the generation of magmas that rise,
and eventually erupts near the surface of the earth. There seems to be a general
consensus that eruption of
flood basalts (covering millions of cubic kilometres)
takes place on to the earth
s surface within a few million years. During the last 250
million years there had been at least a dozen of such eruptions.
Ebinger and Sleep suggested that the Afar plume was probably responsible for
not only the Ethiopian
'
flood basalt, but might also have been instrumental in
promoting rift valleys that radiate from the region. They further envisaged that this
plume was also responsible for rifting of the African continental offshore and south
to the Comoros Islands near Madagaskar, west of the Darfur uplift and probably
further to the Adamawa plateau and Atlantic coast (Fig.
14.5
). This explains the
scattered nature of the associated volcanism due to variation in the thickness of the
lithosphere through this region. The model of Ebinger and Sleep comprises two
mechanisms: (1) the plume tends to rise into the regions of thinner lithosphere
because of its buoyancy, (2) melting is initiated at a critical depth during the ascent
of the plume.
According to this model if the plume rises under a thick lithosphere it may melt
very little or fusion may not occur at all. If the material
flows horizontally it will not
melt any more. If later, the plume reaches thinner lithosphere and can
ow upward
again, it can melt further as a consequent of this a rifting takes place. Subsequently
it may trigger more melting and may promote the
flow of plume material over larger
distance.
George et al. (1998, cited by Davies 1998) however, were of the opinion that
there are actually two plumes in this region. Eruption on the southern Ethiopian
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