Page 45 - Fundamentals of Physical Volcanology

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22 CHAPTER 2

mark two arms and the East African Rift Valley the
third (Fig. 2.7). Along the Red Sea and Gulf of Aden
rifting has advanced to the point where sea-floor
spreading has started. Along the East African Rift
Continental Valley extension and normal faulting have occurred
lithosphere
Plume head in association with continuing volcanism but the
process has not advanced to the point of sea-floor
spreading.
A mantle plume impinges
on continental lithosphere
causing up-doming and
crustal thinning 2.3.1 Tectonic settings, melting processes
and magma composition
(a)
Not only can we link volcanic activity with very
specific tectonic settings, but the types of magma
generated in these different settings are distinct
from each other in terms of composition and phys-
ical properties.
Rift valley
Continental
basaltic lavas
lithosphere
MID-OCEAN RIDGES AND OCEANIC
Plume head
INTRAPLATE SETTINGS
The continental crust has The dominant magma type at mid-ocean ridges is
started to fault and rift. basalt, and mid-ocean ridge basalts (MORBs) are
Copious melting in the
plume head has led to the derived directly from the partial melting of the
eruption of large volumes upwelling mantle beneath the spreading ridge.
of basaltic lava
(b) When a mantle hot spot interacts with oceanic
crust well away from a spreading ridge, melting in
the plume head first gives rise to very large volumes
New
mid-ocean of basaltic magma which erupt to form a Large
ridge Igneous Province (LIP). After this initial phase of
flood volcanism, magmatism continues at a lower
Continental rate and is dominated by ocean island basalts
lithosphere
(OIBs).
We know that both of these settings are zones
in which mantle upwelling is occurring and it is
Continued rifting has led
to the formation of a new thought, therefore, that the dominant melting mech-
spreading centre and anism here is decompression melting. Figure 2.1
production of new oceanic
lithosphere illustrates this process. The first thing to note is that
the geothermal gradient – the variation of tempera-
(c)
ture with depth – is such that the temperature of
the mantle is lower than the solidus temperature of
Fig. 2.5 The consequences of the head of a mantle plume
the mantle material, and so we would expect the
impinging on continental lithosphere. Initially (a) the
mantle to be solid. However, as we have seen, over
lithosphere is up-domed and the crust is thinned and
stretched. Soon (b) the stretching exceeds the strength very long time scales the mantle convects and so
of the rocks, leading to formation of faults that assist the mantle material from deeper, hotter levels is gradu-
process, forming a rift valley. Basaltic lavas erupt into ally brought up to shallower, cooler zones. What
the valley as part of the plume head melts due to the happens to this upwelling mantle depends critic-
decompression (see Fig. 2.1). In (c) the continental
ally on the rate at which it ascends. When the
lithosphere has been rifted to the point where a new
mid-ocean ridge has formed and the eruption of basaltic
lavas is generating new oceanic crust.