Geology Reference
In-Depth Information
The location of volcanoes
run on to the Earth's surface as flood basalt, as occurred
in India during the Cretaceous period when the Deccan
Traps were formed.
Superplumes
may form. One appears to have done
so beneath the Pacific Ocean during the middle of the
Cretaceous period (Larson 1991). It rose rapidly from
the core-mantle boundary about 125 million years ago.
Production tailed off by 80 million years ago, but it did
not stop until 50 million years later. It is possible that
superplumes are caused by cold, subducted oceanic crust
on both edges of a tectonic plate accumulating at the
top of the lower mantle. These two cold pools of rock
then sink to the hot layer just above the core, and a giant
plume is squeezed out between them.
Plume tectonics
may be the dominant style of convection in the major part
of the mantle. Two super-upwellings (the South Pacific
and African superplumes) and one super-downwelling
(the Asian cold plume) appear to prevail (Figure 4.6).
Most volcanoes sit at plate boundaries. A few, includ-
ing the Cape Verde volcano group in the southern
Atlantic Ocean and the Tibesti Mountains in Saharan
Africa, occur within plates. These
'hot-spot' volca-
noes
are surface expressions of thermal mantle plumes.
Hot-spots are characterized by topographic bumps (typ-
ically 500-1,200 m high and 1,000-1,500 km wide),
volcanoes, high gravity anomalies, and high heat flow.
Commonly, a mantle plume stays in the same position
while a plate slowly slips over it. In the ocean, this pro-
duces a chain of volcanic islands, or a
hot-spot trace
,
as in the Hawaiian Islands. On continents, it produces
a string of volcanoes. Such a volcanic string is found in
the Snake River Plain province of North America, where
a hot-spot currently sitting below Yellowstone National
Park, Wyoming, has created an 80-km-wide band across
450 km of continental crust, producing prodigious quan-
tities of basalt in the process. Even more voluminous
are
continental flood basalts
. These occupy large tracts
of land in far-flung places. The Siberian province cov-
ers more than 340,000 km
2
. India's Deccan Traps once
covered about 1,500,000 km
2
; erosion has left about
500,000 km
2
.
Asian
cold
superplume
South Pacific
superplume
Mantle plumes
Inner
core
Mantle plumes
appear to play a major role in plate
tectonics. They may start growing at the core-mantle
boundary, but the mechanisms by which they form and
grow are undecided. They may involve rising plumes
of liquid metal and light elements pumping latent heat
outwards from the inner-core boundary by composi-
tional convection, the outer core then supplying heat to
the core-mantle boundary, whence giant silicate magma
chambers pump it into the mantle, so providing a plume
source. Mantle plumes may be hundreds of kilometres
in diameter and rise towards the Earth's surface. A plume
consists of a leading 'glob' of hot material that is fol-
lowed by a 'stalk'. On approaching the lithosphere, the
plume head is forced to mushroom beneath the litho-
sphere, spreading sideways and downwards a little. The
plume temperature is 250-300
◦
C hotter than the sur-
rounding upper mantle, so that 10-20 per cent of the
surrounding rock is melted. This melted rock may then
Outer core
South
America
0
African
superplume
Lower mantle
0 k
Upper mantle
Mid-Atlantic
ridge
Figure 4.6
A possible grand circulation of Earth materials.
Oceanic lithosphere, created at mid-ocean ridges,
is subducted into the deeper mantle. It stagnates at
around 670 km and accumulates for 100-400 million
years. Eventually, gravitational collapse forms a cold
downwelling on to the outer core, as in the Asian cold
superplume, which leads to mantle upwelling occurring
elsewhere, as in the South Pacific and African hot
superplumes.
Source:
Adapted from Fukao
et al.
(1994)
