Page 124 - Fundamentals of Geomorphology

LARGE-SCALE TECTONIC AND STRUCTURAL LANDFORMS 107

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
Most volcanoes sit at plate boundaries. A few, includ- Traps were formed.
ing the Cape Verde volcano group in the southern Superplumes may form. One appears to have done
Atlantic Ocean and the Tibesti Mountains in Saharan so beneath the Pacific Ocean during the middle of the
Africa, occur within plates. These ‘hot-spot’ volca- Cretaceous period (Larson 1991). It rose rapidly from
noes are surface expressions of thermal mantle plumes. the core–mantle boundary about 125 million years ago.
Hot-spots are characterized by topographic bumps (typ-
ically 500–1,200 m high and 1,000–1,500 km wide), Production tailed off by 80 million years ago, but it did
not stop until 50 million years later. It is possible that
volcanoes, high gravity anomalies, and high heat flow. superplumes are caused by cold, subducted oceanic crust
Commonly, a mantle plume stays in the same position on both edges of a tectonic plate accumulating at the
while a plate slowly slips over it. In the ocean, this pro-
duces a chain of volcanic islands, or a hot-spot trace, top of the lower mantle. These two cold pools of rock
then sink to the hot layer just above the core, and a giant
as in the Hawaiian Islands. On continents, it produces plume is squeezed out between them. Plume tectonics
a string of volcanoes. Such a volcanic string is found in maybethedominantstyleofconvectioninthemajorpart
the Snake River Plain province of North America, where of the mantle. Two super-upwellings (the South Pacific
a hot-spot currently sitting below Yellowstone National and African superplumes) and one super-downwelling
Park, Wyoming, has created an 80-km-wide band across (the Asian cold plume) appear to prevail (Figure 4.6).
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 Asian South Pacific
cold
of land in far-flung places. The Siberian province cov- superplume superplume
2
ers more than 340,000 km . India’s Deccan Traps once
2
covered about 1,500,000 km ; erosion has left about
2
500,000 km .
Mantle plumes
Inner
Mantle plumes appear to play a major role in plate core
tectonics. They may start growing at the core–mantle
Outer core
boundary, but the mechanisms by which they form and Outer core
grow are undecided. They may involve rising plumes South
2,900 km America
of liquid metal and light elements pumping latent heat African Lower mantle
Lower
mantle
outwards from the inner-core boundary by composi- superplume
670 km
tional convection, the outer core then supplying heat to U pper mantle Mid-Atlantic
Upper mantle
the core–mantle boundary, whence giant silicate magma ridge
chambers pump it into the mantle, so providing a plume
source. Mantle plumes may be hundreds of kilometres Figure 4.6 A possible grand circulation of Earth materials.
in diameter and rise towards the Earth’s surface. A plume Oceanic lithosphere, created at mid-ocean ridges,
consists of a leading ‘glob’ of hot material that is fol- is subducted into the deeper mantle. It stagnates at
around 670 km and accumulates for 100–400 million
lowed by a ‘stalk’. On approaching the lithosphere, the years. Eventually, gravitational collapse forms a cold
plume head is forced to mushroom beneath the litho- downwelling on to the outer core, as in the Asian cold
sphere, spreading sideways and downwards a little. The superplume, which leads to mantle upwelling occurring
plume temperature is 250–300 C hotter than the sur- elsewhere, as in the South Pacific and African hot
◦
rounding upper mantle, so that 10–20 per cent of the superplumes.
surrounding rock is melted. This melted rock may then Source: Adapted from Fukao et al. (1994)