DEFINITION OF MANTLE PLUMES

The mantle of the Earth convects with large

cells that generally upwell beneath the oceanic ridges and

downwell with subduction zones. These convection cells are

the main way that the mantle loses heat. In addition to these

large cells, a number of columnar plumes of hot material

upwell from deep within the mantle, perhaps even from the

core-mantle boundary. Heat and material in these plumes

move at high velocities relative to the main mantle convection

cells, and therefore they burn their way through the moving

mantle and reach the surface forming thick sequences of generally

basaltic lava. These lavas are chemically distinct from midocean

ridge and island arc basalts, and they form either as

continental flood basalts, oceanic flood basalts (on oceanic

plateaux), or shield volcanoes.

Mantle plumes were first postulated to be upper mantle

hot spots that were relatively stationary with respect to the

moving plates, because a number of long linear chains of

islands in the oceans were found to be parallel, and all old at

one end and younger at the other end. In the 1960s when

plate tectonics was first recognized, it was suggested that

these hot spot tracks were formed when the plates moved

over hot, partially molten spots in the upper mantle that

burned their way, like a blow torch, through the lithosphere,

and erupted basalts at the surface. As the plates moved, the

hot spots remained stationary, so the plates had a series or

chain of volcanic centers erupted through them, with the

youngest volcano sitting above the active hot spot. The

Hawaiian-Emperor island chain is one of the most exemplary

of these hot spot tracks. They are about 70 million years old

in the northwest near the Aleutian arc, show a sharp bend in

the middle of the chain where the volcanoes are 43 million

years old, and then are progressively younger to essentially

zero age beneath the island of Hawaii. The bend in the chain

is thought to represent a change in the plate motion direction

and is reflected in a similar change in direction of many other

hot spot tracks in the Pacific Ocean.

More recently, geochemical data and seismic tomography

has shown that the hot spots are produced by plumes of deep

mantle material that probably rise from the D” layer at the

core-mantle boundary. These plumes may rise as a mechanism

to release heat from the core, or as a response to greater heat

loss than is accommodated by convection. If heat is transferred

from the core to D”, parts of this layer may become

heated, become more buoyant, and rise as thin narrow plumes

that rise buoyantly through the mantle. As they approach the

base of the lithosphere, the plumes expand outward, forming

a mushroom-like plume head that may expand to more than

600 miles (1,000 km) in diameter. Flood basalts may rise from

these plume heads, and large areas of uplift, doming, and volcanism

may be located above many plume heads.

There are thought to be several plumes located beneath

the African plate, such as beneath the Afar region, which has

experienced uplift, rifting, and flood basalt volcanism. This

region exemplifies a process whereby several (typically three)

rifts may propagate off of a dome formed above a plume head,

and several of these may link up with rifts that propagated off

other plumes formed over a large stationary plate. When several

rifts link together, they may form a continental rift system

that could become successful and expand into a young ocean

basin, similar to the Red Sea. The linking of plume-related rifts

has been suggested to be a mechanism to split supercontinents

that have come to rest (in a geoid low) above a number of

plumes. The heat from these plumes must eventually escape by

burning through the lithosphere, forming linked rift systems

that eventually rip apart the supercontinent.

Some areas of anomalous young volcanism may also be

formed above mantle plume heads. For instance, the Yellowstone

area has active volcanism and geothermal activity and

is thought to rest above the Yellowstone hot spot, which has

left a track extending northwest back across the flood basalts

of the Snake River plain. Other flood basalt provinces probably

also formed in a similar way. For instance, the 65-millionyear-

old Deccan flood basalts of India formed when this

region was over the Reunion hot spot that is presently in the

Indian Ocean, and these may be related to a mantle plume.

Mantle plumes may also interact with mid-ocean ridge

volcanism. For instance, the island of Iceland is located on the

Reykjanes Ridge, part of the Mid-Atlantic Ridge system, but

the height of the island is related to unusually thick oceanic

crust produced in this region because a hot spot (plume) has

risen directly beneath the ridge. Other examples of mantle

plumes located directly beneath ridges are found in the south

Atlantic Ocean, where the Walvis and Rio Grande Ridges

both point back to an anomalously thick region on the present-

day ridge where the plume head is located. As the South

Atlantic opened, the thick crust produced at the ridge on the

plume head was split, half being accreted to the African plate,

and half being accreted to the South American plate.

See also AFAR; CONVECTION AND THE EARTH’S MANTLE;

FLOOD BASALT; HOT SPOT; MANTLE.

Title Post:
Rating: 100% based on 99998 ratings. 99 user reviews.
Author:

Terimakasih sudah berkunjung di blog SELAPUTS, Jika ada kritik dan saran silahkan tinggalkan komentar