DEFINITION OF MANTLE

Volumetrically, the mantle forms about 80 percent

of the Earth, occupying the region between the crust and

upper core, between about 20 and 1,800 miles (35–2,900

km) depth. It is divided into two regions, the upper and lower

mantle, and is thought to be composed predominantly of silicate

minerals in closely packed high-pressure crystal structures.

Some of these are high-pressure forms of more

common silicates, formed under high temperature and pressure

conditions found in the deep Earth. The upper mantle

extends to a depth of about 146 miles (670 km), and the

lower mantle extends from there to the core-mantle boundary

near 1,800 miles (2,900 km) depth.

Most of our knowledge about the mantle comes from

seismological and experimental data, as well as rare samples of

upper mantle material that has made its way to the surface in

kimberlite pipes, volcanoes, and in some ophiolites and some

other exhumed subcrustal rocks. Most of the mantle rock samples

that have made their way to the surface are composed of

peridotite, with a mixture of the minerals olivine, pyroxene,

and some garnet. The velocities of seismic waves depend on

the physical properties of the rocks they travel through. Seismic

experiments show that S-waves (shear waves) propagate

through the mantle, so it is considered to be a solid rocky layer

since S-waves do not propagate through liquids. The temperature

does not increase dramatically from the top to the bottom

of the mantle, indicating that an effective heat transfer mechanism

is operating in this region. Heat transfer by convection, in

which the material of the mantle is flowing in large-scale rotating

cells, is a very efficient mechanism that effectively keeps

this region at nearly the same temperature throughout. In contrast,

the lithosphere (occupying the top of the mantle and

crust) is not convecting but cools by conduction, so it shows a

dramatic temperature increase from top to bottom.

There are several discontinuities in the mantle, where

seismic velocities change across a discrete layer or zone.

Between about 62 and 155 miles (100–250 km) depth, both P

and S-waves (compressional and shear waves) decrease in

velocity, indicating that this zone probably contains a few

percent partial melt. This low-velocity zone is equated with

the upper part of the asthenosphere, upon which the plates of

the lithosphere move. The low-velocity zone extends around

most of the planet; however, it has not been detected beneath

many Archean cratons that have thick roots, leading to

uncertainty about the role that the low-velocity zone plays in

allowing these cratons to move with the tectonic plates. The

asthenosphere extends to about 415–435 miles (670–700 km)

depth. At 250 miles (400 km), seismic velocities increase

rather abruptly, associated with an isochemical phase change

of the mineral olivine ([Mg,Fe]2SiO4) to a high-pressure mineral

known as wadsleyite (or beta-phase), then at 325 miles

(520 km) this converts to a high-pressure spinel known as

ringwoodite. The base of the asthenosphere at 415 miles (670

km) is also associated with a phase change from spinel to the

minerals perovskite ([Mg,Fe]SiO3) and magnesiowustite

([Mg,Fe]O), stable through the lower mantle or mesosphere

that extends to 1,800 miles (2,900 km).

The nature of the seismic discontinuities in the mantle

has been debated for decades. A. E. Ringwood proposed a

model in which the composition of the mantle started off

essentially homogeneous, consisting of the hypothetical composition

pyrolite (82 percent harzburgite and 18 percent

basalt). Extraction of basalt from the upper mantle has led to

its depletion relative to the lower mantle. An alternative

model, proposed by Don Anderson, poses that the mantle is

compositionally layered, with the 670-kilometer discontinuity

representing a chemical as well as a phase boundary, with

more silica-rich rocks at depth. As such, this second model

requires that convection in the mantle be of a two-layer type,

with little or no mixing between the upper and lower mantle

to maintain the integrity of the chemical boundary. Recent

variations on these themes include a two-layered convecting

mantle with subducting slab penetration downward through

the 670-kilometer discontinuity, and mantle plumes that

move up from the core mantle boundary through the 670-

kilometer discontinuity. In this model, the unusual region at

the base of the mantle known as D’’ may be a place where

many subducted slabs have accumulated.

The basal, D’’ region of the mantle is unusual and represents

one of the most significant boundaries in the Earth. The

viscosity contrast across the boundary is huge, being several

times that of the rock/air interface. D’’ is a boundary layer, so

temperatures increase rapidly through the layer, and there is a

huge seismic discontinuity at the boundary. P-waves drop in

velocity from about 8.5 to 4 miles per second (14 to 8 km/s),

and S-waves do not propagate across the boundary since the

outer core is a liquid. Research into the nature of the D’’

layer is active, and several ideas have emerged as possibilities

for the nature of this region. It may be a slab graveyard

where subducted slabs temporarily accumulate, or it could be

a chemical reaction zone between the lower mantle and outer

core. It could be a remnant of chemical layering formed during

the early accretion and differentiation of the Earth, or it

could be material that crystallized from the core and floated

to accumulate at the core/mantle boundary. Whatever the

case, it has been speculated that there may be an analogue to

plate tectonics operating in this region, since it is a viscosity

and thermal boundary layer, subjected to basal traction forces

by the rapidly convecting outer core.

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

LITHOSPHERE; PLATE TECTONICS.

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