Transition zone
Encyclopedia
The transition zone is part of the Earth’s mantle
, and is located between the lower mantle and the upper mantle, between a depth of 410 and 660 km. The Earth’s mantle, including the transition zone, consists primarily of peridotite
, an ultramafic igneous rock
.
The mantle was divided into the upper mantle, transition zone, and lower mantle as a result of sudden seismic-velocity discontinuities at depths of 410 and 660 km. This is thought to occur as a result of rearrangement of atoms in olivine
(which constitutes a large portion of peridotite) at a depth of 410 km, to form a denser crystal structure as a result of the increase in pressure with increasing depth. Below a depth of 660 km, evidence suggests that atoms rearrange yet again to form an even denser crystal structure. This can be seen using body waves from earthquake
s, which are converted, reflected or refracted at the boundary, and predicted from mineral physics
, as the phase changes are temperature and density-dependent and hence depth dependent.
) olivine. From the Clapeyron slope the discontinuity is expected to be shallower in cold regions, such as subducting slabs, and deeper in warmer regions, such as mantle plume
s.
in the pyrolite
mantle.
Other non-global phase transitions have been suggested at a range of depths.
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
, and is located between the lower mantle and the upper mantle, between a depth of 410 and 660 km. The Earth’s mantle, including the transition zone, consists primarily of peridotite
Peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
, an ultramafic igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
.
The mantle was divided into the upper mantle, transition zone, and lower mantle as a result of sudden seismic-velocity discontinuities at depths of 410 and 660 km. This is thought to occur as a result of rearrangement of atoms in olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
(which constitutes a large portion of peridotite) at a depth of 410 km, to form a denser crystal structure as a result of the increase in pressure with increasing depth. Below a depth of 660 km, evidence suggests that atoms rearrange yet again to form an even denser crystal structure. This can be seen using body waves from earthquake
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
s, which are converted, reflected or refracted at the boundary, and predicted from mineral physics
Mineral physics
Mineral physics is the science of materials that compose the interior of planets, particularly the Earth. It overlaps with petrophysics, which focuses on whole-rock properties...
, as the phase changes are temperature and density-dependent and hence depth dependent.
410 km discontinuity
A single peak is seen in all seismological data at 410 km which is predicted by the single transition from α to β(wadsleyiteWadsleyite
Wadsleyite is a high-pressure polymorph of olivine, an orthorhombic mineral found in the Peace River meteorite in Alberta, Canada. In the phase transformations from Mg2SiO4-Fe2SiO4 , olivine is transformed to wadsleyite β-Mg2SiO4 and then to a spinel-structured γ-Mg2SiO4 with increasing pressure...
) olivine. From the Clapeyron slope the discontinuity is expected to be shallower in cold regions, such as subducting slabs, and deeper in warmer regions, such as mantle plume
Mantle plume
A mantle plume is a hypothetical thermal diapir of abnormally hot rock that nucleates at the core-mantle boundary and rises through the Earth's mantle. Such plumes were invoked in 1971 to explain volcanic regions that were not thought to be explicable by the then-new theory of plate tectonics. Some...
s.
660 km discontinuity
This is the most complex discontinuity seen. It appears in PP precursors (a wave which reflects of the discontinuity once) only in certain regions but is always apparent in SS precursors. It is seen as single and double reflections in receiver functions for P to S conversions over a broad range of depths (640 - 720 km). The Clapeyron slope predicts a deeper discontinuity in cold regions and a shallower discontinuity in hot regions.Other discontinuities
There is another major phase transition at 520 km which is possibly split into two transitions at 500 km and 560 km. Multiple phase transitions are needed to explain this, probably transitions of olivine (β to γ) and garnetGarnet
The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
in the pyrolite
Pyrolite
Pyrolite is a theoretical rock considered to be the best approximation of the composition of Earth's upper mantle. The definition varies, but it is generally considered as being 1 part tholeiitic basalt and 3 parts dunite. If fused experimentally, this mix yields high pressure tholeiitic basaltic...
mantle.
Other non-global phase transitions have been suggested at a range of depths.