The transition zone is the part of

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

's mantle that is located between the lower and the

upper mantle The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (geology), crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle (Earth), lower man ...

, most strictly between the seismic-discontinuity depths of about , but more broadly defined as the zone encompassing those discontinuities, i.e., between about depth. Earth's solid, rocky mantle, including the mantle transition zone (often abbreviated as MTZ), consists primarily of

peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high pr ...

, an

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are usua ...

igneous rock Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

. The mantle was divided into the upper mantle, transition zone, and lower mantle as a result of sudden seismic-velocity discontinuities at depths of . This is thought to occur as a result of rearrangement of grains in

olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...

(which constitutes a large portion of peridotite) at a depth of , to form a denser crystal structure as a result of the increase in pressure with increasing depth. Below a depth of , evidence suggests due to pressure changes ringwoodite minerals change into two new denser phases, bridgmanite and periclase. This can be seen using body waves from

earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...

s, which are converted, reflected or refracted at the boundary, and predicted from

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. It provides information that allows interpretation of surface measure ...

, as the phase changes are temperature and density-dependent and hence depth dependent.

410 km discontinuity – phase transition

A peak is seen in seismological data at about as is predicted by the transition from α- to β-Mg₂SiO₄ (

to wadsleyite). From the Clapeyron slope, this change is predicted to occur at shallower depths in cold regions, such as where subducting slabs penetrate into the transition zone, and at greater depths in warmer regions, such as where

mantle plume A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic ho ...

s pass through the transition zone. Therefore, the exact depth of the "410 km discontinuity" can vary.

660 km discontinuity – phase transition

The 660 km discontinuity appears in PP precursors (a wave which reflects off 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 (). The Clapeyron slope predicts a deeper discontinuity in cold regions and a shallower discontinuity in hot regions. This discontinuity is generally linked to the transition from ringwoodite to bridgmanite and periclase. This is thermodynamically an endothermic reaction and creates a viscosity jump. Both characteristics cause this

phase transition In physics, chemistry, and other related fields like biology, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic Sta ...

to play an important role in geodynamical models. Cold downwelling material might pond on this transition.

Other discontinuities

There is another major phase transition predicted at for the transition of olivine (β to γ) and

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, de ...

in the pyrolite mantle. This one has only sporadically been observed in seismological data. Other non-global phase transitions have been suggested at a range of depths.

References

{{DEFAULTSORT:Transition Zone Structure of the Earth