Perovskite

Perovskite is a calcium titanium oxide mineral species composed of calcium titanate, with the chemical formula CaTiO₃.
The mineral was discovered in the Ural Mountains of Russia by Gustav Rose in 1839 and is named after Russian mineralogist Lev Perovski (1792–1856).^[1]
It lends its name to the class of compounds which have the same type of crystal structure as CaTiO₃ (^XIIA^2+VIIB⁴⁺X^2–₃) known as the perovskite structure.^[9] The perovskite crystal structure was published in 1945 from X-ray diffraction data on barium titanate by the Irish crystallographer Helen Dick Megaw (1907–2002).


Basically found in the earth’s mantle, the perovskite’s occurrence at Khibina Massif is restricted to the under saturated ultramafic rocks and foidolites, due to the instability in a paragenesis with feldspar. The complexity is made by an extended series of rocks from early alkaline ultramafic members to late carbonatites that comprise alkaline and mafic igneous rocks such as nepheline syenite, melilitite, kimberlite and rare carbonatites in ultramafites. Perovskite occurs as small anhedral to subhedral crystals filling interstices between the rock-forming silicates

D double-prim

The ~200 km thick layer of the lower mantle directly above the boundary is referred to as the D′′ ("D double-prime" or "D prime prime") and is sometimes included in discussions regarding the core–mantle boundary zone.^[1] The D′′ name originates from the mathematician Keith Bullen's designations for the Earth's layers. His system was to label each layer alphabetically, A through G, with the crust as 'A' and the inner core as 'G'. In his 1942 publication of his model, the entire lower mantle was the D layer. In 1950, Bullen found his "D" layer to actually be two different layers. The upper part of the D layer, about 1800 km thick, was renamed D′ (D prime) and the lower part (the bottom 200 km) was named D′′.
The bottom of D′′ has been observed in some regions to be marked by a seismic velocity discontinuity (sometimes known as the 'Gutenberg discontinuity', after German geophysicist Beno Gutenberg) which besides features ultra-low velocity zones (ULVZs. )^[2].

Today's Mantle, a Guided Tour - From here to "hell," or the D'' layer.

All of the evidence that I've been talking about in this series, all of the experiments and theoretical work, have put a lot of detail into that big blank space between the crust and the core. Let's see what we've got.
The clearest picture we have of the mantle is the one outlined by a century of seismic data. It shows that between the crust and the top of the core 2,888 kilometers down (give or take a few kilometers), there are several distinct boundaries—scientists use the more neutral name "seismic discontinuities," and so will I. You can follow along on this diagram as we move downward. The mantle has at least four layers, and all of them are subjects of vigorous scientific discussion.

_{Cross section from PlanetScapes.}

The Upper Mantle

The crust, of course, is a scrambled mess of rock types, and so is the uppermost mantle beneath it. The strong discontinuity at the bottom of the crust is named to honor the seismologist Andrija Mohorovičić, the man who found it early in this century. I've never once heard a geologist call it the Mohorovičić (mo-ho-ROV-i-chich) discontinuity, but that's its proper name. Everyone calls it the Moho. The rock above and below the Moho is solid, but as we go deeper the upper mantle gradually turns soft—that soft zone is what allows the tectonic plates of the crust to move about. (In plate tectonics, the soft zone is the asthenosphere and the hard rocks above it make up the lithosphere.)
The soft zone bottoms out around 220 km depth, but as with the rocks above, there's a lot of variation in the material there. That comes from the processes of plate tectonics—downward subduction of plates at the deep ocean trenches and formation of plates at the mid-ocean ridges. Slabs of subducted plates going downward are cooler than the rock around them, and in other places hotspots are rising. The upper mantle is as busy in its own way as the surface, constantly mixing and being stirred like stew simmering.

The Transition Zone

There is a worldwide seismic discontinuity at about 410 km depth, which has no name yet, just "the 410-km discontinuity." And there's another at 660 km (or 670 km, depending on who you ask—I prefer to round it off at 666 km). These mark the transition zone. Above is the upper mantle, beneath is the lower mantle. Earthquakes occur all the way down to 660 km, but not below.
These two discontinuities appear at those depths because they mark pressure thresholds, where the abundant mineral olivine suddenly changes to denser crystal forms. Mantle rock gains density by several percent at these discontinuities, and that density change affects the mixing action of the upper mantle. Descending crustal slabs tend to stall in the transition zone, and after a while, a few dozen million years, they mix with surrounding rocks and return to circulation. Many argue that some slabs do seem to push down past the 660-km level, and some deeper rocks may cross it upward, so it's not a complete barrier. Opponents say that the barrier prevents any exchange of material, although exchange of heat takes place. On the whole it appears that most of the mantle's physical and chemical activity is confined to the upper part.

The Lower Mantle and D''

Beneath 660 km, there's nothing much in the seismic picture. The lower mantle appears to have little structure. Many researchers claim to see old crustal slabs there, falling toward the base of the mantle, but the evidence is subtle and disputed. Others argue that the high pressure prevents the rock of the lower mantle from convecting at all. Basic ideas about the Earth's heat budget depend strongly on the lower mantle. Some argue that the minerals of the lower mantle are large enough and clear enough that heat moves through there not by conduction or convection, but mainly by radiation.
Near the bottom of the lower mantle, at 2,700 km or so, we find a strange pair of discontinuities. The top one has no name at all, and the lower one is just called "the core-mantle boundary" or CMB. The rock in between seems to be quite different from the mantle above it. Part of that may, again, reflect a change in mineral structure from the so-called perovskite structure to a recently discovered form, called post-perovskite for the time being. Beneath this layer lies the liquid outer core, an iron-nickel alloy. This lowermost part of the mantle is called D'', "D-double-prime," for lack of a better name (here's the whole story).
The scientific picture of D'' is fuzzy, so I'll be a bit loose describing it. The most popular idea is that it's the dregs of the Earth, a place where slabs from the crust come to die and where iron-silicate slag builds up along the edge of the core. Slow stirrings in the deepest mantle, and chaotic iron swirlings in the core's magnetic dynamo, both seem to push the stuff of D'' into heaps here and thin spots there. Sometimes, it is conjectured, this allows a huge pulse of heat energy to rise from the core, like the activity of a Lava Lamp™. The whole mess is squeezed at 135 gigapascals (20 million pounds per square inch) and is white-hot. If there is a Hell on Earth, D'' is where it is, and maybe "hell" is the name we should give it.

Post-perovskite in D double Prime Layer

The D'' (Dee double prime) layer surrounds the Earth's core and is between 0 and 300 km thick (variable thickness). It is at the interface between two chemically distinct regions, the rocky mantle and the metallic core.
This may be the most dynamic and active zone in the Earth’s interior. It has some strange seismic properties and for some time scientists have discussed its probable main composition.

In 2004 Japanese researchers found that high temperatures and pressures like those existing in the D” layer transform perovskite, the major mineral in Earth's mantle, into a new mineral called post-perovskite. New research published on 17 January 2006 in Proceedings of the National Academy of Science of USA seem to confirm that the seismic properties of post-perovskite are much like the previously inexplicable properties found in the D'' layer. This is convincing evidence that post-perovskite is in the D'' layer and produces its strange seismic properties.

Differentiate between weathering and erosion

Definition of Weathering is the decomposition of rocks, soils and their minerals through direct contact with the Earth's atmosphere. Chemical weathering involves a chemical change in at least some of the minerals within a rock. Mechanical weathering involves physically breaking rocks into fragments without changing the chemical make-up of the minerals within it. It’s important to keep in mind that weathering is a surface or near-surface process. As we know, metamorphism also produces chemical changes in rocks, but metamorphic chemical changes occur at depth where either the temperature and/or pressure are significantly higher than conditions found on the Earth’s surface.

Definition of Erosion or mass wasting is displacement of solids (soil, mud, rock and other particles) usually by the agents of currents such as, wind, water, or ice

External Causes for weathering are atmospheric conditions like air, pressure etc.
External Causes for erosion are wind, water, ice, humans etc

Movement of eroded material occurs.

Movement of eroded weathered material does not occurs.

In short: 
If a particle is loosened, chemically or mechanically, but with no movement, call it weathering. Once the particle starts moving, call it erosion.

GeoGraphy PMS Past Papers 2009