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The crust of the Earth is composed of a great variety of igneous, metamorphic, and sedimentary rocks. The upper part of the mantle is composed mostly of peridotite, a rock denser than rocks common in the overlying crust.
The boundary between the crust and mantle is conventionally placed at the Mohorovi? discontinuity, a boundary defined by a contrast in seismic velocity.
The thickness of the crust ranges between about 20 and 120 km.
Crust on the far side of the Moon averages about 12 km thicker than that on the near side.
A theoretical protoplanet named "Theia" is thought to have collided with the forming Earth, and part of the material ejected into space by the collision accreted to form the Moon.
As the Moon formed, the outer part of it is thought to have been molten, a “lunar magma ocean.” Plagioclase feldspar crystallized in large amounts from this magma ocean and floated toward the surface. The upper part of the crust probably averages about 88% plagioclase (near the lower limit of 90% defined for anorthosite): the lower part of the crust may contain a higher percentage of ferromagnesian minerals such as the pyroxenes and olivine, but even that lower part probably averages about 78% plagioclase. The underlying mantle is denser and olivine-rich.
In other words, it is defined giving the concentration of each component.
Because there are different ways to define the concentration of a component, as a consequence there are also different ways to define the composition of a mixture.
In contrast, the bulk of the continental crust is much older.
Estimates of average thickness fall in the range from about 50 to 60 km.
Most of this plagioclase-rich crust formed shortly after formation of the moon, between about 4.5 and 4.3 billion years ago.
For example it can be expressed as molar fraction, volume fraction, mass fraction, molality, molarity or normality.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equals the total mass of the products leading to the insight that the relations among quantities of reactants and products typically form a ratio of positive integers.
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Partly by analogy to what is known about the Moon, Earth is considered to have differentiated from an aggregate of planetesimals into its core, mantle and crust within about 100 million years of the formation of the planet, 4.6 billion years ago.