Origin of the earth's Crust

Crust is the outermost solid cell of a rocky planet, dwarf planet, or natural satellite.
It is distinguished from the underlying mantle by its chemical composition.
Most terrestrial planets have uniform crusts. However, Earth has two distinct types: continental crust and oceanic crust.

Earth’s Crust

“Earth’s crust is the top component of the lithosphere. It includes the crust and upper mantle.” – EC Robinson (2011) in "The Interior of the Earth".
Earth crust is the thinnest and the most rudimentary layer that makes up the Earth. It contains 1% of earth's volume.

The crust is a dynamic structure. The lithospheric crust is broken into tectonic plates.
The boundary between the crust and mantle is known as the Mohorovicic discontinuity.
The temperature of the crust increases with depth. It reaches to about 100 °C to 600 °C at the mantle boundary. – Robert Peele (1911).

Formation of crust: The crusts of Earth, Mercury, Venus, Mars, Io, Moon and other planetary bodies is formed via igneous processes.
Modification of crust: The crust is later modified by erosion, meteorite impact, sedimentation, volcanism etc.

It is a planet's original crust. It is formed by magma solidification at the end of planetary accretion.
This crust is likely destroyed by large impacts and reformed many times during the Era of Heavy Bombardment. –Ross Taylor (1989). "Growth of planetary crusts".
None of Earth's primary crust has survived till today.
However, information about primary crust can be collected by studying the terrestrial planets. Eg. Mercury's highlands and anorthosite highlands of the Moon are primary crust.

It is formed by partial melting of silicate materials in the mantle. Hence, it is usually basaltic.
It is the most common type of crust in the Solar System.
E.g., Mercury, Venus, Earth, Mars.
On Earth, it is formed primarily at the mid-ocean ridges.

The Earth’s crust was formed during the accretion of the planet.
The lighter and volatile elements formed a thin layer on the primitive planet, which became the crust.

Criticism:

This model suggests that non-volatile elements can only be found in the mantle.
However, non-volatile elements such as uranium and thorium are found on the Earth’s crust.
Hence, this theory is highly unlikely to be true.

The asteroids and other objects that impacted the Earth, were melted and formed the crust.
The oceanic crust, which is mainly composed of basalt, is formed by a basalt asteroid impact.

Criticism:

The basalts in lunar maria were not due to an asteroid collision.
Furthermore, the number of basalts produced from an impact event was too insignificant to form crusts.
A majority of the impact events on Earth happened after oceanic crusts were formed.
Therefore, this theory is also unlikely as well.

The crustal origin of the Earth was due to its internal processes.
After the accretion, heat retained by the earth resulted in the complete melting of the upper mantle. It formed a magma ocean, which covered the surface of the earth.
As the Earth cooled, the magma ocean crystallised to form a widespread crust.
Another possible explanation was that the melted upper mantle rose up to form a crust.

Merits:

The magma ocean could explain some properties of the earth’s crust.
The uniform composition of the crust could be formed by a homogeneous magma ocean.
The layered composition of earth’s crust may be due to the cooling of magma oceans over time.
Thus, the terrestrial model most likely explains the formation of the earth’s crust.

The oceanic crust was first generated along the ocean ridges.
It was formed 4.5 billion years ago, earlier than the first appearance of the continental crust.
The early oceanic crust is likely to be basalts in composition.

The oldest continental crust is about 4 billion years ago.
Granite continental crust only appeared 3 billion years ago.
Earth is the only planet in the solar system that has a continental crust, (mainly because it requires the presence of water and the subduction of crusts)
The seawater cools the hot mantle at the subduction zones. It allows fractional crystallisation to take place to produce a granite crust.

We cannot observe all geological evolutionary phenomena in the span of a human lifetime.
The fragmentary geologic evidence provide the basis for hypothetical solutions to problems relating to the early Earth system.
Only a combination of these theories creates both a framework of current understanding, and also a platform for future study.