A short note on the Crust

Introduction

The Earth comprises three layers – the crust, mantle, and core. The crust is the outermost layer and is defined as the solid Earth above the Mohorovicic discontinuity or, in short, simply Moho. The crust accounts for less than 1% of the Earth’s volume, and the crust’s temperature increases with depth. The crust is thin, a rocky veneer that comprises the continents and the oceans’ floors, and is not a homogenous shell. Based on their composition and physical properties, two distinct types of crust determine the very existence of separate continents and ocean basins.

Crust

The outer skin of the earth’s overlaying mantle varies in thickness from 6 to 90 km is called Crust.

Nature of the Crust

The crust is very thin, averaging only about 33km in thickness. Compared to its lateral extent, the thickness is only of a minuscule proportion. The rocks exposed at the surface show more significant heterogeneity and variation when examined. The densities of the crustal material are generally in the range of 2.5-3.3 g/cm³. Thus, the crust is denser than water but less dense than the materials beneath them. Therefore, they would sink in water but float on the materials beneath it. On a volume basis, oxygen is 94% of the Earth’s crust. Thus the Earth’s crust is composed of relatively light rigid materials than the denser, warmer, and less rigid material. In addition to the crust, the portion of the mantle just beneath the Moho is also relatively cool and rigid. This combined rigid outer region is called the lithosphere (Greek: Lithos-rocks).

Lithosphere

The lithosphere is floating on the underlying asthenosphere. The massive portions of the lithosphere reach deeper to obtain the needed buoyancy. This floating lithosphere can compare to a massive block of ice that extends deeper into the water while its top side extends proportionately higher to the air. The adjustments required to obtain buoyant equilibrium are called isostatic adjustments. The condition of buoyant equilibrium is called isostasy.

Origin and Evolution of the Crust

In the beginning, the surface of the Earth was very much like that of Mercury, Venus, and other planetary bodies. The Earth’s entire surface was littered with craters, and the temperature was also very high. Only at the Precambrian’s end, the planet has continents, ocean basins, and life.

The oldest rocks of the Earth’s crust collected from Greenland, Australia, and South Africa are continental rocks. Most of these rocks are metamorphosed sedimentary rocks. That means 

1. The oceans had already formed by this time.
2. Sediments demonstrate that some high-standing continental platforms evolved very early in the Earth’s history.

Also, the origin of the crust can be explained by three groups of theories:

1. Inhomogeneous earth accretion model.
2. Catastrophic model.
3. Non-catastrophic model.

Types of Crust

1. The continental crust
2. The Oceanic Crust

The Continental Crust

Structure and Composition of Continental Crust

• Continental crust has a range of thickness from 30 to 50 km. 
• The depth of the Moho beneath the continents averages about 35 km.
• Continental crust has a density ranging from 2.5 to 2.7 g/㎤.
• It is also referred to as being ‘granitic’; it is an assortment of various rocks which, if all melted together, could be converted to granites or, more specifically, granodiorite.

The structure of the continental crust comprises three layers.

1. Upper sedimentary layer

It consists of sediments that include a thickness of 15 km. These sediments may be absent, mainly where the ancient metamorphic and magmatic rocks come out onto the surface, such as the Baltic shield and the Aldan shield.

2. Granitic layer

It consists of metamorphic rocks and has a highly complex sequence of metamorphosed sediments such as granitic intrusions and granite gneiss.

3. Basaltic layer

Below the granitic layer is the basaltic layer. One of the essential characteristics of continental igneous rocks is that they are relatively richer in silicon and potassium but poorer in iron, magnesium, and calcium. These rocks are also the oldest rocks on the Earth and are as old as 3.8 billion years. This layer has a higher density compared to the upper layer. Based on the character, it is divided into two types.

1. Regions-cratons
2. Orogenic belts

The Oceanic Crust

• The oceanic crust is approximately 5 to 12 km thick.
• It has an average density of 3.0 g/㎤.
• Oceanic crust is known for its remarkable uniformity.
• It is the uppermost layer of the tectonic plates. They have a denser structure.

The upper sedimentary layer varies in thickness and composition. They consist of calcareous and siliceous shells of microscopic marine organisms together with red clay. 

The upper layer consists of basaltic flows, which are 1 to 2.5 km thick. These basaltic flows are pillow-shaped and were initially fed by numerous fissures. The pillow lava represents the eruption of volcanoes on the seafloor. The next layer consists of dykes and coarse-grained gabbro. Below the gabbro are peridotites composed of olivines and pyroxenes. The boundary between the crust and the mantle is called the Moho. The oceanic crust has relatively heavier iron, magnesium, and calcium elements.

Most abundant element in the Earth’s Crust

1. Oxygen
2. Silicon
3. Aluminum
4. Iron
5. Calcium

Conclusion

We are fortunate to have the crust because that is the one that makes the Earth a perfect planet to live on. The crust consists of a highly superficial layer; it appears to be a solid rock covering our world. It is a collection of enormous rock plates of heterogeneous composition floating upon an equally colossal ocean of magma that is the mantle. Though thin, it is an essential layer where tectonic and chemical activities occur. Finally, it provides a home for us hence we should realise its value and protect ourselves by reducing pollution.