Lithosphere

The lithosphere is where the story of our Earth begins, from dangerous volcanoes to diverse and mesmerising landscapes, it holds the history of millions of years under its surface. Shaped and powered by the dynamic activities of tectonic plates, the lithosphere is not just the ground we walk on; it is the face of our planet, and a beautiful one in this case. Here in these notes, we will explore the activities, composition, and science behind the natural process happening on this layer.  

1.0Lithosphere Definition

The Earth's lithosphere is the outermost hard layer, made up of the crust and the uppermost part of the mantle layer. The depth of this layer extends up to almost 100 Km and is broken into pieces known as the tectonic plates. These plates float on the asthenosphere, a semi-fluid layer of Earth’s mantle below. 

The movement of tectonic plates causes several natural phenomena such as earthquakes, volcanoes, and continental drift. Although the average width of the lithosphere is approximately 100 km, it can reach up to 200 km under the stable continental regions, known as cratons. See the following diagram of the lithosphere to understand it better.

2.0Types of Lithosphere 

Based on the location, composition, thickness and density, the Earth’s lithosphere is divided into two main types, which are: 

Continental Lithosphere

As the name suggests, the continental lithosphere is found under the continents, which are Asia, Africa, Europe, etc. It is generally composed of granite and other silica-rich rocks, also known as felsic rocks. The thickness of this layer ranges from 30 to 70 km and is less dense (almost 2.7g/cm³) than the oceanic lithosphere. Continental lithosphere includes some of the oldest regions of the Earth, especially in the areas of stable continental cores or cratons. This is the type of lithosphere where all the beautiful landscapes of Earth are present, such as mountains, plateaus, deserts, and valleys. 

Oceanic Lithosphere

The oceanic lithosphere can be found beneath the ocean floors. It is generally composed of basalt and gabbro rocks, which are iron and magnesium-rich rocks, also known as mafic. The oceanic lithosphere is thinner than the continental one, with a thickness of almost 5 to 10 km. The density of this type is 3.0 g/cm³, which is the reason why this layer sinks under the continental plates at the subduction zones (meeting point of oceanic and continental lithosphere). These layers are younger than the continental ones due to the process of seafloor spreading, a process where the old layer is destroyed and created at the plate boundaries. The example of the lithosphere includes mid-ocean ridges such as the Mariana Trench, etc. 

3.0Layers of the Lithosphere

The lithosphere is made up of distinct layers that together form the rigid outer shell of the Earth. The lithosphere is mainly composed of two layers, each with a certain depth and thickness: 

• The Crust:
• • It is the outer layer of the Earth.
  • It is further categorised into two categories: continental crust and oceanic crust.
  • The continental crust is thicker (30–70 km) and consists of granite predominantly, while the oceanic crust is thinner (5–10 km) and comprises basalt mostly.
• The Uppermost Mantle:
• • It is situated just below the crust.
  • Although it is of the mantle, the topmost part is hard and solid and constitutes the bottom of the lithosphere.
  • In conjunction with the crust, it acts like an individual rigid layer.

4.0Components of the Lithosphere

The lithosphere consists of several vital constituents that make up the framework of the Earth's exterior. These consist of rocks, minerals, soil, fossil fuels, tectonic plates, landforms, and natural resources. Each of them plays a vital function in Earth's processes.

1. Rocks: Rocks are the major construction material of the lithosphere and are categorised into igneous, sedimentary, and metamorphic rocks. They constitute the solid crust of the Earth and contain geological history in their layers, structure, and composition.
2. Minerals: Minerals are naturally occurring inorganic substances having a definite chemical composition and crystal structure. Minerals are the raw materials of rocks and are vital for use in industry, construction, and even for the development of advanced technologies such as electronics and batteries.
3. Soil: Soil develops as the result of long-term weathering of rock and organic matter decomposition. Soil is an essential lithosphere layer that nurtures plants, holds nutrients, and is vital to agriculture and the natural ecosystem.
4. Fossil Fuels: Fossil fuels like coal, oil, and natural gas are located in sedimentary layers of the lithosphere. They are created from the remains of dead organisms and are essential sources of energy, but they cause pollution and climate change.
5. Tectonic Plates: The lithosphere is composed of a number of hard tectonic plates which travel gradually across the more supple asthenosphere. It is they, which in motion, account for the most important geologic activities, including earthquakes, volcanism, the construction of mountains and the basin-building that fills oceanic basins.

5.0Properties of Lithosphere

• The lithosphere is the rigid and brittle layer of the Earth, meaning it can crack or break under pressure. 
• The temperature of this layer is less than that of other underlying layers beneath it, like the mantle and core. 
• It varies in thickness under different surfaces. For instance, the lithosphere is thicker under continents, while it is thinner beneath the ocean floors. 
• The lithosphere is broken into tectonic plates that move slowly over the asthenosphere for about 2 to 5 cm per year. 
• This is the layer of Earth where all the lifeforms exist, from terrestrial and marine to microorganisms.

6.0Lithosphere’s Relationship with Other Spheres

The Earth doesn’t consist of only the lithosphere but also other spheres of life, which interact with the lithosphere to make this layer the way we see it today. The lithosphere forms a relation with every other layer of Earth like this: 

• Lithosphere and Atmosphere: The lithosphere significantly affects the atmosphere through natural phenomena like volcanic eruptions, which release gases like carbon dioxide and sulfur oxides. 
• Lithosphere and Hydrosphere: The waters from rivers, oceans, and other forms of the hydrosphere interact and reshape the lithosphere through the process of erosion, weathering, and sediment transport. 
• Lithosphere and Biosphere: All the components of the biosphere, including plants and animals, depend upon the resources of the lithosphere. 
• Lithosphere and Cryosphere: Environmental changes lead to the melting of glaciers, exposing new lithospheric landforms, ultimately affecting the sea level and human settlements.