The lithosphere refers to the hard, rocky outermost layer of any terrestrial planet or natural satellite. On planet Earth, the lithosphere consists mainly of the crust and the solid outer part of the upper mantle. One of the major spheres of the Earth, the lithosphere is primarily the terrestrial component that includes solid land masses such as continents and islands on which all biological life exists. The lithosphere is severely affected by human activities such as mining, deforestation, agriculture, overgrazing and urbanization.
Why is it called Lithosphere?
The term lithosphere came from the Greek words stonemeaning stones or stones, and of bullets, meaning sphere. The lithosphere or lithosphere thus refers to the hard and rocky outer layer of the Earth, consisting of the crust and the upper mantle. The lithosphere can extend to a depth of over 100 km. Below the lithosphere is the asthenosphere, which refers to the weaker, warmer, and much deeper part of the upper mantle. The lithosphere remains hard for longer periods of time and deforms elastically, while the asthenosphere deforms viscously. The lithosphere is, therefore, less ductile than the asthenosphere.
History of the concept of lithosphere
The concept of the lithosphere as the strong outer layer of the Earth was first proposed by the English mathematician Augustus Edward Hough Love in his monograph in 1911. The idea was further developed by the American geologist Joseph Barrell, who introduced the term lithosphere. Canadian geologist Reginald Aldworth Daly reinforced these concepts even further. The work of Love, Barrell and Daly is widely respected by the geophysical and geological communities, serving as the backbone of the theory of plate tectonics.
Composition of the Lithosphere
The composition of the lithosphere varies depending on whether it is on land or under the oceans. It is known that the Earth’s crust is not homogeneous and consists of different rock layers, including sedimentary (top), metamorphic (middle) and basaltic rocks (bottom). In addition, the lithosphere is broken up into many large tectonic plates that move slowly but continuously at an average rate of about 10 cm.
Types of Lithosphere
There are mainly two types of lithosphere: oceanic and continental.
Oceanic lithosphere refers to the lithosphere associated with the oceanic crust that exists beneath the seas and oceans. Oceanic lithosphere includes mafic crust and ultramafic mantle and tends to be comparatively denser than continental lithosphere. Young oceanic lithosphere is usually found at mid-ocean ridges, while old oceanic lithosphere thickens as it ages and moves away from the mid-ocean ridge, recycling at subduction zones.
This thickening of the oceanic lithosphere usually occurs by conductive cooling, where the hot asthenosphere is converted into the lithospheric mantle. It should be noted that the oceanic lithosphere is comparatively younger than the continental lithosphere and the oldest oceanic lithosphere is about 170 million years old.
Continental Lithosphere refers to the lithosphere associated with the continental crust. The average thickness of this lithosphere varies between 40km and 280km. The continental lithosphere makes up about 40% of the Earth’s surface and 70% of the volume of the Earth’s crust. Scientists believe that Earth originally had no continental crust, but eventually the fractional differentiation of oceanic crust led to the formation of continental crust.
Continental lithosphere is therefore considerably older than oceanic lithosphere, and the oldest parts of continental lithosphere lie beneath the cratons. However, due to its relatively low density, continental lithosphere is not recycled into subduction zones, as it cannot subduct more than 100 km.
Several geophysical studies of the 21st century have revealed that there may be many large pieces of recycled lithospheric elements that have descended up to 2900 km into the mantle. There is a strong belief that some pieces of lithosphere can still float in the upper mantle, and that some pieces can descend about 400 km while still being physically attached to continental plates higher up on the Earth’s surface.
Importance of the Lithosphere
Being one of the most important spheres of the Earth, the lithosphere helps a lot in the flourishing of life on the planet. The upper part of the lithosphere that chemically interacts with the other three spheres is called the pezosphere. In addition to being a rich source of minerals, the lithosphere provides forests, grasslands, farmland, and land for human settlements. The movement of tectonic plates is also responsible for the formation of mountains, volcanoes and continents.