Lithosphere-Definition, Properties, Types, Practice problems, FAQs

The term lithosphere in geology refers to the outermost layer of the Earth. The same term can also be applied to any similar layer on terrestrial planets like Earth. The lithosphere consists of the crust and the uppermost part called the mantle. The mantle layer behaves elastically as if it is a very viscous liquid. However, such movements require thousands of years to happen. The tectonic plates are present on this layer, and this is the reason why the plates move. The crust and the mantle are differentiated from each other based on their chemical structure and mineral composition. In this article, we will explore the lithosphere in detail.

Table of contents

• History
• Properties
• Types
• Tectonic plates
• Practice problems
• FAQs

History

In 1911, A.E.H. Love first brought about the concept that the Earth consists of a hard outer layer. Hence, he composed a monograph called ‘Some Problems of Geodynamics’. Joseph Barrell then further developed these ideas, thereby writing a series of papers on the subject. He also introduced the term ‘lithosphere.’ His assumption was that the outer layer must be situated on a plastic layer. The latter behaves like a liquid. The source of his hypothesis was anomalies in gravitational forces found across the crust. In 1940, Reginald Aldworth Daly agglomerated these ideas and named his work ‘Strength and Structure of Earth.’

Properties

The layer which lies directly below the lithosphere is called the asthenosphere. The asthenosphere is a viscous layer and is the reason behind why the tectonic plates move on the surface of the Earth. The rocks found in the lithosphere are plastic in nature. However, these rocks in the lithosphere are not as ductile as in the asthenosphere. Ductility can be defined as the ability of a material to change its shape under stress or pressure. When an external force is applied, the rocks deform due to stress. Lithosphere-Asthenosphere Boundary (LAB) is the boundary where scientists distinguish between the lithosphere and the asthenosphere.

Types

• Oceanic lithosphere: Oceanic lithosphere is older than the continental lithosphere and is associated with the oceanic basins. They are denser than the continental lithosphere. The mid-ocean ridges contain the youngest oceanic lithosphere layer. Moving farther away from the mid-ocean ridges, the thickness of the oceanic lithosphere increases. The oldest oceanic lithosphere layer is about 140 km in thickness. It also acts as the boundary for the flow of convection currents in the mantle. Hence, as a result of this, some parts of the asthenosphere cool into the oceanic lithosphere. The newer lithosphere layers are less dense compared to the asthenosphere. However, the older layers are denser than the asthenosphere. The reason behind this is thermal contraction.
• Subducted lithosphere: It is a term we use to refer to pieces of lithosphere that have broken off from the upper layer and later have sunken deep into the mantle. They extend as deep as 2900 km near the core-mantle boundary. Some of them are located in the upper regions of the mantle, and others have sunken below the level of the lithosphere but are still attached to it.
• Continental lithosphere: Continental lithosphere is located near the continental crust. This is also where the dry land is located. The thickness of this region varies anywhere from 40 km to 280 km. The periphery thickness of 30 km to 50 km is termed crust. The crust and the mantle are differentiated on the basis of their chemical composition. The continental lithosphere cannot further fall down to lower regions. However, it resurfaces after going to a depth of about 100 km. Hence, the continental lithosphere is a permanent fixture on the Earth.

Tectonic plates

Tectonic plates are large chunks of the landmass that float on the mantle layer. All the activities related to tectonic plates take place near the boundary of the plates. They collide, then move apart, and also slide under each other. This activity also triggers many geological events like earthquakes, the eruption of volcanoes, orogeny (the process of building up mountains) and deep-sea trenches. The tectonic plates are thin near the rift valleys and mid-ocean ridges.

The major tectonic plates are located in the USA, Caribbean, Latin America, Nova Scotia, Antarctic circle, Eurasia, Africa, India, Philippines, Australia, Pacific, Juan de Fuca, Cocos, and Nazca.

Xenoliths are minerals which are located in the deep regions of the Earth. They have come out of the surface as a result of volcanic eruptions. Studies show that a few lithospheric plates have been in existence for about 3 billion years.

Practice problems

Q1. Identify and write about two types of the lithosphere.
Answer. There are two types of lithosphere–oceanic and continental. The oceanic lithosphere exists in the ocean basins. The continental lithosphere is made of the continental crust and the parts of the upper mantle which are non-convecting in nature.

Q2. Discuss about the composition of the lithosphere.
Answer. The earth’s crust is not a uniform body. It is made up of different sections of rocks which include sedimentary rocks (on the top). Granitic rocks and metamorphic rocks make up the space in the middle. Basaltic rocks located at the bottom.

The earth’s crust is also loaded with several large dynamic tectonic plates. These plates move slowly at an average rate of 10 cm per year.

Q3. Explain how hot the lithosphere is.
Answer. The temperature at the earth's surface is close to 00 C (freezing temperature of water). The temperature increases to about 15000 C at 100 km depth. From 100 km onwards, the temperature increases slowly by only 300 C for every 100 km and reaches a maximum temperature of about 25000 C.

FAQs

Q1. How old is the lithosphere?
Answer. The age of the oceanic lithosphere is much less compared to the continental lithosphere. The oldest oceanic lithosphere is approximately 170 million years old. On the other hand, parts of the continental lithosphere are billions of years old.

Q2. Why is the lithosphere important?
Answer. The term lithosphere refers to the Earth’s outermost rigid, rocky layer. It consists of the crust and the uppermost solid layer is called the mantle. Additionally, it extends to a depth of about 96 miles. It also disintegrates into many separate rigid blocks or plates. It is crucial for living things on the Earth to thrive on. The lithosphere provides us with forests and grasslands for agriculture and human habitation.

Q3. What is the pedosphere?
​​​​​​​Answer. The pedosphere is the outermost layer of the lithosphere. Through the soil-forming process, it chemically reacts with the atmosphere, hydrosphere, and biosphere. It can also be termed as the soil mantle of the earth.

Q4. What is the composition of the earth’s crust?
​​​​​​​Answer. The earth’s crust is composed of many layers with sedimentary rocks on top, granitic plus metamorphic rocks in the centre, and basaltic rocks located at the bottom. Additionally, a large number of tectonic plates make up the earth’s crust.