Our World i.e. Earth, is the third planet from the sun and the only planet known to maintain life. This layer that maintains life on earth is called the lithosphere. The Lithosphere is composed of the crust and upper most solid mantle. While the Asthenosphere, which lies beneath the lithosphere, is composed of the upper most weaker part of the mantle. As we move from the lithosphere to the asthenosphere the temperature increases. This increase in temperature as well as extreme pressure causes rocks to become plastic. In time these semi molten rocks will flow. The aforementioned occurrence, at a certain depth and temperature gives rise to the asthenosphere layer. These two layers are crucial due to the mechanical changes that occur within these layers, as well their effects on society. Their differences and interactions will be further discussed in the following article.
History/Formation
The lithosphere concept began in 1911 by A. E. H. Love, and was further developed by other scientists such as J. Barrell, and R. A. Daly[i]. Whereas the asthenosphere concept was proposed at a later stage in history i.e. 1926, and confirmed in 1960 by seismic waves resulting from the Great Chilean earthquake. They proposed gravity anomalies over the continental crust, where a strong upper layer floated over a weak lower layer i.e. asthenosphere. As time passed these ideas were expanded. However, the basis of the concept consisted of the strong lithosphere which rested on the weak asthenosphere[ii].
Structure
The lithosphere consists of the crust and uppermost mantle (consisting largely of peridotite), which makes up the rigid outer layer that is divided by tectonic plates (large slabs of rocky material). The movement (collision and sliding past each other) of these tectonic plates is said to cause geologic events such as deep-sea rifts, volcanoes, lava flows, and mountain building. The lithosphere is surrounded by the atmosphere above and the asthenosphere below. Although the lithosphere is considered to be the most rigid of layers, it is also considered elastic. However, its elasticity and ductility, is much less than the asthenosphere and is dependant on the stress, temperature, and the earths curvature. This layer ranges from a depth of 80km to 250km below surface, and is considered a cooler environment than its neighbour (asthenosphere), approximately 400 degrees Celsius[iii].
In contrast to the lithosphere, the asthenosphere is believed to be much hotter, i.e. between 300 to 500 degrees Celsius. This is due to the asthenosphere being mostly solid with some regions containing partially molten rock. Which contributes to asthenosphere being regarded as viscous and mechanically weak. Thus it is considered more fluid in nature than the lithosphere which is its’ upper border, while its’ lower border is the mesosphere. The asthenosphere may extend to a depth of 700km below the earths’ surface. Hot materials that make up the mesosphere heat up the asthenosphere, causing melting of rocks (semi-fluid) in asthenosphere, provided temperatures are high enough. The semi fluid areas of the asthenosphere allow for movement of the tectonic plates in the lithosphere[iv].
Chemical composition
The lithosphere is divided into two types, namely:
- Oceanic lithosphere – a denser oceanic crust, with an average density of 2.9 grams per cubic centimetre
- Continental lithosphere – a thicker crust that stretches 200km below the surface of the earth, with an average density of 2.7 grams per cubic centimetre
The chemical composition of the lithosphere contains approximately 80 elements and 2000 minerals and compounds, while the slush-like rock in the asthenosphere is made of iron-magnesium silicates. This is almost identical to the mesosphere layer. The oceanic crust is darker than the continental crust due to less silica, and more iron and magnesium[v].
Plate tectonics/Activity
The lithosphere contains 15 major tectonic plates, namely:
- North American
- Nazca
- Scotia
- Caribbean
- Antarctic
- Eurasian
- African
- Indian
- Australian
- Pacific
- Juan de Fuca
- Philippine
- Arabian
- South American
- Cocos
Convection caused by heat from lower layers of the earth, drives the asthenospheric flow, which causes the tectonic plates in the lithosphere, to start to move. Tectonic activity occurs mostly at the boundaries of said plates, resulting in collisions, sliding against each other, even tearing apart. Producing earthquakes, volcanoes, orogeny, as well as ocean trenches. The activity in the asthenosphere under the oceanic crust, creates new crust. By forcing the asthenosphere to the surface, at mid ocean ridges. When the molten rock extrudes, it cools, forming the new crust. Convection force also causes the lithosphere plates at the ocean ridges to move apart[vi].
The Lithosphere – Asthenosphere boundary (LAB)
The LAB can be found between the cool lithosphere and warm asthenosphere. Therefore, represents a rheological boundary, i.e. containing rheological properties such as thermal properties, chemical composition, extent of melt, and difference in grain size. LAB depicts the transition from hot mantle in the asthenosphere to the colder and more rigid lithosphere above. The lithosphere is characterised by conductive heat transfer whereas the asthenosphere is a boundary with advective heat transfer[vii].
Seismic waves moving through the LAB, travel faster across the lithosphere than the asthenosphere. Accordingly wave speeds in some areas are reduced by 5 to 10%, 30 to 120km (oceanic lithosphere). This is due to the different densities and viscosity of the asthenosphere. The boundary (where seismic waves slow down) is known as the Gutenberg discontinuity which is believed to be inter-related to the LAB, due to their common depths. In oceanic lithosphere the LAB depth, can range between 50 to 140km, except at mid-oceanic ridges where it isn’t any deeper than the new crust that is being formed. Continental lithosphere LAB depths are a source of dispute, scientists estimate a depth ranging from 100km to 250km. Ultimately continental lithosphere and the LAB in some older parts, are thicker as well as deeper. Suggesting that their depths are age dependant[viii].
Comparison of the Lithosphere and Asthenosphere
| Lithosphere | Asthenosphere |
| The lithosphere concept was proposed in 1911 | The asthenosphere concept was proposed in 1926 |
| Lithosphere is composed of the crust and upper most solid mantle | Asthenosphere is composed of the upper most weaker part of the mantle |
| Lies beneath the atmosphere and above the asthenosphere | Lies beneath the lithosphere and above the mesosphere |
| The physical structure consists of a rigid outer layer that is divided by tectonic plates. It is regarded as rigid, brittle, and elastic. | The physical structure is mostly solid with some regions containing partially molten rock, which exhibits plastic properties |
| Characterized as elastic and less ductile | Has a higher degree of ductility than the lithosphere |
| Ranges from a depth of 80km and 200 km below the earths surface | Extends to a depth of 700km below the earths’ surface |
| Approximate temperature of 400 degrees Celsius | Approximate temperature ranging from 300 to 500 degrees Celsius |
| Has a lower density than the asthenosphere | Asthenosphere is denser than the lithosphere |
| Allows for conductive heat transfer | Allows for advective heat transfer |
| Seismic waves travel at faster speeds across lithosphere | Seismic waves travel 5 to 10% slower in asthenosphere than in lithosphere |
| Rocks are under much less pressure forces | Rocks are under immense pressure forces |
| Chemical composition consists of 80 elements and approximately 2000 minerals | Asthenosphere is mainly composed of iron-magnesium silicates |
Conclusion
The earth is composed of 5 physical layers namely; lithosphere, asthenosphere, mesosphere, outer core, and inner core. This article focused on the first two layers, and their differences. Which forms a part Geology; the science that deals with the earths structure, history, and its’ processes. Geology facilitates study surrounding some of humanities formidable issues, such as climate change, natural disasters (tsunamis, earthquakes, volcanic eruptions, landslides, etc.), as well as resource depletion (water, energy, mineral). The solutions to our current environmental challenges require a knowledge of our earth structures and systems. This world is our home. We are completely reliant on earth for our survival. Therefore it is only logical for us to understand our environment in order to promote sustainable living.