What Is The Lithosphere?

The lithosphere is the rigid, solid outer layer of the Earth. We explain its structure and composition, its tectonic make-up and its lithification of carbon and other chemicals.
San Andreas Fault Aerial View
San Andreas Fault, South California extends 1,200 kms. Photo: © picryl/public domain

The lithosphere (named after the Greek word “lithos” meaning “rocky”) is the rigid, solid outer layer of the Earth which can extend to a depth of about 100 km (60 mi). The actions of this part of the planet are rarely visible and are typically encountered only during moments of dramatic seismic activity involving earthquakes and volcanic eruptions. Even so, it plays a much underestimated role in climate change mitigation through the lithification of large amounts of heat-trapping carbon dioxide, that might otherwise lead to rising temperatures and other effects of global warming.

The lithosphere consists of the crust and the uppermost portion of the mantle, and is made up of a number of irregular-shaped blocks of rock (think jigsaw pieces) known as tectonic plates, including seven large and numerous other smaller plates. 1

The top part of the lithosphere is called the pedosphere. This is the slice of Earth’s crust that interacts directly with the atmosphere (air), hydrosphere (water), cryosphere (frozen water) and biosphere (all living things) through the soil forming process.

The lithosphere is the interface between the surface and the geological depths, and plays an important part in the planet’s climate system through its involvement in the slow carbon cycle. 2 3

What Is Earth’s Structure?

The centre of the Earth is known as the “core“. It comprises a solid inner core (1270 km thick) and a liquid outer core (2200 km thick). The latter is composed of nickel, iron and molten rock and temperatures can reach 50,000 degrees Celsius. Surrounding the Earth’s outer core is the “mantle” (up to 2900 km thick), a part-solid, part-viscous layer consisting of hot, iron- and magnesium-rich rock. The outside layer of the earth is known as the “crust“. This is made of solid rock, mainly basalt and granite. The crust is subdivided into two types: oceanic and continental. Oceanic crust is dense but thin and is mostly made from basalt. Continental crust is less dense but thicker, and mainly made of granite. 4

What Is The Astenosphere?

Immediately underneath the solid lithosphere is a partially-melted and much hotter layer of slowly moving rock, known as the asthenosphere. Geologists believe that the inflexible lithosphere “floats” on the ductile, mobile asthenosphere, permitting the movement of the lithosphere’s tectonic plates. 5 6 7

What Are Tectonic Plates?

The lithosphere is known for its tectonic activity – the interaction of its huge component blocks, known as tectonic plates (or lithospheric plates). Generally made up of both continental and oceanic crust, these tectonic plates vary in size from two hundred to several thousand kilometers across, and in thickness from 10-200 km (6-125 mi) or more. So how do these massive tectonic plates interact with each other? To answer this question, we need to know more about the two types of crust: oceanic and continental.

What’s The Difference Between Oceanic And Continental Crust?

Oceanic crust lies under oceans, and is typically about four miles thick in most places. The exceptionally thin nature of oceanic crust allows for the formation of “mid-ocean ridges” – caused by subterranean molten rock (magma) shooting up through weak spots in the ocean floor. As the magma cools, it hardens into new rock, which forms new oceanic crust.

Continental crust is found under continents and is generally between 10-75 km (6-47 mi) in thickness. Continents need these deep “roots” to support their elevations. Continental crust is generally much older than oceanic – up to 4 billion years old compared to 170 million years old for oceanic. Most importantly it is less dense than oceanic crust. As a result, it floats higher on the asthenosphere, rather like a piece of Styrofoam floats higher on water than a denser piece of wood. To compensate for this, nature has made the continental crust much thicker.

What Causes Tectonic Plates To Move?

Tectonic plates developed in the very early stages of the Earth’s 4.6-billion-year history, and ever since have been drifting about on the surface of the asthenosphere, like slow-moving bumper cars, repeatedly clustering together and then separating. 8 So what causes this bumping, these collisions between plates? The answer is, gravity.

Because oceanic crust is denser (heavier) than continental crust, it is constantly sinking and moving under continental crust. As a result, when a plate which is mostly made up of oceanic crust meets another plate made up of more continental plate, the heavier oceanic plate is forced beneath the lighter opponent (a process called subduction). The oceanic plate gradually sinks into the mantle dragging the rest of the plate down with it. This is the main reason why plates move.

Tectonic plates also move about in the lithosphere because of slow convection currents deep within the mantle, caused by radioactive heating of the interior.

What Are The Effects Of Plate Tectonics? Earthquakes And Volcanoes

The movement of tectonic plates can cause significant changes to the lithosphere and to the surface of the Earth. During subduction, for instance, the descending plate often causes considerable seismic and volcanic activity in the plate above it. Major movements of sub-oceanic and subterranean rock formations are also possible, as in the case of the collision between the Indian and Eurasian continental plates which began about 50 million years ago.

As the northward-proceeding Indian plate was forced under the static Eurasian plate, the thick sediments on the top of the Indian plate were scraped off and thrust upwards onto the Eurasian continent, where they now form the Himalayan mountain range.

Other violent subterranean activity can result when two plates slide against each other. A typical situation is when one plate moves one way while its opponent moves another. If the two edges stop sliding and catch on one another, pressure builds up and up until the rocks break. The sudden release of energy causes seismic waves that trigger earthquakes. 9

The movement of tectonic plates is also the most common cause of volcanic eruptions and other volcanic activity. These occur when molten rock under high pressure in the mantle rises through cracks or weak-spots in the Earth’s crust, caused by seismic waves.

Note: Seismology is the study of earthquakes and a seismologist is a scientist who studies earthquakes and seismic waves. Seismic waves move through and around the earth, and are caused by the sudden breaking of a rock within the earth or an explosion. The energy they create can be recorded on a seismograph.

Here is a list of the biggest super volcanic eruptions ever:

  1. Lake Toba, North Sumatra, Indonesia – 75,000 years ago
  2. Pacana Caldera, northern Chile – 4 million years ago
  3. Whakamaru, North Island of New Zealand – 340,000 years ago
  4. Taupo, North Island, New Zealand – 26,500 years ago
  5. Cerro Galán, Argentina – 2,200,000 years ago
  6. Yellowstone Creek, Yellowstone National Park, Wyoming, erupted 640,000 years ago.

How Does The Lithosphere Interact With Earth’s Other Spheres to Form Soil?

As mentioned above, the solid but brittle lithosphere is one of the five main subsystems of the Earth. All these spheres interact to influence a variety of planetary features, such as climate, weather, ocean salinity, terrestrial topography, landscape and of course the lithosphere’s main preoccupation -soil formation. The section of the lithosphere that interacts most with the other spheres in the formation of soil, is the pedosphere – the topmost layer of Earth’s surface.

Soil is formed through the comined efforts of all the spheres. For example, volcanic or sedimentary rocks (lithosphere) are ground down by the movement of a glacier (cryosphere), by weathering and erosion caused by wind (atmosphere) or rain (hydrosphere). The rocky residues resulting from this erosion process are then enriched by the organic components of plant and animal remains (biosphere), to create fertile soil.

In total, five separate factors influence soil formation: (1) Minerals in the rocks comprise the soil basis. (2) Climate determines the rate of weathering and erosion. (3) Topography affects drainage, erosion and deposition. (4) Living organisms (animals and plants) affect the organic content and richness of the soil. (5) Time allows everything to happen.

Soybeans Texas Drought
Soybeans show the effect of drought near Navasota, Texas. Photo: Bob Nichols, USDA/CC BY 2.0 (Flickr)

Why Is Soil Important?

Nearly all plants obtain their nutrients from the soil, and since plants are the main source of food for humans, animals and birds, one can say that nearly all living things on land depend directly or indirectly on soil for their existence. Soils store water for plants to make use of during photosynthesis, without which few living things would survive. Soils also play an important role in several biogeochemical cycles, including the Nitrogen Cycle and the Phosphorus Cycle, to name but two. For more about soils, see our in-depth article: Why is Soil So Important to the Planet?

What Part Does The Lithosphere Play In The Carbon Cycle?

The lithosphere is involved in all three variants of the slow carbon cycle, in which carbon is locked up for thousands, if not millions, of years. It is the second-largest carbon reservoir after Earth’s mantle.

VARIANT 1. CHEMICAL WEATHERING OF ROCK SURFACES

Rainfall acidified by carbon dioxide in the air gradually dissolves exposed rock surfaces (especially silicates, or carbonate rocks like limestone or dolomite), producing calcium and bicarbonate which are then washed away via rivers to the ocean. In the water, these products are utilized by calcifying marine creatures to create skeletons and shells. After death, some of these calcified structures end up on the ocean floor.

Here, over 20 million years or more, through a combination of heat and pressure, these calcium and carbon-rich sediments are turned into sedimentary rock, or else organic deposits within sandstones or shales. Some of these deposits may become fossil fuels, such as petroleum or natural gas. This overall process of compaction and cementation is known as lithification. 10

To return the lithified carbon to the atmosphere, the sedimentary rocks are slowly carried along by their tectonic plate until their plate collides with another plate, whereupon they may be ejected into the atmosphere during a volcanic episode or possibly uplifted as exposed rock. Once in the atmosphere, a significant proportion of their carbon will cross over into the fast carbon cycle for immediate recycling.

VARIANT 2. CHEMICAL WEATHERING IN THE DEEP OCEAN

A recent study has demonstrated that rock weathering also takes place in the deep oceans, where young, hot, volcanic ocean crust is subjected to chemical weathering from the circulation of CO2-rich seawater through cracks and fissures in the crust. This leads to the formation of carbon-rich calcites forming within the crust which, over the usual geological time span, follow the same route back into the atmosphere as that taken by sedimentary rocks. 11

Research shows that this ocean floor weathering mechanism is regulated by water temperature – the warmer it is, the more carbon dioxide is stored in the ocean crust.

VARIANT 3. DEVELOPMENT OF FOSSIL FUELS

When living organisms die, their CO2-rich remains are usually broken down by decomposers such as fungi, bacteria, worms and maggots. Any remaining material is recycled back into the soil. Over years, the organic material builds up on the forest floor, where it slowly sinks into the soil as it becomes compacted by new layers pressing down from above. Depending on the time span involved and the degree of heat and pressure applied, this material is transformed into peat (the wettest and least compact fossil fuel) coal, or sedimentary rock. (Oil and gas originated mainly in the sediments of coastal marine basins and inland seas.)

Substantial deposits of fossil fuel has been discovered and retrieved from their underground locations. However, if left undisturbed long enough, all these carbon rich compounds will be fully lithified and returned to the atmosphere as outlined above. 12 13

The Lithosphere and the Sulfur Cycle

The lithosphere also plays an important role in the sulfur cycle, linking the ocean or soil with the atmosphere.

Sulfur that enters the ocean sometimes falls into the ocean depths and becomes sedimented. Over millions of years, these sediments are compacted, heated, lithified and geologically uplifted, after which the sulfur is outgassed into the atmosphere by volcanic action, or chemically weathered back into the biosphere.

How Does The Lithosphere Affect Global Warming?

Lithospheric processes influence climate change in five ways:

Long term storage of carbon dioxide (CO2)
By locking up large quantities of carbon dioxide in rocks, the lithification process helps to regulate the greenhouse effect, thus maintaining a stable climate. It also preserves a substantial stock of CO2 for future use.

Exposed rock cooling mechanism
The more CO2 that is emitted into the atmosphere, the more acid rain there is. And the more acid rain, the faster that rocks are weathered and the more CO2 is locked up in the slow carbon cycle.

Mountain uplift leads to cooling
Plate tectonic activity may result in an uplift of exposed rock (like the Himalayas), which leads directly to more CO2 being removed from the atmosphere through chemical weathering. Paleoclimatologists believe that the formation of the Himalayan Mountains led to the abrupt 40 percent fall in carbon dioxide levels that caused global cooling about 34 million years ago. This cooling led directly to the formation of the Antarctic ice sheets.

Fossil fuel deposits provide instant energy
By processing and preserving carbon-rich organic deposits in rock for millions of years, the lithosphere has created a potent store of instant energy. Unfortunately, its very ease of use has so far proved too much of a temptation for energy-greedy governments and consumers, who see global warming as more of a nuisance than an existential threat.

Subterranean heat sources provide geothermal energy
Reservoirs of hot water, heated by geothermal energy, can be tapped to make electricity. Geothermal energy typically emits about 1 percent of the greenhouse gases released by coal.

For more about the timeline of our planet and its rocky lithosphere, see: History of Earth in One Year (Cosmic Calendar).

References

  1. Environmental Science: Systems & Solutions. (6th Edition) ML McKinney, RM Schoch, L Yonavjak, GA Mincy. Jones & Bartlett Learning. Burlington MA 01803. []
  2. Slow carbon cycle.” Communicating Science[]
  3. Slow carbon cycle.” Tutor2u []
  4. Earth’s Structure.” Geological Survey []
  5. Daly, R. (1940) Strength and structure of the Earth. New York: Prentice-Hall. []
  6. 4-D evolution of SE Asia’s mantle from geological reconstructions and seismic tomography”. Replumaz, Anne; Karason, Hrafnkell; Van Der Hilst, Rob D; Besse, Jean; Tapponnier, Paul (2004). Earth and Planetary Science Letters. 221 (1–4): 103–115. []
  7.  “A new global model for P wave speed variations in Earth’s mantle”. Li, Chang; Van Der Hilst, Robert D.; Engdahl, E. Robert; Burdick, Scott (2008). Geochemistry Geophysics Geosystems. []
  8. What is a tectonic plate?” USGS. []
  9. See also: “The Science of Earthquakes.” USGC []
  10. Riebeek, Holli. “The Carbon Cycle”. Earth Observatory. NASA. 16 June 2011. []
  11. Oceanic crustal carbon cycle drives 26-million-year atmospheric carbon dioxide periodicities,” Science Advances (2018). R.D. Muller el al. []
  12. The role of soil microbes in the global carbon cycle: tracking the below-ground microbial processing of plant-derived carbon for manipulating carbon dynamics in agricultural systems.” Journal of the Science of Food and Agriculture. 2014 Sep; 94(12): 2362–2371. Published online 2014 Mar 6. Christos Gougoulias, Joanna M Clark, Liz J Shaw. []
  13. United States Carbon Cycle Science Program. (PDF) []
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