- Convection currents: origin of the hypotheses
- What are convection currents?
- You criticize these theories
- References
The convection currents are continuously moving constantly performed Earth 's plates. Although they tend to occur on a large scale, there are studies that show that there are also on a smaller scale.
The planet earth is made up of a core, the mantle and the earth's crust. The mantle is the layer that we can find between the core and the crust. The depth of this varies, depending on the point of the planet where we are, and can extend from a depth of 30 km with respect to the surface, up to 2,900 km.
Image via blog.educastur.es
The mantle is distinguished from the core and the crust because it has a mechanical behavior. It is made up of a solid viscous material. It is in a viscous state due to the high pressures to which it is subjected.
Mantle temperatures can range from 600ºC to 3,500ºC. It has colder temperatures the closer it is to the surface and higher temperatures the closer it is to the core.
We can separate the mantle into two parts, the upper and the lower. The lower mantle converges from the Mohorovičić discontinuity to a depth of about 650 km.
This discontinuity, commonly known as Moho, is located at an average depth of 35 km, and can be found only 10 km below the bottom of the oceans. The lower mantle would be the part between 650 km deep, up to the limit with the inner core of the planet.
Due to the thermal difference between the core and the earth's crust, convective currents are produced throughout the entire mantle.
Convection currents: origin of the hypotheses
In 1915, a hypothesis developed by Alfred Wegener, postulated the movement of the continental masses. Wegener said that the continents moved on the ocean floor, although he did not know how to prove it.
In 1929, Arthur Holmes, a renowned British geologist, hypothesized that under the earth's crust we could find a mantle of molten rock, which caused convection currents of lava that had the force to move the tectonic plates and, therefore, the continents.
Although the theory was consistent, it was not accepted until the 1960s, when plate tectonics theories began to develop.
In these formulations it was maintained that the earth's plates moved due to the convection forces of the earth, causing shocks, which are responsible for shaping the earth's surface.
What are convection currents?
Convection currents are the currents of materials that are produced in the Earth's mantle with the help of gravity. These currents are responsible for displacing not only the continents, as Wegener postulated, but all the lithospheric plates that are found above the mantle.
These currents are produced by differences in temperature and density. Helped by gravity they make the hottest materials rise towards the surface, since they are less heavy.
This therefore means that the coldest materials are denser and heavier, which is why they descend towards the Earth's core.
As we discussed before, the mantle is made of solid materials, but it behaves as if it were a viscous material that deforms and stretches, which moves without breaking. It behaves in this way due to the high temperatures and great pressure to which these materials are subjected.
In the area near the Earth's core, temperatures can reach 3,500ºC, and the rocks found in that part of the mantle can melt.
As solid materials melt, they lose density, so they become lighter and rise to the surface. The pressure of the solid materials above it, makes them try to descend due to their weight, allowing the hottest materials to exit towards the surface.
These upward-shaped streams of materials are known as thermal plumes or plumes.
The materials that reach the lithosphere can cross it, and that is what forms the fragmentation of the continents.
The oceanic lithosphere has a temperature much lower than that of the mantle, so large cold chunks sink into the mantle, causing downdrafts. These downdrafts can move chunks of cold oceanic lithosphere close to the core.
These currents produced, whether ascending or descending, act like a roller, creating convection cells, which gives rise to explain the movement of the tectonic plates of the earth's crust.
You criticize these theories
New studies have modified the convection cell theory a bit. If this theory were true, all the plates that make up the Earth's surface should have a convection cell.
However, there are plates that are so large that a single convection cell should have a large diameter and a large depth. This would cause some of the cells to go deep into the nucleus.
Through these latest investigations, the idea has been reached that there are two separate convective systems, this being the reason why the earth has maintained heat for so long.
Studies of seismic waves have made it possible to obtain data on the internal temperature of the earth and to carry out a heat map.
These data obtained by seismic activity support the theory that there are two types of convection cells, some closer to the Earth's crust and others closer to the core.
These studies also suggest that the movements of the tectonic plates are not only due to the convection cells, but that the force of gravity helps by pushing the innermost parts towards the surface.
When the plate is stretched by convection forces, the force of gravity exerts pressure on it and they end up breaking.
References
- Dan, Mckencie; Frank Ritcher (1997) Convection currents in the Earth's mantle. Research and Science Magazine Nº4.
- Archibald Geikie (1874) Geology.
- JACKSON, Julia A. Glossary of geology. Glossary of Geology, by JA Jackson. Berlin: Springer.
- DAVIS, John C.; SAMPSON, Robert J. Statistics and data analysis in geology.
- DAVIS, George Herbert; REYNOLDS, Stephen J. Structural geology of rocks and regions. In Structural geology of rocks and regions. Wiley, 1996.
- SUPPE, John. Principles of structural geology. Prentice Hall, 1985.
- BILLINGS, Marland P. Structural geology. Prentice-Hall, 1954.