Why is The Earth Divided into Continents and Oceans?
Oceans and continents exist as a result of complex geological and geophysical processes that have shaped the Earth’s surface over billions of years.
Rocks that form the continents are different from those that form the ocean floors; this shows that the division of the Earth into continents and oceans is not a matter of chance.
Continental rock (granite) consists of the minerals alkali feldspar and quartz. Ocean floor rock consists of the minerals pyroxene and calcium feldspar. They do not have the same density and thickness: continental rock has an average thickness of 35 km compared to an average thickness of 6 to 7 km for oceanic rock.
These rocks form the Earth’s crust which is underlain by mantle, consisting mainly of the minerals olivine and pyroxene. The upper 100 km of the mantle is relatively cool and rigid and together with the Earth’s crust, form what is called the lithosphere (or outer shell) . The lithosphere is divided into large, rigid plates called tectonic plates that ‘float’ on the asthenosphere beneath them. These minor and major plates move and interact with each other.
The asthenosphere is about 200 km thick and shows a gradual increase in temperature with depth. There, the mantle is close to its melting point and is therefore deformable and semi-fluid. The tectonic plates that float on this putty-like layer caused the formation of continents and ocean basins over geological timescales. Tectonic plates, which can be minor or major, are in constant movement. There are several types of boundaries between tectonic plates: divergent boundaries occur where two plates move away from each other. Then, new crust is created at mid-ocean ridges or rift valleys. They can also have convergent boundaries which occur where two plates move towards each other. This can lead to one plate being forced beneath another in a process called subduction. Convergent boundaries can form mountain ranges, deep ocean trenches, and volcanic arcs. There are also transform boundaries which slide past each other in a horizontal manner, the friction of which can cause earthquakes (for example, San Andreas Fault in California).
As tectonic plate movements began to shape the Earth’s crust, ocean basins formed and evolved. The movement of tectonic plates created the current configuration of continents and ocean basins, and, in order to understand why they are configured the way they are, it is important to understand the following: an object of low density floats better than an object of high density. However, if the density of an object is higher than that of water, it will sink. Thus the thicker, lower-density continental crust floats higher on the asthenosphere than the thinner and denser oceanic crust. Water therefore covers the denser oceanic crust and fills the ocean basins leaving the continents to project above sea level. The water that originally filled the early oceans likely came from a combination of volcanic outgassing (vapour released by volcanic activity which condensed into liquid water), impacts from comets and asteroids (which contained ice and other compounds), atmospheric condensation (which condensed and fell as precipitation) and chemical reactions, with volcanic outgassing being a major source in the initial stages of Earth’s history.
Nowadays, there are seven continents (Europe, Antarctica, North America, South America, Asia, Africa and Australia) stated by Albert Wegener, a German geophysicist who proposed the continental drift theory, to be mainly sections of Pangea, the supercontinent present previously and formed through complex processes involving multiple geological and tectonic events over billions of years. There are also five main oceans basins (Atlantic, Southern, Arctic, Pacific, and Indian) formed due to the rifting and stretching of continental crust and the rise of magma and mantle materials.
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