Did you know that more than half of the surface of the planet Earth is occupied by oceanic crust? It is incredible, but the truth is that precisely the oceanic crust constitutes more than 55% of the planetary surface. Due to the difficulty it represents, studying the oceanic crust constitutes a great challenge for researchers. However, its study is essential to understand the geology of planet Earth.
Before we get into the subject, we will begin by understanding what the oceanic crust is. This is actually the Earth’s crust that forms the bottom of the oceans . Regarding the composition of the oceanic crust, basalt is one of the main rocks that make up the crust. It is a type of igneous rock , or also called magmatic rock, made up of magnesium, iron and silica.
Now, how is oceanic crust formed? Precisely in the areas where oceanic tectonic plates diverge , magma flows from the interior of the Earth. As magma solidifies , magmatic rocks form that give rise to the oceanic crust. Due to the simple fact that the oceanic crust is denser, when it meets the continental crust, it sinks under it until it reaches the earth’s mantle, that is, the terrestrial layer that is between the core and the crust. Upon reaching the mantle, it melts and transforms into magma again. Therefore, the oceanic crust is constantly recycling.
Now that we know what it is and how it is formed, we are ready to discover the characteristics of the oceanic crust .
Division of the oceanic crust
As we will see below, the oceanic crust is not a homogeneous layer, but rather 3 layers can be distinguished:
- First layer: made up mainly of sediments and volcanic rocks. It is approximately 1.5 kilometers thick.
- Second layer: like the previous layer, it is approximately 1.5 kilometers thick. It is made up mostly of mafic rocks, a type of rock made up of a large percentage of iron and magnesium.
- Third layer: it has a thickness of almost 5 kilometers. It is composed of gabbros and peridotites, which are formed by a slow cooling of magma at great depth.
Reliefs of the oceanic crust
Like the earth’s crust, the oceanic crust has different reliefs. Let’s see some of them here:
- Mid-ocean or mid-ocean ridges: these are elevations on the ocean floor, which can reach 3,000 meters in altitude. They are generally found on the edges of tectonic plates and are associated with undersea volcanoes because they have a central groove through which magma flows continuously. The oceanic ridge systems generate new oceanic crust, an average of 17 cubic kilometers of oceanic crust per year.
- Abyssal plains: they are flat extensions located between 3000 and 6000 meters deep. It is estimated that the abyssal plains represent 50% of the ocean floor and there are various reliefs similar to the reliefs existing in the continental crust. Here you can learn more about what the abyssal plains are and their characteristics .
- Guyots: They are considered seamounts with flat tops. It is believed that at some point they were at sea level and that is why their tops were eroded until they were flat.
- Abyssal trenches: they are narrow but deep fissures in which large amounts of sediment accumulate. They are found on the edges of tectonic plates or near a continent or island zone. One of the best known abyssal trenches is the Mariana Trench .
The crust of the Earth , or terrestrial crust , can be differentiated into continental crust and oceanic crust , each with its own characteristics and peculiarities. The main differences between the two lie in the composition and density.
- In principle, the oceanic crust is composed, as we mentioned before, by basaltic rocks while the earth’s crust, although it has a heterogeneous composition, is mainly composed of granite-type rocks, with large percentages of aluminum and silicon.
- Another point is the density, the oceanic crust is denser than the continental one. As we developed in previous sections, the difference in density allows the oceanic crust to be in a constant recycling process. This phenomenon determines the age of the different crusts, the oceanic being considerably younger than the continental one.
- Finally, another difference is the thickness of both crusts: the oceanic one presents a thickness between 8 to 10 kilometers while the continental one varies between 30 to 70 kilometers.