On Earth, there is no ocean older than 200 million years, which testifies to the great cycle of tectonic processes. Very regularly, some oceans close while others open. But how is the birth of a new ocean?
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[EN VIDÉO] A billion years summed up in 40 seconds: plate tectonics Researchers have modeled the motions of plate tectonics over the past billion years.
if it was a movement Tectonic plates can cause major continental collisions and generate mountain ranges such as the Alps or the Himalayas, and they are also closely linked to another major phenomenon in the tectonic cycle: the opening of new oceans.
Over the course of geological time, the Earth has in fact experienced several major tectonic cycles that led to the formation of Super Continents. During these stages where everything masses The continents are grouped together, and one ocean occupies the majority of the Earth’s surface. Deep operations but also limitations related to the continuous creation of oceanic crust and his Recycling at the level of regions subduction It means that this configuration cannot remain static. After the reassembly phase inevitably follows a phase ofContinental Opening. These different stages are part of Wilson cycle.
Continental rifting and rupture: the previous stages of the ocean opening
However, a continent that separates (completely) into two spontaneously gives birth to a new ocean. This is the new method, after the stage of the continental extension ocean edge. For example, the Mid-Atlantic mountain range was born as a result of a turbulence The supercontinent Pangea. 180 million years ago, the plates of South America and Africa began to separate. will follow them Separation Plates of North America and Eurasia.
thinning and breaking continental crust It is part of the cracking process. The creation of a new oceanic ridge will occur only at the end of this rupture process, the mechanisms of which can be varied and take more or less time, in particular depending on the Speed To stretch the plates no more than a few centimeters per year. The mechanism leading to the formation of a new ocean thus extends over tens of millions of years.
Local elevation of the auricle
Oceanic ridges are defined by a highly localized elevation Coat hot. this ascent peridot At more than 1300 ° C accompanied by a process partial fusion who is born the silhouette Morp type. By crystallizing at different depths, this magma will produce new crust, whose composition is significantly different from that of the continental crust. This is the oceanic crust. Talking about the ocean When a ridge enters the activity and produces an oceanic crust.
But how is the ridge formed?
It’s all about stretching and equal balance. continental crust It has a lower density than the mantle, it “floats” on top of this denser medium. But when thinned, this variation in thickness will cause a deep material flow, with the goal of “compensating” for this loss of thickness. Therefore, the fault zones are characterized by the elevation of the Mori mantle layer below the zone of greatest thinning. This is also why Alsace, an ancient incomplete rift, is so strong regression Geothermal energy that allows in particular Exploiting the basement heat.
As the continental crust gradually declines, and with it all of the continental lithosphere (which includes the upper mantle),Acinosphere It rises and can begin to produce magma if thermal conditions permit. This is how continental edges of magma or poor magma are obtained, depending on the ability of the system to extract magma from the mantle at this point.
Some ocean births take longer than others
When the continental extension reaches a critical point, the continental crust, which is weakening to the maximum, will have no choice but to break. The story may end there in some cases at the coat lithosphere It was weak at the same time, if not before, the continental crust. Then the asthenosphere is high enough to generate a large pot Sizes from magma. In this case, the crustal rupture is accompanied by the creation of a stationary magmatic system and thus the initiation of a ridge and the creation of a new ocean, between the two recently separated continental masses.
In other cases, We still have to wait Before seeing the real ridge begins. In fact, if the continental mantle is very cold (and therefore very dense), it will not rupture at the same time as the crust. Then the elevation in the asthenosphere is not high enough for active and stable magma to begin immediately. The extension will continue to absorb itself by exposing undissolved mantle rock from the rest of the subcontinental mantle, until it is exhausted. In this type of formation, the creation of an oceanic ridge is delayed and the final rupture of the subcontinental mantle will follow. Delivery, in this case, is a little longer!
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