Usually the term sea it refers to a mass of salt water that covers a relatively large extent of the earth's surface.
When we are at the edge of a sea, we cannot help wondering what has happened on the planet so that so much water is accumulated before us. There are various mechanisms for the generation of a sea, although all of them are related to the tectonics of the earth's crust, the most superficial and external layer of our planet.
In a very simplistic way, we will say that there are huge fractures of the cortex that are called dorsals (if they occur under the sea) or cross faults, if they occur in emergent lands. In both cases, magma rises from the interior of the Earth that generates volcanoes and the creation of new rocks, when it comes out in the form of lava to the surface.
These new materials, when cooling and becoming rigid, press on the two sides of the fracture that were once united, separating them and forming a new, thinner crust. This stage is told rift valley.
Although the distance between the two edges that separate increases very millimeters each year, at the end of thousands of years the distance is appreciable. As the new crust is usually at a lower height than the two separate continental ends, if the water of a nearby sea bursts into this huge step, it floods. Being at a lower height than its edges, rainwater also tends to accumulate in this depression. And we already have a new sea formed.
If the volcanic activity continues, the separation between the old edges of the fracture will continue to increase. When the magma comes out under the sea water, we are facing a dorsal, as this stage is called. The crust that is created now is oceanic.
There will come a time when the fracture cools and the lava that is in the duct (or chimney) solidifies, plugging the fracture. When magma stops ascending, an inland sea of more or less stable extension will be formed.
That said, could you tell me a sea that is currently forming in this way? no? Well I think yes. Let's go to Africa, specifically near the majestic Kalahari. If we look at it with the eyes of a geologist, we will see that we are facing a series of stepped terraces created by transforming faults. The central and lowest height step is the most recent. The wonderful snowy mountain Kalahari is actually a volcano. And we are facing what will undoubtedly be, thousands of years later, a great Red Sea.
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