What is below that thin layer of dirt and water that man has learned to understand in such a difficult manner throughout his history? The center of our planet is located 6 400 kilometers below the surface, but till now, we have descended only a few times into the deepest oceanic trenches to a depth of 10 000 meters while the wells dug into the bowels of the Earth have never exceeded 11 kilometers in length. And because it is impossible to dig a hole to the center of the planet to directly remove samples of materials, scientists have relied on a little trick to study the heart of our world: earthquakes. During an earthquake, the Earth is shaken by various types of impact waves, called seismic waves, which originate from an epicenter and spread out at different speeds. Some of these waves, called primary waves, are very important because they cross through the entire planet and, striking different materials, modify their behavior. Recorded with special seismographs, they can determine if such waves have passed through dense rocks, like basalt, or lighter types, such as sandstone, liquid deposits or even gas. Naturally, studies of the Earth are not only based on earthquakes: geologists can produce "artificial" quakes by setting off underground explosive charges in strategic locations. This is how it was discovered that our planet consists of four main concentric layers: the inner core, the outer core, the mantle and the crust. The inner core is a kind of solid "nut" with a diameter of 2 600 kilometers and a density that is 13 and a half times greater than water. Its temperature is around 4 000 degrees Celsius and the pressure is mind-boggling: three million times that of the atmosphere. An external transition area, that extends outward for 500 kilometers, leads to an external and fluid core, with a thickness of 1 700 kilometers and a temperature that ranges between 2 200 to 2 750 degrees Celsius. The core mainly consists of iron and nickel in a solid and liquid state respectively in the inner and outer core. Continuing towards the surface, we find the mantle, composed of rocks which are rich in silicon, magnesium and iron, which extends for 2 900 kilometers. Overall, the mantle is solid, but in a concentric segment with a thickness of 150 kilometers located at a depth of about 100 kilometers, called the asthenosphere, it may be partially molten. Slightly higher is the lithosphere, including the crust and part of the upper mantle, which is divided into a series of plates moving on the flexible and hot asthenosphere. The plates diverge and move apart in relation to the flow of hot material which, rising from the ocean ridges, forms a new crust, and they approach each other in the trenches where the lithosphere sinks into the mantle. This, as we have seen, is how the continents wander around on the crust that is the external part of the Earth's shell, with a thickness that varies between 10 and 40 kilometers and an average density of 2.8 times that of water. The most prevalent element in the Earth's crust is oxygen (46.6%) followed by silicon (27.7%), aluminum (8.1%), iron (5%), calcium (3.6%), sodium (2.8%), potassium (2.6%) and magnesium (1.4%). The other elements, some of which are very important such as carbon, sulphur, nitrogen and precious metals, represent less than 3% of the crust, a shell that is as thin as an apple peel compared to the dimensions of the planet.
What is below that thin layer of dirt and water that man has learned to understand in such a difficult manner throughout his history? The center of our planet is located 6 400 kilometers below the surface, but till now, we have descended only a few times into the deepest oceanic trenches to a depth of 10 000 meters while the wells dug into the bowels of the Earth have never exceeded 11 kilometers in length. And because it is impossible to dig a hole to the center of the planet to directly remove samples of materials, scientists have relied on a little trick to study the heart of our world: earthquakes. During an earthquake, the Earth is shaken by various types of impact waves, called seismic waves, which originate from an epicenter and spread out at different speeds. Some of these waves, called primary waves, are very important because they cross through the entire planet and, striking different materials, modify their behavior. Recorded with special seismographs, they can determine if such waves have passed through dense rocks, like basalt, or lighter types, such as sandstone, liquid deposits or even gas. Naturally, studies of the Earth are not only based on earthquakes: geologists can produce "artificial" quakes by setting off underground explosive charges in strategic locations. This is how it was discovered that our planet consists of four main concentric layers: the inner core, the outer core, the mantle and the crust. The inner core is a kind of solid "nut" with a diameter of 2 600 kilometers and a density that is 13 and a half times greater than water. Its temperature is around 4 000 degrees Celsius and the pressure is mind-boggling: three million times that of the atmosphere. An external transition area, that extends outward for 500 kilometers, leads to an external and fluid core, with a thickness of 1 700 kilometers and a temperature that ranges between 2 200 to 2 750 degrees Celsius. The core mainly consists of iron and nickel in a solid and liquid state respectively in the inner and outer core. Continuing towards the surface, we find the mantle, composed of rocks which are rich in silicon, magnesium and iron, which extends for 2 900 kilometers. Overall, the mantle is solid, but in a concentric segment with a thickness of 150 kilometers located at a depth of about 100 kilometers, called the asthenosphere, it may be partially molten. Slightly higher is the lithosphere, including the crust and part of the upper mantle, which is divided into a series of plates moving on the flexible and hot asthenosphere. The plates diverge and move apart in relation to the flow of hot material which, rising from the ocean ridges, forms a new crust, and they approach each other in the trenches where the lithosphere sinks into the mantle. This, as we have seen, is how the continents wander around on the crust that is the external part of the Earth's shell, with a thickness that varies between 10 and 40 kilometers and an average density of 2.8 times that of water. The most prevalent element in the Earth's crust is oxygen (46.6%) followed by silicon (27.7%), aluminum (8.1%), iron (5%), calcium (3.6%), sodium (2.8%), potassium (2.6%) and magnesium (1.4%). The other elements, some of which are very important such as carbon, sulphur, nitrogen and precious metals, represent less than 3% of the crust, a shell that is as thin as an apple peel compared to the dimensions of the planet.
