What is the use of locating seismic discontinuities? Locating these disturbances enable scientists to map the inner regions of the Earth. This science, known as tomography originates from the knowledge gained from discontinuities. Tomographists have found that this planet is divided into six regions: the inner core, the outer core, the lower mantle, the upper mantle, the transition region, and the crust (oceanic and continental).

0- 40 Crust Upper mantle Transition region Lower mantle D'' layer Outer core Inner core

We know that the crust plunges down to adepth that varies between 5 to 70 km, making it the thinnest layer. It is mainly composed of rocks, which are subdivided into two groups – sial and sima. The continental crust is mainly composed of silica and alumina, hence called sial. On the other hand, oceanic crust, which is mainly composed of silica and magnesium, is called sima.

is the smallest part of Earth, only 0.099% of its mass and reaching a small depth of 0-6 miles (0- 10 kilometers). In the beginning of time, it was possible that this area did not exist for through frequent volcanic activity does only the crust form. Evidence of this is marked by the oceanic ridge system, which is a 25,000 mile (40,000-kilometer) array of many volcanoes which creates layer after layer of new crust at the rate of 17 km3 per year. The ocean floor is covered in basalt originating from volcanic activity and as a matter of fact, Iceland and Hawaii are two island systems that emerged from the accumulated basalt.

The second smallest area of the Earth is the Continental crust, making up only 0.374% of the Earth's mass and extending a short depth of miles (0-50 kilometers). Looking at the percent by composition, the continental crust makes up only 0.554% of the mantle-crust mass. The layer is composed primarily of crystalline rocks made of low-density buoyant minerals dominated mostly by quartz (SiO2) and feldspars (metal-poor silicates). This is the outer part of the Earth composed essentially of crystalline rocks. The continental crust and the oceanic crust are also referred to as the lithosphere because of the cool and rocky conditions that exist in its chemical composition.

We know that the mantle reaches down to a depth of about 2,890 km. While the crust is the thinnest layer, the mantle is the thickest. The upper mantle is represented by the asthenosphere, a highly viscous layer that supports the lithosphere. Like the lithosphere, the asthenosphere plays a significant role in plate tectonics.asthenosphere Despite the extremely high temperatures in the mantle (up to 4,000 degrees Celsius), even higher than the melting points of the rocks that constitute it, the very high pressures there prevent melting from taking place.

 its chemical composition includes silicon, magnesium, and oxygen.  also contains some iron, calcium, and aluminum.  is comprised of 72.9% of the antle-crust mass, making the Earth abundant in the chemical elements of silicon, magnesium and oxygen, the layer's primary components.

Through excavations in volcanoes, scientists have found that this part of the crust composes of 15.3% of the total mantle-crust mass and is made of crystalline forms of Olivine (Mg,Fe)2SiO4 and pyroxene (Mg,Fe)SiO3. The upper mantle makes up 10.3% of the Earth's mass, extending a depth of miles ( kilometers). A relatively large portion when compared to the other interior layers. This layer is not completely made of solid minerals for scientists speculate that the asthenosphere could be partly liquid molten.

The next layer, the Transition region comprises 7.5% of Earth's mass with a depth of miles ( kilometers). This layer is also known as the mesosphere and is 11.1% of the mantle-crust. It is made of mainly basaltic magmas with amounts of calcium, aluminum and garnet (an aluminum-bearing silicate mineral. The layer becomes dense when the garnet mineral cools but is buoyant and light when subject to heat due to the low melting points.

At the center of the Earth is the inner core, a solid sphere believed to be made up of an iron-nickel alloy. That the solid inner core is distinct from the liquid outer core was suggested by Inge Lehmann, a seismologist who based his theories on earthquake-generated seismic waves.earthquake

The inner core is a solid section of the Earth and is unattached to the mantle, being suspended by the molten outer core. This solidified state is the result of a very intense pressure-freezing process that occurs in most liquids when temperature decreases or pressure increases.

is mainly composed of melted iron and nickel. They are melted because the pressures at these depths are not sufficient to prevent the much higher temperatures, which range between 4,400 degrees Celsius to 6,100 degrees Celsius, from melting the rocks there. Convection in the outer core, which move the liquefied metals about, is believed to be the source of the magnetic field that encapsulates the Earth and protects it from cosmic radiation.

The outer core of Earth is a scorching hot, electrically conductive liquid in which convection takes place. The outer core is in the range of 200 to 300 kilometers (125 to 188 miles) thick and represents about 4% of the mantle-crust mass. This layer is sometimes identified as part of the lower mantle due to its geographical nature. However, studies on seismic discontinuities suggest that this "D" layer might differ chemically rom the lower mantle lying above it.

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