Earth

Internal Structures

Earth's internal structure can be separated into four layers as shown in Figure 09-05a and explained in more details in the followings.


1. Crust - The outermost part of the Earth; this is what we walk around on. It is made of cold, brittle, and relatively light material. Under the continents, the crust averages about 30-40 km thick (more under tall mountains, somewhat less in other areas) and under the ocean, it averages about 5-6 km thick.
   Continental crust is on average older, more silica-rich and thicker than oceanic crust, but is also more variable in each of these respects. The oldest parts of the continental crust, known as 'shields' or 'cratons', include some rocks that are nearly 4 billion years old. Most of the

   		rest of the continental crust consists of the roots of mountain belts, formed at different stages in Earth history. Oceanic crust underlies most of the two-thirds of the Earth's surface, which is covered by the oceans. It has a remarkably uniform composition (mostly 49% 2% SiO2 ) and thickness (mostly 7 1 km). The ocean floor is the most dynamic part of the Earth's surface. As a result, no part of the oceanic crust existing today is more than 200 million years old, which is less than 5% of the age of the Earth itself. New oceanic crust is constantly being generated by sea-floor spreading at mid-ocean ridges, while other parts of the oceanic crust are being recycled into the mantle at subduction zones.
   Figure 09-05a Earth, Structure [view large image]	Figure 09-05b Earth, Structure (new)

   The boundary between the crust and the mantle is known as the 'Mohorovicic discontinuity', or 'moho'. The mantle material beneath the moho is not generally molten or even partially molten. The mantle only becomes partially molten in special circumstances such as in mid-ocean ridges, subduction zones or 'hotspots'. The crust is firmly attached to the uppermost part of the mantle and together they make up a rigid layer known as the 'lithosphere'. The rigid surface of the Earth is made up of 'plates' in the lithosphere, they move relative to one another and relative to the underlying part of the mantle, known as the 'asthenosphere'. The asthenosphere is also solid, but over millions of years it deforms in a manner similar to Plasticine (although it is actually many times more viscous).



2. Mantle - Immediately below the crust is the mantle. It is made of rocky material similar to the crust, but it is very hot and not brittle. The material of the mantle acts like a solid over timescales of a second, hour, week, and up to several thousand years. Over hundreds of thousands to millions of years, however, mantle material acts as a very viscous fluid and can flow from one place to another in a process called convection. The mantle makes up about 70% of Earth's mass and about 45% of its radius. The bulk of the lower mantle is termed the mesosphere and is stronger than the asthenosphere
   New study in 2010 reveals that there is a relatively thin layer at the bottom of the lower mantle, which has perovskite (MgSiO₃) as its main composition. This thin layer underwent a phase transition to another crystal form at the specific temperature and pressure prevalent in that location (Figure 09-05b). According to computer simulations, it makes the mantle more dynamic and carries heat more efficiently than previously thought and thus explains the fast growth rate of the continents in the last 2 billion years. It may also be responsible for the hot spot in Hawaii, the evolution of the Earth's magnetic field, and the periodic precession of the Earth's axis of rotation.



3. Outer Core - Next is the outer core, which is made of very different material from the crust and mantle. The outer core is mostly iron, and is very hot. The iron mix which makes up the outer core is a fluid which moves around significantly at a rate of about 10 km/year.

   		The fluid motion of the outer core generates the Earth's magnetic field. According to the theory of Geodynamo, the Earth's magnetic field is generated by the convection of liquid iron in the outer core. The pattern of up and down circulation is twisted around by the Earth's rotation (Figure 09-05c). The helical flow would trap the magnetic flux which would create more current and more current would beget more magnetic flux in a bootstrap or positive feedback loop until the process is terminated by lack of more energy. Such energy is supplied by the solidification of the inner core and or the decay of the radioactive elements. Figure 09-05d shows a three dimensional view of the current flow in the outer core (without the helixes). Recent (2014) research indicates that the conduction of heat in this region is more effective than previous thought. Such discovery will presents a problem with the theory of Geodynameo which relies on strong convective flow.
   Figure 09-05c Geodynamo [view large image]	Figure 09-05d Outer Core [view large image]




4. Inner Core - Finally, there is the innermost part of the Earth, called the inner core. The inner core is mostly iron, similar to the outer core, but because the pressure is so much higher near the center of the Earth, the inner core is solidified. There is some evidence that the inner core may be spinning at a faster rate than the rest of the planet.

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