The Structure of the Earth & Lithosphere

The Earth is the largest of the rocky planets moving around the Sun by mass and by size. The Earth’s shape is a spheroid: not quite a sphere because it is slightly squashed on the top and bottom. The shape is called an oblate spheroid. As the Earth spins around itself, the centrifugal force forces the equator out a little and pulls the poles in a little making it a unique type of spheroid. As this shape is unique it is also called ‘geoid’ or Earth-like.
The deepest hole ever dug is only about 12.3 kilometres or 7.6 miles. Still, we know something about the inside of the Earth, because we can learn things from earthquakes and the volcano eruptions. We are able to see how quickly the shock waves move through the Earth in different places and determine the type of materials making up that area.
The interior of the Earth is divided into layers. These layers are both physically and chemically different. The Earth has an outer solid crust, a highly viscous mantle, a liquid outer core, and a solid inner core.
The boundaries between these layers were discovered by seismographs which showed the way vibrations bounced off the layers during earthquakes. Between the Earth’s crust and the mantle is a boundary called the moho. It was the first discovery of a major change in the Earth’s structure as one goes deeper.
The crust is the outermost layer of the Earth. It is made of solid rocks. It is mostly made of the lighter elements, silicon, oxygen, aluminium. Because of this, it is known as sial (silicon = Si; aluminium = Al) or felsic. The thickness of the crust varies under the oceanic and the continental areas. Oceanic crust is thinner as compared to the continental crust. The mean thickness of oceanic crust is 5 km whereas that of the continent is around 30 km. The continental crust is thicker in the areas of major mountain systems. It is as much as 70 km thick in the Himalayan region.
The mantle is the layer of the Earth right below the crust. It is made mostly of oxygen, silicon and the heavier element magnesium. It is known as siam (Si + am for magnesium) or mafic.The mantle extends from Moho’s discontinuity to a depth of 2,900 km. The mantle itself is divided into layers.
The uppermost mantle plus overlying crust are relatively rigid and form the lithosphere(lithos=rock), an irregular layer with a maximum thickness of perhaps 200 km, of which the uppermost mantle is 120 to 50 km thick.
Below the lithosphere, the upper mantle becomes notably more plastic, called the asthenosphere, and is composed of flowing rock in the state of plasticity, about 200 km thick. It behaves as a hot viscous liquid and consists of hot, weak material that can be deformed like silly putty. That means it is capable of gradual flow. The lower mantle is much thicker than the upper mantle. It is made of magma, under great pressure, and so is thicker (higher viscosity) and flows less easily.
The Earth’s core is the part of Earth in the middle of our planet. It has a solid inner core and a liquid outer core.
The outer core of the Earth is a liquid layer about 2,260 kilometres thick. It is made of iron and nickel. This is above the Earth’s solid inner core and below the mantle. Its outer boundary is 2,890 km (1,800 mi) beneath the Earth’s surface. The transition between the inner core and outer core is approximately 5,150 km beneath the Earth’s surface.
The temperature of the outer core ranges from 4400 °C in the outer regions to 6100 °C near the inner core. The outer core is not under enough pressure to be solid, so it is liquid even though it’s mostly made of the same stuff as the inner core. Sulphur and oxygen could also be in the outer core.
The inner core of the Earth, as detected by seismology, is a solid sphere about 1,216 km (760 mi) in radius, or about 70% that of the Moon. It is believed to be an iron–nickel alloy and may have a temperature similar to the Sun’s surface, approximately 5778 K (5505 °C).

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