◇ The interior of the earth is made up of several concentric layers of which the crust, the mantle, the outer core and the inner core are significant because of their unique physical and chemical properties.
◇ The crust is a silicate solid, the mantle is a viscous molten rock, the outer core is a viscous liquid, and the inner core is a dense solid.
◇ Mechanically, the earth’s layers can be divided into lithosphere, asthenosphere, mesospheric mantle (part of the Earth's mantle below the lithosphere and the asthenosphere), outer core, and inner core.
◇ Chemically, Earth can be divided into the crust, upper mantle, lower mantle, outer core, and inner core.
■ THE CRUST
◇ The crust is the outermost layer of the earth making up 0.5-1.0 per cent of the earth’s volume and less than 1 per cent of Earth’s mass.
◇ Density increases with depth, and the average density is about 2.7 g/cm3 (average density of the earth is 5.51 g/cm³).
◇ The thickness of the crust varies in the range of 5-30 km in case of the oceanic crust and as 50- 70 km in case of the continental crust.
◇ The continental crust can be thicker than 70 km in the areas of major mountain systems. It is as much as 70-100 km thick in the Himalayan region.
◇ The temperature of the crust increases with depth, reaching values typically in the range from about 200 °C to 400 °C at the boundary with the underlying mantle.
◇ The temperature increases by as much as 30 °C for every kilometre in the upper part of the crust.
◇ Outer covering of the crust is of sedimentary material and below that lie crystalline, igneous and metamorphic rocks which are acidic in nature.
■ MOST ABUNDANT ELEMENTS OF THE EARTH'S CRUST
ELEMENT - APPROXIMATE % BY WEIGHT
◇ Oxygen - 46.6%
◇ Silicon (Si) - 27.7%
◇ Aluminium (Al) - 8.1%
◇ Iron - 5.0%
◇ Calcium (Ca) - 3.6%
◇ Sodium (Na) - 2.8%
◇ Potassium (K) - 2.6%
◇ Magnesium (Mg) - 1.5%
■ ELEMENTS OF EARTH
1. Iron – 34.6%
2. Oxygen – 29.5 %
3. Silicon – 15.2 %
4. Magnesium – 12.7%
5. Nickel – 2.4 %
6. Sulphur – 1.9%
7. Titanium – 0.05%
▪︎Note - This is an approx data. Different sources have given different data.
■ THE MANTLE
◇ It forms about 83 per cent of the earth's volume and holds 67% of the earth’s mass.
◇ It extends from Moho’s discontinuity to a depth of 2,900 km.
◇ The density of the upper mantle varies between 2.9 g/cm3 and 3.3 g/cm3.
◇ The lower mantle extends beyond the asthenosphere. It is in a solid state.
◇ The density ranges from 3.3 g/cm3 to 5.7 g/cm3 in the lower mantle.
◇ The mantle is composed of silicate rocks that are rich in iron.
◇ Regarding its constituent elements, the mantle is made up of 45% oxygen, 21% silicon, and 23% magnesium (OSM).
◇ Temperatures range from approximately 200 °C at the upper boundary with the crust to approximately 4,000 °C at the core-mantle boundary.
◇ Because of the temperature difference, there is a convective material circulation in the mantle (although solid, the high temperatures within the mantle cause the silicate material to be sufficiently ductile).
■ THE OUTER CORE
◇ The outer core, surrounding the inner core, lies between 2900 km and 5100 km below the earth's surface.
◇ The outer core is composed of iron mixed with nickel (nife) and trace amounts of lighter elements.
◇ The outer core is not under enough pressure to be solid, so it is liquid even though it has a composition similar to the inner core.
◇ The density of the outer core ranges from 9.9 g/cm3 to 12.2 g/cm3 .
◇ The temperature of the outer core ranges from 4400 °C in the outer regions to 6000 °C near the inner core.
◇ Dynamo theory suggests that convection in the outer core, combined with the Coriolis effect, gives rise to Earth's magnetic field.
■ THE INNER CORE
◇ Inner core extends from the centre of the earth to 5100 km below the earth's surface.
◇ Inner core is generally believed to be composed primarily of iron (80%) and some nickel (nife).
◇ Since this layer can transmit shear waves (transverse seismic waves), it is solid. (When P-waves strike the outer core – inner core boundary, they give rise to S-waves).
◇ Earth's inner core rotates slightly faster relative to the rotation of the surface.
◇ The solid inner core is too hot to hold a permanent magnetic field.
◇ The density of the inner core ranges from 12.6 g/cm3 to 13 g/cm3.
◇ The core (inner core and the outer core) accounts for just about 16 per cent of the earth's volume but 33% of earth’s mass.
◇ Scientists have determined the temperature near the Earth's centre to be 6000֯C.
◇ At 6000°C, this iron core is as hot as the Sun’s surface, but the crushing pressure caused by gravity prevents it from becoming liquid.
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