The origin of Earth and universe is incompletely understood and accepted that the universe began with a cosmic event called the big bang. First and foremost, the Earth is the only planet which supports lives of all forms. It is the third planet from the sun by its position and fifth largest planet by its size and mass. The age of the Earth is over 4.5 billion years and the shape represents a rough sphere (named as geoid). The diameter of the Earth is about 12,740 kilometres and its mass is about 5.97×1024 kilograms. The highest point is Mount Everest of the Himalayas with 8,848 metres above sea level and the lowest point is Mariana Trench of the Pacific Ocean with 11,034 metres below sea level.
Interior of the Earth
The interior of the earth is made up of layers which can be explained through either by its mechanical or chemical properties.
- Mechanically (Structure): Earth can be split into lithosphere, asthenosphere, mesospheric mantle, outer core, and the inner core.
- Chemically (composition): Earth can be split into the crust, the mantle (upper and lower mantle), and the core (outer core and inner core) (Fig. 1).
Fig. 1 – The Layers (Strata) of the Earth’s Interior
i) The Crust
The crust is the outermost layer of the Earth, consists of a broad mixture of rock types. The average thickness of the crust ranges from 6.5 kilometres (oceanic crust - under oceans) to 35 kilometres (continental crust - under continents). The temperature of the crust in the desert regions might be as hot as 35° Celsius and in the Arctic and Antarctic it would be below freezing point.
Continental crust tends to be much more variable in composition like silicate and aluminium rocks – granitic type (SIAL - lower in density with 2.7 g/cm3). Oceanic crust is typically composed of silicate and magnesium igneous rocks - basaltic type (SIMA - higher in density with 3.5 g/cm3).
However, the crust can be further divided into upper and lower layers. SIAL refers to earth’s crust's upper layer, made up of silicate and aluminium based minerals. SIMA refers to the earth’s crust's lower layer with an abundance of silicate and magnesium minerals.
Fig. 2 – Cross-section of Continental and Oceanic Crusts
ii) The Mantle
The mantle is the Earth’s layer that lies between the crust and core. Below the crust a narrow zone with a change in mineral composition is called the Mohorovičić (Moho) discontinuity which separates crust and mantle. The mantle extends downward to a depth of approximately 2,900 kilometres from the Moho discontinuity and makes up a volume of 84% of the total (two-thirds of Earth’s total mass; half the distance from the surface to the centre of Earth). Further, the mantle is divided into two parts as upper mantle and lower mantle (Fig. 3).
Fig. 3 – Cross-section of Earth's Layers (Crust to Lower Mantle)
The upper mantle is relatively thin but hard and rigid. The average thickness of this layer is 600 kilometres, extending down to a depth of 650 kilometres from the Moho discontinuity (deeper under the continents than under the ocean floors). The layers upper mantle and crust together is called the lithosphere. The temperature near the crust ranges from 480º Celsius to 870º Celsius and it increases towards the greater depths.
The lower mantle is hotter and denser than the upper mantle. The thickness of the lower mantle is about 2,200 kilometres beneath the crust. Comparatively, the lower mantle is much less malleable than the upper, and the extreme pressure retains the lower mantle solid. A greater variation of temperature in the mantle ranges from 1000° Celsius (near the crust) to 3700° Celsius (near the core).
iii) The Core
The core and mantle are separated by a layer called Wiechert–Gutenberg Discontinuity. The core is a ball-shaped layer with the thickness of approximately 3,400 kilometres beneath the mantle. Further the core is divided into two layers as outer core and inner core and dominantly made up of nickel and iron (also called NIFE). These two zones together make up about 15 percent of Earth’s volume and 32 percent of its mass.
Fig. 4 – Cross-section of Earth’s Interior
The outer core is liquid and made up of liquid metals (nickel and iron). In this layer, the temperature ranges from 2,204° Celsius to 4,982° Celsius and the depth is about 2,900 kilometres to 5,150 kilometres below the inner mantle.
The inner core is solid and made up of solid metals (nickel and iron). The maximum temperature of the layer is 5,400° Celsius and the thickness ranges from 5,150 kilometres to 6,370 kilometres below the outer core.