Consider the following statements: 1. The continental crust is composed of iron magnesium silicate igneous rocks, like basalt. 2. Gutenberg Discontinuity forms the boundary between the crust and the asthenosphere. Which of the statements given above is are correct?
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
​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.
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 is composed of lighter (felsic) sodium potassium aluminium silicate rocks, like granite. Hence, statements 1 is NOT correct.
The oceanic crust, on the other hand, is composed of dense (mafic) iron magnesium silicate igneous rocks, like basalt.
Most abundant element in the earth's crust is oxygen.
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 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 and magnesium relative to the overlying crust.
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 density of the outer core ranges from 9.9 g/cm3 to 12.2 g/cm3.
The Inner Core
The inner core extends from the centre of the earth to 5100 km below the earth’s surface.
The inner core is generally believed to be composed primarily of iron (80%) and some nickel (nife).
The density of the inner core ranges from 12.6 g/cm3 to 13 g/cm3.
The core accounts for just about 16 per cent of the earth’s volume but 33% of earth’s mass.
Seismic Discontinuities
​Mohorovicic Discontinuity (Moho): separates the crust from the mantle.
Asthenosphere: highly viscous, mechanically weak and ductile part of the mantle.
Mistake Point
Gutenberg Discontinuity: lies between the mantle and the outer core. Hence, statements 2 is NOT correct.