Q1. If you travelled from the surface to the centre of the Earth, describe the four major
layers you would encounter in terms of their physical and chemical properties. Suggest how
each layer influences (or may influence) our life on the Earth’s surface.
THE HISTORY OF EARTH
The Earth, as we know it today, is thought to have formed approximately 4.6 billion years ago.
With the passing of hundreds of millions of years, the early Earth was created by accretion. It
continued to transform due to asteroidal barrage by a special kind of meteorite known as a
Carbonaceous Chondrite (a carbon-rich stony meteorite). These chondrites in particular have
much more of the element Iron (Fe) than typical mantle rocks and are very similar in
composition to the Sun (except for the absence of a few gaseous elements). They are known to
have an age of roughly 4550 million years which is early in the evolution of the solar system.
Therefore, the Earth was built up by the successive impacts of these carbonaceous chondrite
asteroids drawn into it as a result of gravity. Eventually, the interior of the Earth would begin to
heat up under gravitational pressure, heat of asteroidal impacts as well as the enormous heat
generated by the decay of naturally occurring radioactive elements such as Potassium and
Uranium. The hot interior of the Earth serves the role of a blast furnace where liquid iron is freed
from the interior of carbonaceous chondrite as carbon draws off iron from iron-rich ores. This
liquid iron then gradually falls to the Earth’s centre due to its high density where it forms the
Core. So, deep in the Earth while the iron is separating out and sinking, water vapor, gases and
other light compounds are expelled and move upwards towards the surface where they form the
Earth’s Crust, Oceans and Atmosphere. The material that is left behind has a composition and
density very close to the Earth’s Mantle. This entire process by which the heating of a uniform
starting material produces different compositional fractions that separate out according to their
density is known as differentiation.
Therefore, as we go down to the centre of the Earth, the following four layers can be
distinguished based upon their physical and chemical composition:
The Crust is the outermost layer of the Earth made up of silicate rock materials. The crust makes
up only about 0.1% to 1% of the Earth’s radius and is the thinnest of all the layers. Geologists
distinguish between two different types of crust – oceanic crust which underlies the sea floor,
and continental crust which underlies continents. The crust is not simply cooled mantle, but
rather consists of a variety of rocks that differ in chemical composition from mantle rock. The
thickness of the crust varies on the land and on the ocean. For example, oceanic crust is only 7 to
10 km thick and consists of fairly uniform layers. The top layer consists of a blanket of sediments,