Evolution of the Earth
EVOLUTION OF THE EARTH
Since, 4.6 billion years, Earth has evolved from being a barren, rocky, and hot object having an atmosphere of hydrogen and helium to its present form of a layered structure in increasing order of density towards the core. These layers are made up of different zones with materials having different characteristics. The layers of the atmosphere have the least density whereas the core has the highest density.
EVOLUTION OF LITHOSPHERE
Earth in the primordial stage was very volatile. Gradually, the density increased as the materialsstarted to separate and get organized. Therefore, heavier metals sink towards the center (like iron) whereas, lighter ones moved towards the surface. As time passes materials solidify and condensed into smaller sizes. This outermost layer is known as the crust. This gave multiple layering in the interior of the earth i.e., Crust, Mantle, and Core.
CRUST- The crust is the outermost solid part of the earth. The crust is divided into two parts i.e., continental crust and oceanic crust. The thickness of the crust varies as the oceanic crust is thinner than the continental crust. The mean thickness of the oceanic crust is 5 Km whereas the mean thickness of the continental crust is 30 Km and it can reach up to 70 Km in the Himalayan Mountain region. The mean density of continental crust is 2.7 g/cm3 and it is majorly made up of granite rock whereas the oceanic crust is made up of basalt rock with a mean density of
MANTLE- Crust, and Mantle separated from Moho’s discontinuity. The mantle can extend to a depth of 2900km with a mean density of 3.4 g/cm3. The upper part of the mantle is known as the asthenosphere. The word “astheno” means weak. It is a kind of liquid form on which plates move. It extends up to 400 Km. It is also the main source of magma that comes out from a vent during a volcanic eruption.
The crust and the asthenosphere combined are known as the lithosphere. The lower mantle is in a solid state.
CORE- The core of the earth is divided into two parts outer and inner core. The outer core is in the liquid state while the inner core is in the solid state. This evidence is acquired from the waves of earthquakes. The core is made up of nickel and iron. Thus, the mean density of the core is very high (5 g/cm3).
NOTE- The density of the materials increases from the crust to the core.
EVOLUTION OF ATMOSPHERE
Earth’s initial atmosphere was composed of hydrogen and helium, the passage of time got replaced by Nitrogen and Oxygen due to the impact of solar winds. This process gets completed through three stages.
STAGE 1- MODIFICATION OF ATMOSPHERE THROUGH SOLAR WINDS
The first stage of the formation of the Atmosphere is known as the loss of primitive atmosphere. Initially, the earth’s atmosphere is composed of hydrogen and helium. All the terrestrial planets including earth lost their primordial atmosphere due to solar winds.
NOTE- The solar winds refer to the stream of charged particles coming from the outer atmosphere of the sun and traveling in the entire solar system.
STAGE 2- MODIFICATION OF ATMOSPHERE THROUGH EARTH’S INTERIOR
In the second stage, the heat coming from the interior of the earth change and evolve the atmosphere. When the earth started cooling various gases and water vapor was released from the earth’s core. At this time atmosphere consists of water vapor, nitrogen, methane, carbon dioxide, ammonia, and very little oxygen. These gases and water vapor are coming out with the help of volcanic eruptions.
NOTE- Degassing is the process through which the gases were outpoured from the interior of the earth.
STAGE 3- MODIFICATION OF ATMOSPHERE THROUGH PHOTOSYNTHESIS
Further chances occurred in the atmosphere due to the living organism through photosynthesis. Living organisms were dominate only oceans for a long period. It helps in flooding the
atmosphere with oxygen. Thus, nitrogen and oxygen dominate the earth’s atmosphere.
EVOLUTION OF HYDROSPHERE
Hydrosphere refers to the total water available on earth. Oceans, seas, rivers, lakes, and glaciers are part of the hydrosphere. The water on our planet is presented in three forms solid, liquid, and gas. The evolution of the hydrosphere has the following stages-
STAGE 1- AGE OF DEGASSING
The degassing accumulates water vapors and gases into the atmosphere. It is believed that these gases and water vapors are released from within the interior of the earth through volcanic eruptions, hot springs, and geysers.
STAGE 2- AGE OF CONDENSATION AND PRECIPITATION
As the earth cooled, the water vapor starts to condense, and carbon dioxide mixed with rainwater further reduces the temperature causing more precipitation in the form of rain, snow, hail, etc.
STAGE 3- AGE OF RAINWATER HARVESTING
The rainwater collected in depressions gives rise to the present form of the world’s oceans. It takes 500 million years to form oceans and they are 4000 million years old.
Continents and oceans are considered “relief features of the first order”. About 71% of the total surface area of the earth is dominated by oceans. The oceans contain 97% of the total volume of water. Water dominates the southern hemisphere. Thus, the southern hemisphere is known as the “Water Hemisphere”. The ocean of the world can further be divided into two parts i.e., the aqua sphere and cryosphere. The word aqua sphere refers to the liquid form of water and the word cryosphere refers to the solid form of water (Ice), respectively.
Average depth (m)
The area of the earth’s surface (%)
ORIGIN OF LIFE
Scientists found microscopic fossils of blue algae which were traced back from Pre- Cambrian era i.e.,3000 million years old. Thus, it is believed that the origin of life on the planet earth started with unicellular bacteria sometime 3800 million years ago. Since then, it starts evolving into a modern-day man. The evolution from unicellular bacteria to modern-day man is depicted in the above flow chart.