Chapter II: The Earth (NCERT Class 11 Geography Solutions)
1. Multiple choice questions.
(i) Which one of the following figures represents the age of the earth?
(a) 4.6 million years
(b) 13.7 billion years
(c) 4.6 billion years
(d) 13.7 trillion years
Correct answer: (c) 4.6 billion years
Explanation: The correct representation of the age of the earth is 4.6 billion years. This is the estimated age of the earth based on scientific evidence and geological dating methods.
(ii) Which one of the following has the longest duration?
(a) Eons
(b) Period
(c) Era
(d) Epoch
Correct answer: (a) Eons
Explanation: Eons have the longest duration among the given options. Geological time is divided into several units, with eons being the largest division of time, followed by eras, periods, and epochs.
(iii) Which one of the following is not related to the formation or modification of the present atmosphere?
(a) Solar winds
(b) Differentiation
(c) Degassing
(d) Photosynthesis
Correct answer: (d) Photosynthesis
Explanation: Photosynthesis is not related to the formation or modification of the present atmosphere. Photosynthesis is a biological process carried out by plants and some bacteria, which converts carbon dioxide and water into oxygen and glucose.
(iv) Which one of the following represents the inner planets?
(a) Planets between the sun and the earth
(b) Planets between the sun and the belt of asteroids
(c) Planets in gaseous state
(d) Planets without satellite(s)
Correct answer: (a) Planets between the sun and the earth
Explanation: The inner planets are the ones that lie between the sun and the asteroid belt. They include Mercury, Venus, Earth, and Mars. These planets are rocky and terrestrial in nature.
(v) Life on the earth appeared around how many years before the present?
(a) 13.7 billion
(b) 3.8 million
(c) 4.6 billion
(d) 3.8 billion
Correct answer: (d) 3.8 billion
Explanation: Life on Earth is estimated to have appeared around 3.8 billion years before the present. The origin of life is believed to have started with simple unicellular organisms like bacteria and algae, which eventually evolved into more complex forms over time.
2. Answer the following questions in about 30 words.
(i) Why are the terrestrial planets rocky?
Ans: Terrestrial planets are rocky because they formed closer to the Sun, where it was too warm for gases to condense into solid particles. This allowed heavier materials like rocks and metals to dominate.
(ii) What is the basic difference in the arguments related to the origin of the earth given by: (a) Kant and Laplace (b) Chamberlain and Moulton
Ans: (a) Kant and Laplace proposed the nebular hypothesis, suggesting that planets formed from a rotating cloud of material around the young Sun. According to their theory, a cloud of gas and dust slowly rotated, and gravity caused this material to accrete into planets over time.
(b) Chamberlain and Moulton proposed the “giant impact” theory, suggesting that the Moon formed from debris after a collision between Earth and a Mars-sized body. They argued that a wandering star approached the Sun, resulting in a cigar-shaped extension of material being separated from the solar surface. The material then revolved around the Sun and eventually condensed into planets.
(iii) What is meant by the process of differentiation?
Differentiation refers to the process by which the Earth’s interior materials separated based on their densities during its early molten stage. The Earth was initially in a volatile state, and due to gradual increase in density, the temperature inside increased. This caused heavier materials, like iron, to sink towards the center of the Earth, forming the core, while lighter materials moved towards the surface, forming the crust. The process of differentiation led to the formation of distinct layers within the Earth, such as the mantle, outer core, and inner core.
(iv) What was the nature of the Earth’s surface initially?
The nature of the Earth’s surface initially was hot, barren, and rocky with a thin atmosphere of hydrogen and helium. During its primordial stage, the Earth was mostly in a volatile state. As it cooled and solidified, a crust began to form on the outer surface.
(v) What were the gases which initially formed the Earth’s atmosphere?
The initial atmosphere of Earth mainly consisted of hydrogen, helium, water vapor, nitrogen, carbon dioxide, methane, and ammonia. However, the early atmosphere was not conducive to supporting life. Over time, volcanic eruptions and other geological processes released gases like water vapor and carbon dioxide into the atmosphere. Eventually, the composition of the atmosphere was modified by the living world through the process of photosynthesis, leading to the oxygen-rich atmosphere we have today.
3. Answer the following questions in about 150 words.
(i) Write an explanatory note on the ‘Big Bang Theory’.
(ii) List the stages in the evolution of the earth and explain each stage in brief.
(i) Explanatory note on the ‘Big Bang Theory’:
The Big Bang Theory is the prevailing scientific explanation for the origin and evolution of the universe. It suggests that the universe began as a singularity, a point of infinite density and temperature, around 13.7 billion years ago. This initial state expanded rapidly in a tremendous explosion, creating space, time, and all matter in the universe. As the universe expanded, it cooled, allowing subatomic particles to form. Over time, these particles combined to form atoms, then molecules, and eventually stars and galaxies.
The evidence supporting the Big Bang Theory comes from various observations, such as the cosmic microwave background radiation, which is the faint glow of radiation left over from the early universe. Additionally, the redshift of galaxies indicates that the universe is still expanding.
The Big Bang Theory not only explains the origin of the universe but also its evolution. It accounts for the abundance of light elements like hydrogen and helium, the formation of galaxies and clusters of galaxies, and the cosmic background radiation. This theory is widely accepted by the scientific community and has become the foundation of modern cosmology.
(ii) Stages in the evolution of the earth and brief explanation:
Formation of the Earth: The Earth’s formation began around 4.6 billion years ago from a rotating cloud of gas and dust, according to the nebular hypothesis. As the cloud contracted due to gravity, it formed a flattened disk. The center of this disk became the young Sun, while the material in the disk coalesced to form the planets.
Differentiation and Layered Structure: During its early molten stage, the Earth underwent differentiation, where materials separated based on their densities. Heavier materials, like iron and nickel, sank to the center to form the core, while lighter materials accumulated at the surface to create the crust. The process of differentiation gave rise to a layered structure with the crust, mantle, outer core, and inner core.
Evolution of the Atmosphere and Hydrosphere: The early atmosphere mostly consisted of hydrogen, helium, water vapor, nitrogen, carbon dioxide, methane, and ammonia. Volcanic eruptions released gases, and as the Earth cooled, water vapor condensed, leading to the formation of oceans. Life began to evolve around 3.8 billion years ago, and through photosynthesis, oxygen began to accumulate in the atmosphere.
Origin of Life: Life on Earth is believed to have originated around 3.8 billion years ago, likely from chemical reactions that formed complex organic molecules. These molecules assembled in a way that allowed them to replicate, giving rise to the first living organisms. Over billions of years, life evolved from simple unicellular bacteria to complex multicellular organisms.
These stages in the evolution of the Earth have shaped our planet into the diverse and vibrant world we know today.
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