Maarg Manthan · Topic 1.5

Evolution of the Lithosphere, Atmosphere and Hydrosphere

Geography › Earth's Origin, Interior and Motions · Topic 1.5

The atmosphere, hydrosphere and lithosphere of the Earth did not exist in their present form at the start. The crust formed as the molten surface cooled. The first atmosphere of hydrogen and helium was blown away, and a second atmosphere built up from gases released by volcanoes: water vapour, carbon dioxide, nitrogen and traces of others, with almost no oxygen. As the Earth cooled, water vapour condensed and rain filled the hollows to form oceans within about 500 million years of the Earth's formation. Oxygen came much later, from photosynthesis by early organisms.

5 min read · UPSC Prelims · Geography · Earth's Origin, Interior and Motions

At a Glance

First atmosphere Hydrogen and helium, lost to the solar wind
Second atmosphere Gases from volcanic degassing
Oceans formed Within about 500 million years of the Earth’s birth
Oxygen rise From photosynthesis, mainly after about 2.4 billion years ago
Oldest known rocks About 4 billion years old
Exam link UPSC Prelims, Geography: Earth’s Origin, Interior and Motions

Where Does It Fit?

This topic follows the Earth’s differentiation and explains how its solid, gaseous and watery layers arose. It leads into the topics on the Pangaea, about 300 to…">geological history of the Earth and the origin of life, since oceans and oxygen made life possible. Previous topic: Layered Structure of the Earth (Five Layers). Next topic: Geological History of the Earth.

How Did the Lithosphere Evolve?

In short: as the molten surface cooled and hardened, a thin crust formed, and over time it was recycled and enlarged to produce the continents.

First crust: within a few hundred million years of its birth, the Earth’s surface cooled enough to harden into a thin crust. Most of this early crust was destroyed by heavy bombardment from space and by remelting.
Oldest rocks: the oldest known rock bodies, such as the Acasta Gneiss in Canada, are about 4 billion years old. Tiny zircon crystals from the Jack Hills in Western Australia are about 4.4 billion years old and are the oldest known Earth material.
Growth of continents: lighter granite-rich crust built up in patches called cratons, which became the cores of the continents. Plate movements then joined and split them, as in Continental Drift Theory (Alfred Wegener, 1912): Pangea, Panthalassa, Laurasia, Gondwanaland.
Recycling: the oceanic crust is continuously created and destroyed, so it is young, but the continental crust survives for billions of years.

How Did the Atmosphere Evolve?

The history of the atmosphere is usually described in three stages.

Stage	What happened
1. Loss of the first atmosphere	The hydrogen and helium that surrounded the young Earth were blown away by the strong solar wind from the young Sun, and by the Earth’s heat
2. Degassing	Gases trapped inside the hot Earth came out through volcanoes. They were mainly water vapour, carbon dioxide, nitrogen, methane and ammonia, with little or no free oxygen
3. Photosynthesis	Early life in the oceans, especially cyanobacteria, used sunlight, water and carbon dioxide to make food and released oxygen. Oxygen built up slowly in the sea and then in the air

The rise of oxygen is called the Great Oxidation Event, and it took place roughly 2.4 to 2.2 billion years ago. Early oxygen first reacted with dissolved iron in the sea, which formed the banded iron formations that are the main source of iron ore, including the ores of India. Free oxygen reached about its present level of 21 per cent much later. A layer of ozone, which absorbs harmful ultraviolet rays, formed from oxygen and later allowed life to move onto the land.

How Did the Hydrosphere Evolve?

Water vapour: volcanic eruptions kept adding water vapour to the air.
Cooling and condensation: as the Earth cooled, the water vapour condensed and fell as rain. The rain also dissolved carbon dioxide from the air, which cooled the Earth further and caused more rain.
Collection: rainwater gathered in the depressions on the surface and filled them to make the first oceans. The oceans were formed within about 500 million years of the Earth’s formation, so they are roughly 4 billion years old.
Salts: rivers carried dissolved minerals to the sea and volcanoes added more, giving the oceans their salinity.
Life: the oceans provided the setting for the first life and for photosynthesis.

Scientists still debate how much of the water came from inside the Earth and how much was delivered by icy comets and water-rich asteroids. Both sources are probably important.

Why Do These Three Spheres Matter Together?

The lithosphere, atmosphere and hydrosphere interact continuously, and the biosphere joins them once life begins. Volcanoes pass gases from the lithosphere to the air, rain weathers rock and carries minerals to the sea, and the oceans absorb and store carbon dioxide. This is the basis of the later study of climate, the carbon cycle and weathering, such as Weathering: Chemical, Physical/Mechanical and Biological Processes. As an analytical point, the chemistry of the modern air is the result of life itself, since a planet without photosynthesis would not hold free oxygen.

Why Is This Topic Important?

Sequence Order of events is asked
Oxygen Link between life and air
Climate Basis for the carbon cycle

What Came Next?

The next topic tells the long story of the Earth’s geological history, era by era. Next topic: Geological History of the Earth.

Key People and Terms

Degassing Release of gases from the Earth’s interior
Craton Ancient stable core of a continent
Cyanobacteria Early photosynthetic microbes
Great Oxidation Event Rise of oxygen in the air
Banded iron formation Layered rocks of iron oxide and silica
Ozone layer Shield against ultraviolet rays

Exam Corner

Points to Remember

The first atmosphere of hydrogen and helium was lost.
The second atmosphere came from degassing by volcanoes.
Photosynthesis added oxygen.
Oceans formed within about 500 million years of the Earth’s birth.
The oldest known Earth material is a zircon crystal about 4.4 billion years old.

Do Not Confuse With

First and second atmosphere Primordial gases versus volcanic gases
Degassing and photosynthesis Gases from inside versus oxygen from life
Oceanic and continental crust Young and recycled versus old and lasting

Memory Hook

Hydrogen fled, volcanoes bled; rain made seas, microbes breathed out ease.

Mains Angle

Questions ask for the evolution of the atmosphere and oceans. These points add depth.

Stages Loss, degassing, photosynthesis
Role of life Oxygen and ozone
Link Iron ores and carbon storage

A Question You May Face

An original practice question, not a past paper question.

“Trace the evolution of the Earth’s atmosphere and explain the role of life in changing its composition.”

How to Answer

Stage one Loss of the first atmosphere.
Stage two Degassing.
Stage three Photosynthesis and oxygen.
Impact Ozone, oceans and life.

GS Relevance

Prelims: stages in the evolution of the atmosphere, origin of oxygen and oceans. GS Paper 1: origin and evolution of the Earth.

Frequently Asked Questions

How did the Earth's atmosphere originate?

The Earth's first atmosphere of hydrogen and helium was lost to the solar wind. A second atmosphere built up as volcanoes released water vapour, carbon dioxide, nitrogen and other gases from the interior, a process called degassing. Photosynthesis later added oxygen and changed the air into its present form.

How were the oceans formed?

Volcanoes released large amounts of water vapour. As the Earth cooled, the vapour condensed into clouds and fell as rain, which collected in hollows on the surface. The oceans formed within about 500 million years of the Earth's formation and are therefore roughly four billion years old.

Where did oxygen in the atmosphere come from?

Almost all of the free oxygen came from photosynthesis. Early cyanobacteria in the oceans released oxygen as a by-product, and it built up gradually, especially during the Great Oxidation Event about 2.4 to 2.2 billion years ago, until the air held about 21 per cent oxygen.

What is degassing?

Degassing is the release of gases that were trapped inside the hot young Earth. The gases escaped through volcanic eruptions and included water vapour, carbon dioxide, nitrogen, methane and ammonia. This formed the second atmosphere and supplied the water for the oceans.

What are the oldest rocks on the Earth?

The oldest known rock bodies, such as the Acasta Gneiss in Canada, are about four billion years old. Tiny zircon crystals from Western Australia are about 4.4 billion years old, which makes them the oldest known material formed on the Earth.

PYQ Practice — Statement Analysis

1 The Earth's first atmosphere was rich in oxygen.

False

It was hydrogen and helium, and was lost.

2 Degassing from volcanoes contributed to the second atmosphere.

True

Water vapour and carbon dioxide were major gases.

3 Free oxygen in the air is mainly the product of photosynthesis.