Evolution of the atmosphere and hydrosphere

Detail

1. Key terms

• Atmosphere: the layer of gases around the Earth.
• Hydrosphere: all the water on the Earth. This includes oceans, seas, rivers, lakes and water vapour.
• Evolution of the atmosphere: the slow change in the gases around the Earth. The Earth started with a hydrogen-helium envelope. Today the air is mainly nitrogen and oxygen.
• Terrestrial planets: the four inner, rocky planets: Mercury, Venus, Earth and Mars.

2. The three stages in the evolution of the atmosphere

Stage	What happened	Main gases	Driving force
1. Primordial atmosphere	The first gas envelope was lost	Hydrogen, helium	Solar wind
2. Degassed atmosphere	Gases escaped from the hot interior	Water vapour, nitrogen, carbon dioxide, methane, ammonia; very little free oxygen	Degassing and volcanic eruptions
3. Biological atmosphere	Living things changed the mix of gases	Free oxygen added; today mainly nitrogen and oxygen	Photosynthesis

Stage 1: loss of the primordial atmosphere

• Primordial atmosphere: the very first gas envelope. It was made of the light gases hydrogen and helium.
• Solar wind: a stream of charged particles that flows out from the Sun all the time.
• The young Sun sent out a strong solar wind.
• This wind stripped away the hydrogen-helium envelope.
• This happened to all the terrestrial planets, not only to the Earth.

Stage 2: degassing

• Degassing: gases and water vapour escaping from inside a cooling planet.
• The young Earth's interior was very hot.
• As the Earth cooled, gases trapped inside came out to the surface.

• Constant volcanic eruptions added more water vapour and gases to the air.

• The result was a second atmosphere. It had water vapour, nitrogen, carbon dioxide, methane and ammonia.

• It had very little free oxygen. Free oxygen means oxygen gas (O₂) that is not locked into rocks or water.

Stage 3: change by living things

• Photosynthesis: plants and some microbes use sunlight, water and carbon dioxide to make food. They release oxygen as a by-product.
• Living things slowly added free oxygen to the air and changed its make-up.
• Today's atmosphere is mainly nitrogen and oxygen (see atmosphere-composition-weather).

3. Origin of the oceans (the hydrosphere)

How the oceans formed:

• The Earth cooled, so the water vapour in the air condensed. This means it turned from gas into liquid drops.
• It fell as rain. The rainwater dissolved carbon dioxide from the air, so the air lost some of its carbon dioxide.
• The temperature fell further. This caused more condensation and more rain.
• The rainwater collected in low-lying basins (depressions) on the surface, and these became the oceans.

Timing:

• The oceans formed within 500 million years of the Earth's birth (about 4.6 billion years ago).
• So the oceans are about 4,000 million years (4 billion years) old.

4. Oxygenation of the atmosphere

• Oxygenation of the atmosphere: the slow build-up of free oxygen in the air through photosynthesis.

Step-by-step sequence:

• Life was confined to the oceans for a very long time.
• Photosynthesis evolved about 2,500–3,000 million years ago.
• The oceans filled up with oxygen first.
• Ocean organisms released oxygen into the seawater.

• The oceans took in this oxygen until they were saturated, which means they could hold no more.

• Then the oxygen moved into the air. About 2,000 million years ago, oxygen began to flood the atmosphere.

Great Oxidation Event (GOE)

• The Great Oxidation Event was the first large and lasting rise of free oxygen in the atmosphere. It happened about 2.4–2.0 billion years ago.
• The oxygen rise was fast in geological time. Oxygen in the air rose from about 1 per cent to 15 per cent within about 200 million years [4].
• The oceans and the air did not gain oxygen at the same pace [4]:

Period	Atmosphere	Oceans
Before 2.3 billion years ago	Little oxygen	Little oxygen; much dissolved iron
2.3–1.8 billion years ago	Oxygen increasing	Still mostly oxygen-free
After 1.6 billion years ago	Oxygenated	Modern, oxygen-rich conditions

• The oxygen rise may have started earlier than once thought:
• In South Africa, fossil microbial mats mixed with ancient soil are about 2.6 billion years old. They show that there was life on land and some oxygen in the air by then [3].
• The older view was that the air had enough oxygen for an ozone layer only by about 2.2 billion years ago. That is about 400 million years later [3].

5. Side effects of oxygenation

(a) Banded iron formations (BIFs)

• Banded iron formation: an old sedimentary rock made of thin, alternating layers. Iron-rich bands (haematite or magnetite) alternate with silica-rich bands (chert or jasper).
• How BIFs formed:
• The early oceans had a lot of dissolved iron.
• Oxygen from photosynthesis reacted with this iron. The iron became an oxide that does not dissolve, so it sank to the sea floor as an iron-rich layer [4].
• When oxygen in the water fell, iron stopped settling. Then only rock without iron formed [4].

• These layers repeated many times. This gave the striped rocks, which can be "read" like the rings of a tree [4].

• Link to India:

• India's haematite belongs to the Precambrian Iron Ore Series. The ore occurs within banded iron formations [2]. The Precambrian is the long period of Earth history before about 541 million years ago.
• Total iron ore resources (haematite + magnetite) are over 33.276 billion tonnes (IMYB 2021) [2].
• Haematite resources are 24,057 million tonnes (as on 1.4.2020) [2].
• About 79% of haematite is in the Eastern Sector (Assam, Bihar, Chhattisgarh, Jharkhand, Odisha, Uttar Pradesh) [2].
• Magnetite resources are 11,227 million tonnes (as on 1.4.2020). Karnataka alone has 69.50% [2].
• Iron ore production was 204.48 million tonnes in 2020-21 [2].
• Major mechanised public-sector mines include Kiriburu, Barsua, Gua, Bailadila, Donimalai, Daitari and Dalli-Rajhara [2].
• See minerals-rocks-rock-cycle.

Rank	State	Haematite resources (1.4.2020) [2]
1	Odisha	9,409 million tonnes (39%)
2	Jharkhand	4,710 million tonnes (20%)
3	Chhattisgarh	4,592 million tonnes (19%)
4	Karnataka	2,835 million tonnes (12%)
5	Goa	1,197 million tonnes (5%)

(b) The ozone layer

• Ozone (O₃): a form of oxygen with three atoms instead of two.
• How the ozone layer formed:
• Free oxygen built up in the air.
• The Sun's ultraviolet (UV) rays split some oxygen molecules. The single atoms joined other oxygen molecules to form ozone.

• This made the ozone layer in the upper atmosphere. It absorbs harmful UV rays.

• Result: with a UV shield in place, life could later move out of the oceans and live on land.

Prelims Hooks

• The primordial atmosphere was hydrogen + helium. It was removed by the solar wind, and this happened to all terrestrial planets (Mercury, Venus, Earth, Mars).
• Degassing = release of gases from the cooling interior, helped by volcanic eruptions.
• The degassed atmosphere had water vapour, N₂, CO₂, CH₄ and NH₃. Trap: it had very little free oxygen, not "no gases other than CO₂".
• Correct order: hydrogen-helium atmosphere lost → degassing → photosynthesis adds oxygen.
• The oceans formed within 500 million years of the Earth's birth and are about 4,000 million years old.
• Rain dissolved carbon dioxide from the early atmosphere.
• Photosynthesis evolved about 2,500–3,000 million years ago. Oxygen began to flood the atmosphere about 2,000 million years ago, after the oceans were saturated first.
• Great Oxidation Event ≈ 2.4–2.0 billion years ago. Banded iron formations are its record in the rocks.
• Indian haematite occurs in banded iron formations of the Precambrian Iron Ore Series. Ranking of haematite resources (2020): Odisha > Jharkhand > Chhattisgarh > Karnataka > Goa [2].
• Karnataka leads in magnetite (69.50%, 2020) [2]. Trap: Odisha leads in haematite, not magnetite.

Mains Points

• Life and atmosphere shaped each other (GS-I): Volcanic degassing made an air with no free oxygen. Photosynthesis then turned it into an oxygen-rich air. Oxygen made the ozone layer, which let life spread onto land. So the biosphere is a cause of today's atmosphere, not only a result of it.
• The hydrosphere as a carbon sink (GS-I): Rain dissolved carbon dioxide, and the oceans stored it. This lowered the CO₂ in the early air and helped the Earth cool. The same process links to the oceans' role as a carbon sink in today's climate change debates.
• Ancient chemistry, modern resource (GS-I/III): About 2.4–2.0 billion years ago, oxygen reacted with dissolved iron. This laid down banded iron formations. These rocks hold India's 24,057 million tonnes of haematite (2020), mostly in the Odisha–Jharkhand–Chhattisgarh belt [2]. This explains why steel plants cluster in eastern and central India.
• Mining has costs (GS-III): The iron ore belts lie in forested, tribal regions such as Bailadila in Chhattisgarh. There are also dust problems in Ballari-Hosapete in Karnataka [2]. Old geology therefore shapes today's debates on environment and development.