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This lesson explores the composition and characteristics of the Earth's early atmosphere in greater detail, as required by AQA GCSE Chemistry specification (5.9.1). You need to understand what gases were present, where they came from, and why the early atmosphere was so different from the one we have today. This lesson builds on the overview of atmospheric history and focuses specifically on the first phase — the period dominated by volcanic outgassing.
When the Earth first formed about 4.6 billion years ago, it was an extremely hot, molten mass. As the surface began to cool, a thin solid crust formed. Beneath this crust, the mantle remained hot and partially molten, driving intense volcanic activity. These volcanoes released vast quantities of gases in a process called outgassing (or degassing).
The early atmosphere was formed almost entirely from volcanic emissions. There was no life on Earth at this stage, and the conditions were extremely hostile — the surface temperature was very high, and the atmosphere contained no free oxygen.
Exam Tip: "Outgassing" is the key term for the release of gases from volcanoes that formed the early atmosphere. If you see this word in an exam question, it is asking about volcanic emissions.
The early atmosphere is thought to have consisted mainly of the following gases:
| Gas | Approximate Proportion | Source |
|---|---|---|
| Carbon dioxide (CO₂) | Very high (possibly 50–95%) | Volcanic outgassing |
| Water vapour (H₂O) | Very high | Volcanic outgassing |
| Nitrogen (N₂) | Small but increasing proportion | Volcanic outgassing; breakdown of ammonia |
| Methane (CH₄) | Small amounts | Volcanic outgassing |
| Ammonia (NH₃) | Small amounts | Volcanic outgassing |
| Oxygen (O₂) | None or trace | No photosynthetic organisms existed |
| Sulfur dioxide (SO₂) | Small amounts | Volcanic outgassing |
| Hydrogen (H₂) | Small amounts | Volcanic outgassing; may have escaped into space |
Exam Tip: When comparing the early atmosphere to today's atmosphere, the two biggest differences are: (1) the early atmosphere had far more CO₂, and (2) there was no oxygen. These are the two most important points to make in any comparison question.
Volcanic outgassing is the process by which gases trapped in the interior of the Earth are released to the surface through volcanic eruptions. This was the primary source of atmospheric gases for the first billion years of Earth's history.
Modern volcanoes provide a model for understanding ancient volcanism. The typical composition of volcanic gases today is:
| Gas | Typical Proportion of Volcanic Emissions |
|---|---|
| Water vapour | 60–90% |
| Carbon dioxide | 5–25% |
| Sulfur dioxide | 1–10% |
| Nitrogen | 1–5% |
| Hydrogen | 0.5–2% |
| Carbon monoxide | Trace |
| Hydrogen sulfide | Trace |
| Hydrogen chloride | Trace |
The exact mixture varies between volcanoes and even between eruptions of the same volcano. However, the dominance of water vapour and carbon dioxide is consistent.
graph TD
A["Volcanic Outgassing"] --> B["Water Vapour (H₂O)"]
A --> C["Carbon Dioxide (CO₂)"]
A --> D["Nitrogen (N₂)"]
A --> E["Sulfur Dioxide (SO₂)"]
A --> F["Ammonia (NH₃)"]
A --> G["Methane (CH₄)"]
B --> H["Condensed to form Oceans"]
C --> I["Dominated early atmosphere"]
D --> J["Accumulated over time"]
style A fill:#c0392b,color:#fff
style H fill:#2980b9,color:#fff
style I fill:#e67e22,color:#fff
style J fill:#27ae60,color:#fff
One of the most significant events in early Earth history was the formation of the oceans. As the Earth gradually cooled over millions of years, the temperature eventually dropped below 100°C at the surface. At this point, the vast quantities of water vapour in the atmosphere began to condense and fall as rain. This torrential rain continued for millions of years and filled the low-lying basins on the Earth's surface, forming the first oceans.
The formation of the oceans had a profound effect on the atmosphere:
Exam Tip: The formation of the oceans is the first major mechanism by which carbon dioxide was removed from the atmosphere. Make sure you can explain this clearly — water vapour condensed, CO₂ dissolved in the oceans, and marine organisms used it to make calcium carbonate shells.
Carbon dioxide was the most significant gas in the early atmosphere (by volume, excluding water vapour). Its gradual removal over billions of years is one of the most important stories in atmospheric chemistry.
Carbon dioxide was removed from the early atmosphere by several processes:
| Process | How It Removes CO₂ |
|---|---|
| Dissolving in oceans | CO₂ is soluble in water, especially cold water. As oceans formed, vast quantities of CO₂ dissolved. |
| Photosynthesis | Photosynthetic organisms (first cyanobacteria, then algae and plants) absorbed CO₂ and released O₂. |
| Formation of sedimentary rocks | Marine organisms used dissolved CO₂ / carbonates to build shells (CaCO₃). These organisms died and their shells accumulated on the ocean floor, eventually forming limestone and chalk. |
| Formation of fossil fuels | Dead organisms (plants and marine creatures) were buried and compressed over millions of years, trapping carbon in coal, oil, and natural gas. |
The removal of carbon dioxide from the atmosphere and its storage in rocks and fossil fuels is part of the long-term carbon cycle. This carbon remained locked away for hundreds of millions of years — until humans began burning fossil fuels, releasing it back into the atmosphere.
Understanding the early Earth's atmosphere is aided by comparing it with the present-day atmospheres of Mars and Venus:
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