Define carbonisation.

Explanation

**Step-by-Step Solution:**

1. **Understanding Carbonization**: Carbonization is a process that involves the transformation of organic material into carbon-rich substances.

2. **Source of Carbonization**: The organic material primarily comes from decomposed plant material, which contains carbon-based deposits.

3. **Conditions for Carbonization**: This transformation occurs under conditions of tremendous heat and pressure, typically found below the Earth's surface.

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