Obsidian is a naturally occurring volcanic glass. Simply put, it is a dark natural glass formed by the cooling of molten lava.
Defining Obsidian
Based on geological classification, obsidian is an extrusive igneous rock. Its definition highlights key characteristics:
- Natural Glass: Unlike crystalline rocks, obsidian cools so rapidly from viscous lava that mineral crystals do not have time to grow. This results in a glass-like texture and appearance.
- Dark Color: Obsidian is typically black or very dark brown, though variations with hints of red, green, or even iridescence (known as 'rainbow obsidian') exist due to inclusions or specific mineral compositions. The term "dark" in the definition reflects its most common appearance.
- Formed by Cooling Lava: Its origin is intrinsically linked to volcanic activity. When felsic lava (rich in silica) erupts and cools very quickly, it can solidify into obsidian rather than forming crystalline rock types like granite or rhyolite.
Key Characteristics from the Definition
The core definition provides the essential properties of this material:
- Type: Natural Glass
- Color: Typically dark
- Origin: Volcanic (derived from molten lava)
- Formation Process: Rapid cooling
These points can be summarized in a simple table:
| Characteristic | Description |
|---|---|
| Material Type | Natural Glass |
| Typical Color | Dark |
| Source | Molten Lava |
| Formation | Rapid Cooling |
The Formation Process
The formation of obsidian involves molten rock, known as magma when underground and lava when erupted onto the surface. When this molten lava, particularly silica-rich types, cools extremely quickly – perhaps in contact with water or cool air – the atoms within it cannot arrange themselves into an ordered crystalline structure. Instead, they remain in a disordered state, characteristic of glass. This swift solidification process is crucial for obsidian to form.
Understanding obsidian means recognizing it as a volcanic product with a unique, non-crystalline structure, resulting in its characteristic dark, glassy appearance derived directly from the cooling of molten lava.
