1. Magma Rises:
* Molten rock, called magma, originates deep within the Earth's crust or mantle.
* Due to its lower density, the magma rises through cracks or volcanic vents.
2. Eruption and Lava Flow:
* When the magma reaches the surface, it erupts as lava.
* The lava flows outwards, cooling and solidifying as it goes.
3. Rapid Cooling and Fine Grains:
* Cooling on the surface happens much faster than inside the Earth.
* This rapid cooling leads to the formation of small, fine-grained crystals.
4. Formation of Extrusive Rocks:
* The solidified lava forms extrusive igneous rocks, which are typically characterized by their:
* Fine-grained texture: Due to rapid cooling, crystals have less time to grow large.
* Porphyritic texture: Some rocks might have larger crystals embedded in the finer-grained matrix, indicating a two-stage cooling process.
* Vesicular texture: Trapped gas bubbles within the lava can form holes or cavities in the rock.
Examples of Extrusive Igneous Rocks:
* Basalt: A dark-colored, fine-grained rock common in volcanic eruptions.
* Rhyolite: A light-colored, fine-grained rock formed from felsic lava.
* Andesite: An intermediate-colored, fine-grained rock with a composition between basalt and rhyolite.
* Scoria: A dark, porous volcanic rock containing gas bubbles.
* Pumice: A very light, porous rock formed from frothy lava.
Key Takeaway: The key difference between extrusive and intrusive igneous rocks is the location of cooling. Extrusive rocks cool on the Earth's surface, while intrusive rocks cool underground.
