Volcanic rocks (also called extrusive rocks) form when magma erupts onto the Earth's surface as lava and then cools and solidifies. This rapid cooling results in:
* Fine-grained texture: Individual crystals are small and often hard to see with the naked eye.
* Vesicular texture: Gas bubbles trapped in the lava during cooling form holes called vesicles.
* Often glassy: Rapid cooling sometimes doesn't allow crystals to form, resulting in a glassy texture.
Examples of volcanic rocks:
* Basalt
* Rhyolite
* Andesite
* Scoria
* Pumice
Plutonic rocks (also called intrusive rocks) form when magma cools and solidifies slowly beneath the Earth's surface. This slow cooling allows for:
* Coarse-grained texture: Large, visible crystals form due to the extended cooling time.
* No vesicles: The magma loses its gases before cooling, resulting in a solid, dense texture.
* No glassy texture: Crystals have ample time to form.
Examples of plutonic rocks:
* Granite
* Gabbro
* Diorite
Here's a table summarizing the key differences:
| Feature | Volcanic Rocks | Plutonic Rocks |
|---|---|---|
| Formation | Lava cools on the surface | Magma cools underground |
| Cooling Rate | Fast | Slow |
| Texture | Fine-grained, vesicular, sometimes glassy | Coarse-grained, solid |
| Examples | Basalt, Rhyolite, Andesite, Scoria, Pumice | Granite, Gabbro, Diorite |
It's important to note that the same chemical composition can result in both volcanic and plutonic rocks. For instance, basalt (volcanic) and gabbro (plutonic) share similar mineral compositions but differ in texture due to their cooling environments.
