Igneous rocks are the type of rock that cools while inside the Earth.
Igneous rocks form when magma, or melted rock from inside the Earth, cools and hardens. This process can occur either deep within the Earth's crust or on its surface, leading to two distinct types of igneous rocks:
Intrusive Igneous Rocks
When magma cools slowly while still deep inside the Earth's crust, it forms intrusive igneous rocks (also known as plutonic rocks). The slow cooling process, insulated by the surrounding rock, allows for the formation of large, easily visible crystals within the rock structure. Over vast spans of geological time, these rocks can become exposed at the surface as the overlying layers wear away due to erosion.
- Formation Environment: Deep within the Earth's crust.
- Cooling Rate: Slow, due to insulation.
- Crystal Characteristics: Large, well-formed crystals.
- Texture: Coarse-grained.
- Common Examples:
- Granite: A widely recognized intrusive igneous rock, often light-colored and rich in quartz and feldspar.
- Diorite: Medium-colored, with a composition between granite and gabbro.
- Gabbro: A dark, coarse-grained rock, rich in magnesium and iron.
Extrusive Igneous Rocks (For Comparison)
In contrast, when magma erupts onto the Earth's surface (becoming lava) and cools rapidly, it forms extrusive igneous rocks (also known as volcanic rocks). The quick cooling prevents large crystals from forming, resulting in rocks with very fine grains or even a glassy texture.
Key Differences Between Igneous Rock Types
The cooling location and rate are crucial in determining the final characteristics of an igneous rock.
| Feature | Intrusive Igneous Rocks | Extrusive Igneous Rocks |
|---|---|---|
| Cooling Location | Inside the Earth (Earth's crust) | On or near the Earth's surface |
| Cooling Speed | Slow | Rapid |
| Crystal Size | Large (macroscopic) | Small or microscopic (fine-grained/glassy) |
| Texture | Coarse-grained | Fine-grained or glassy |
| Example Rocks | Granite, Diorite, Gabbro | Basalt, Obsidian, Pumice |
For further exploration of how these rocks form and their characteristics, you can learn more about igneous rocks.
