1. Magma Formation:
* Magma is molten rock found beneath the Earth's surface.
* It forms when the Earth's mantle or crust melts due to:
* Heat from the Earth's core: The Earth's interior is incredibly hot, and this heat can melt rock.
* Pressure changes: When pressure decreases, rock can melt. This happens at plate boundaries where plates move apart.
* Water content: Water lowers the melting point of rocks, and the presence of water can cause melting.
* Heat from magma intrusions: Magma rising from deeper in the Earth can melt surrounding rocks.
2. Cooling and Solidification:
* Intrusive igneous rocks: Magma cools and solidifies slowly underground. This allows large crystals to form, resulting in coarse-grained rocks like granite.
* Extrusive igneous rocks: Lava, which is magma that erupts onto the Earth's surface, cools quickly. This rapid cooling results in small crystals, forming fine-grained rocks like basalt.
3. Classification of Igneous Rocks:
* Composition: The minerals present in igneous rocks depend on the chemical composition of the magma or lava. Common minerals include quartz, feldspar, mica, and pyroxene.
* Texture: The size and arrangement of crystals in igneous rocks are called texture.
* Coarse-grained: Large crystals, indicating slow cooling.
* Fine-grained: Small crystals, indicating rapid cooling.
* Porphyritic: A mixture of large and small crystals, indicating a change in cooling rate.
Examples of Igneous Rocks:
* Intrusive: Granite, Gabbro, Diorite
* Extrusive: Basalt, Rhyolite, Andesite
Key Takeaways:
* Igneous rocks form from the cooling and solidification of molten rock.
* The process can occur both underground (intrusive) and on the surface (extrusive).
* The texture and composition of igneous rocks depend on the cooling rate and the chemical composition of the magma or lava.
