Definition: Rocks are hard, naturally occurring substances composed of minerals.
Minerals: Typically crystalline substances formed by geological processes. Crystalline structure means atoms are arranged in a definite manner, though some minerals are non-crystalline.
Classification of Rocks: Based on the geological processes by which they are formed.
Major Classes of Rocks: Igneous, Sedimentary, Metamorphic
Igneous Rocks
Formation: Formed when molten rock (magma) from deep within the Earth's crust cools and hardens.
Characteristics: Do not occur in layers, mostly crystalline.
Intrusive Igneous Rocks
Formation: Magma cools and hardens within the Earth's crust.
Features: Slow cooling allows large crystals to form, making them coarse-grained.
Examples: Granite, Gabbro
Visibility: Large crystals visible to the naked eye; reach surface when overlying rocks erode.
Extrusive Igneous Rocks
Formation: Lava (magma that reaches Earth’s surface) cools and hardens quickly.
Features: Fine-grained due to rapid cooling preventing large crystal formation.
Examples: Basalt, Obsidian (volcanic glass)
Sedimentary Rocks
Formation: Formed from material deposited by agents like rivers, compacted over time.
Typical Environments: Often form in water bodies like seas.
Characteristics: Non-crystalline but may contain fossils, visible layering.
Types of Sedimentary Rocks
Mechanically Formed (Clastic)
Origin: Made from pieces of other rocks damaged by weathering and erosion.