Extrusive igneous rocks
Explanation
Igneous rocks are formed from the cooling and solidification of molten rock. They are primarily classified based on their place of origin and the rate at which the molten material cools. The distinction lies between magma cooling beneath the surface and lava cooling upon the surface.
Option Analysis
- Intrusive igneous rocks is incorrect: Intrusive igneous rocks (also known as Plutonic rocks) form when magma cools and solidifies deep within the Earth's crust. Due to the insulation provided by the surrounding rocks, the cooling process is slow, allowing large crystals to form (e.g., Granite).
- Extrusive igneous rocks is correct: Extrusive igneous rocks (also known as Volcanic rocks) form when molten lava erupts onto the Earth's surface. Exposure to the atmosphere or water causes the lava to cool and solidify rapidly. This rapid cooling prevents the growth of large crystals, resulting in a fine-grained or glassy texture (e.g., Basalt).
- Sedimentary rocks is incorrect: Sedimentary rocks are formed through the accumulation, compaction, and cementation of sediments (weathered fragments of other rocks) or organic matter over geological time, not by the cooling of molten lava.
- Metamorphic rocks is incorrect: Metamorphic rocks are formed when existing rocks (igneous, sedimentary, or metamorphic) undergo physical or chemical changes due to intense heat and pressure, without melting into magma.
Key Takeaway
Extrusive igneous rocks are characterized by rapid cooling on the surface and a fine-grained texture, whereas intrusive igneous rocks are characterized by slow cooling beneath the surface and a coarse-grained texture.