Very fine-grained structure
Explanation
Igneous rocks are classified into two main types based on where the molten magma solidifies: Intrusive (Plutonic) and Extrusive (Volcanic). Extrusive igneous rocks form when magma erupts onto the Earth's surface as lava and cools.
Option Analysis
- Large grains is incorrect: Large grains are characteristic of Intrusive Igneous Rocks (e.g., Granite). These rocks form deep within the Earth's crust where magma cools slowly, allowing sufficient time for large crystals to develop.
- Very fine-grained structure is correct: Extrusive igneous rocks form from lava that cools rapidly upon exposure to the atmosphere or water. This rapid cooling rate prevents the growth of large crystals, resulting in a very fine-grained or even glassy structure (e.g., Basalt, Obsidian).
- Formed deep inside the crust is incorrect: Rocks formed deep inside the crust are classified as Intrusive Igneous Rocks. Extrusive rocks are formed on the surface of the crust.
- Contains fossils is incorrect: Igneous rocks originate from molten magma at extremely high temperatures, which destroys any organic remains. Consequently, fossils are typically found in Sedimentary rocks, not Igneous rocks.
Key Takeaway
Extrusive Igneous Rocks are characterized by a fine-grained texture due to the rapid cooling of lava on the Earth's surface, distinguishing them from the coarse-grained texture of Intrusive Igneous Rocks.