Large grains
Explanation
Intrusive igneous rocks, also known as plutonic rocks, form when magma cools and solidifies deep beneath the Earth's surface. The physical characteristics of these rocks are primarily determined by the rate at which the molten material cools.
Option Analysis
- Fine-grained structure is incorrect: A fine-grained structure (aphanitic texture) is characteristic of extrusive igneous rocks (volcanic rocks). These form when lava cools rapidly on the Earth's surface, preventing the formation of large crystals (e.g., Basalt).
- Large grains is correct: Intrusive rocks cool very slowly because they are insulated by the surrounding country rock deep underground. This slow cooling process allows mineral crystals sufficient time to grow, resulting in a coarse-grained or large-grained structure (phaneritic texture). A common example is Granite.
- Layered structure is incorrect: A layered or stratified structure is the defining characteristic of sedimentary rocks, which form through the deposition and cementation of sediments over time. Igneous rocks are generally massive and unstratified.
- Contains fossils is incorrect: Fossils are typically found in sedimentary rocks. The extremely high temperatures of the magma from which igneous rocks crystallize would destroy any organic remains or fossils.
Key Takeaway: The texture of igneous rocks depends on cooling time; intrusive rocks cool slowly underground yielding large grains, while extrusive rocks cool rapidly on the surface yielding fine grains.