ASBOG Practice Exam 2025 – Comprehensive All-in-One Guide to Ace Your Geology Licensure!

Igneous rocks are categorized based on their formation process, which primarily distinguishes them into two main types: intrusive (plutonic) and extrusive (volcanic) rocks.

Intrusive igneous rocks are formed from magma that cools and solidifies beneath the Earth's surface, resulting in a coarse-grained texture due to the slower cooling rate that allows larger crystals to grow. Granite is a classic example of an intrusive rock. In contrast, extrusive igneous rocks form when lava cools and solidifies quickly on the Earth's surface, leading to a fine-grained texture with smaller crystals. Basalt is a typical extrusive rock formed through this rapid cooling process.

Understanding this classification is vital for geologists and earth scientists, as it helps in interpreting geological history and processes related to volcanic activity, tectonics, and the Earth's crust development. The other options, while relevant to geology, do not accurately represent the two main types of igneous rocks.