Which type of igneous rock forms small crystals due to the quick cooling of magma at the Earth's surface?

Extrusive igneous rocks are formed from magma that cools and solidifies at or near the Earth's surface. The rapid cooling process associated with these rocks causes the formation of small crystals, as there is insufficient time for larger crystals to grow. This is in contrast to intrusive igneous rocks, which cool slowly beneath the Earth's surface, allowing for the development of larger crystals.

The formation of extrusive igneous rocks typically occurs during volcanic eruptions, where lava flows spread and cool relatively quickly upon exposure to the air or water. Common examples of extrusive igneous rocks include basalt and rhyolite, which display a fine-grained texture due to this rapid cooling. Understanding the formation process of these rocks is crucial in geology, as it helps in interpreting geological history and volcanic activity.