What is the primary characteristic of extrusive rocks?

Extrusive rocks are primarily characterized by their ability to cool quickly on the Earth's surface. This rapid cooling occurs when magma erupts from a volcano and interacts with the cooler temperatures of the atmosphere or water. As a result, the minerals do not have sufficient time to grow large crystals, which contributes to the fine-grained texture commonly found in these rocks.

The other characteristics mentioned in the question do not accurately describe extrusive rocks. For instance, rocks formed underground are known as intrusive or plutonic rocks, which cool slowly and develop larger crystals. Fossils are typically not found in extrusive rocks as they cool too quickly for organic materials to be preserved within the solidifying lava. Similarly, extrusive rocks tend to have small crystal sizes due to the rapid cooling process, unlike the larger crystals formed in intrusive rocks. Thus, the swift cooling process is the defining feature of extrusive rocks.