Extrusive rock, or volcanic rock, forms from magma that erupts onto the Earth’s surface and cools rapidly, resulting in fine-grained textures. Examples include basalt, andesite, and rhyolite. Extrusive rocks are important in the study of volcanology, plate tectonics, and the interpretation of past volcanic activity.
Reference: Winter, J. D. (2010). “Principles of Igneous and Metamorphic Petrology.” Pearson.