What is a primary characteristic of volcanic rocks?

Volcanic rocks are primarily formed from the rapid cooling and solidification of lava that erupts from a volcano. When lava cools quickly, such as during an explosive volcanic eruption, gases that are dissolved in the magma can become trapped as the rock solidifies, leading to the formation of air bubbles, or vesicles, within the rock. This characteristic is particularly notable in rocks like pumice, which can be very light and porous due to these air-filled cavities.

In contrast, volcanic rocks generally do not form beneath the Earth's surface; that characteristic is more typical of intrusive igneous rocks, which cool slowly underground. While volcanic rocks can exhibit a crystalline structure, it is not as consistent or pronounced as in some other rock types, such as granite or metamorphic rocks, which can feature larger, more developed crystals. Furthermore, volcanic rocks are not formed by sedimentation; this process is characteristic of sedimentary rocks, which form from the accumulation of sediments over time. Therefore, the presence of air bubbles is a defining characteristic of volcanic rocks, making it the correct response.