A volcanic block is a solid piece of rock that has been expelled from a volcano during an eruption. The term covers rocks of various sizes, but the minimum size is 2.5 in (64 mm) in diameter. Volcanic eruptions can be immensely powerful, and for this reason, a volcanic block can be very large with some measuring 16 feet (5 m) or more in diameter. They are sometimes confused with another type of volcanic ejecta, the volcanic bomb, which is a blob of material ejected from an eruption in a molten state rather than as a solid rock. During violent eruptions, some volcanoes can throw volcanic blocks as far as several miles away.
The type of rock that can make up a volcanic block can vary widely depending on the volcano and the region of the world where it is found. Many volcanic blocks are simply pieces of the surrounding rocks that are broken and ejected by the violence of the volcanic eruption. These rocks can be of almost any type and composition. More commonly, however, these rocks are part of the volcano itself and can be part of old magma tunnels, hardened lava from older eruptions, or sections of the wall of the main vent, which is the primary point of the ejection of magma from within the volcano.
It is important to understand the difference between a volcanic block and a volcanic bomb. A volcanic block is a rock that existed before being ejected by a volcanic eruption while a volcanic bomb is a rock that is ejected from the volcano in a molten state. Volcanic bombs most often partially or completely solidify before striking the ground sometimes causing them to be confused with volcanic blocks.
Geologists and vulcanologists use several means to differentiate volcanic bombs from volcanic blocks, but shape is the simplest method. A typical volcanic bomb tends to have an elongated shape and may have fairly smooth surfaces while a volcanic block tends to have angular surfaces that are characteristic of shattered or fractured rocks. The basic shape of a volcanic block is more spherical than a volcanic bomb as well, although this is merely a tendency, and volcanic blocks with other shapes are not uncommon. Scientists study volcanic blocks and their dispersion to learn about volcanoes, their eruptions, and their history.