What Is a Pyroclastic Rock?

A pyroclastic rock is a type of rock composed of compacted fragments of volcanic materials. Often the result of an explosive volcanic eruption, pyroclastic rocks can be giant or quite small, and may amalgamate with non-volcanic rocks as they travel. The shape and size of a pyroclastic rock can tell much about its origin, as well as explain how the rock traveled to its resting place. The combination of volcanic gases and exposure to heat and air may cause some types of pyroclastic rock to take on a bubbled appearance, leading to unusual density levels.

When a volcano explodes, fragments of volcanic material, called tephra, are thrown into the air or borne away on powerful flows of gas and magma. In a highly explosive eruption, fragments may remain extremely small, creating a fine powder known as volcanic ash. Eruptions that are somewhat less violent may allow volcanic rock to remain in larger chunks, or coalesce into sizable fragments as it cools. These distinct pieces of volcanic material make up the categories of pyroclastic rock.

The size of pyroclastic rocks can vary extensively, and helps vulcanologists define their distinct characteristics. Smaller rocks, known as lapilli, are between 0.7- 2.5 inches (2 and 64 mm) in diameter, and resemble gravel. Anything larger than this size is known as a volcanic bomb or block, depending on its shape. Some volcanic bombs and blocks can reach more than 16 ft (4.8 m) in diameter, and may weight thousands of pounds.

In addition to a variety in size, pyroclastic rocks have many distinct shapes. In lapilli pyroclastic rock, fragments may be tear-drop shaped, spherical, or drawn into stringy filaments. Volcanic bombs have streamlined shapes and often are covered with a ribbon-like pattern, a result of remaining semi-molten when expelled from the volcano. If a large pyroclastic rock is ejected after fully coalescing, however, it has the angular, bulky shape known as a volcanic block.

As lava mixes with tephra during a volcanic explosion, the sudden loss of heat and pressure can create an fascinating type of pyroclastic rock known as pumice or scoria. As the rock rapidly cools, gas bubbles may become trapped within the rock, creating a porous internal and external structure, leaving both scoria and pumice with an unusual, bubbled surface. The simplest way to tell pumice from scoria is by dropping a sample into a bucket of water; pumice, which has thinner walls and more internal bubbles, possesses such a low density level that it will actually float on the surface of water.