Lapilli are rock fragments and particles violently ejected from a volcano during an explosive eruption. The term comes from the Latin word lapillus, meaning “little stone,” which describes these small, solidified pieces of erupted matter. These fragments are part of the broader group known as tephra, which includes all airborne material that falls back to the ground. Studying lapilli provides volcanologists with a direct window into the intensity and chemical processes occurring deep within a volcano.
Defining Characteristics and Size Classification
Lapilli are defined by their size, ranging in diameter from 2 millimeters up to 64 millimeters. This size range is equivalent to fragments roughly the size of a pea up to a small walnut, making them easily identifiable in volcanic deposits.
This metric boundary places lapilli between the finer volcanic ash (particles smaller than 2 millimeters) and the coarser volcanic blocks and bombs (larger than 64 millimeters). This precise size classification allows geologists to consistently categorize and compare deposits from different eruptions worldwide. Lapilli can be irregular and angular from violent fragmentation, or they may exhibit spherical, teardrop, or button-like shapes if they were ejected as semi-molten droplets that cooled during flight.
Mechanisms of Formation and Types of Lapilli
The formation processes for lapilli are diverse, resulting in different types that provide clues about specific eruption dynamics. The presence and shape of these various lapilli types allow scientists to reconstruct the violence, magmatic composition, and water involvement of the ancient eruption.
Juvenile Lapilli
Juvenile lapilli are formed directly from fresh, liquid magma that is rapidly fragmented by expanding gas during the eruption. These fragments cool quickly as they travel through the air. They often form vesicular materials like pumice (if the magma was silica-rich) or scoria (if it was iron and magnesium-rich).
Accretionary Lapilli
A distinct type is accretionary lapilli, which forms through a process similar to how hailstones develop. These are spherical aggregates of fine volcanic ash particles that clump together around a nucleus within a moist eruption cloud. Water vapor, often from external sources like groundwater, causes the sticky ash to build up in concentric layers before falling to the ground.
Lithic Lapilli
Lithic lapilli consist of fragments of older, solidified rock torn from the walls of the volcano’s conduit or the surrounding country rock. They are sometimes called accidental lapilli because they are not formed from new magma. This tearing occurs during the explosive ascent of magma.
Geological Deposits and Importance
Once deposited, lapilli accumulate to form distinct geological structures. When layers of lapilli and finer ash become compressed and cemented over time, they lithify into a rock known as a lapilli tuff. A rock is classified as a lapilli tuff when it contains 25% to 75% lapilli fragments within a matrix of finer ash.
If the deposit is dominated by lapilli (containing more than 75% of the fragments), it is classified as a lapillistone. The arrangement of these deposits, often observed as layers called lapilli beds, provides a chronological record of the eruption. Thicker, poorly sorted beds might indicate a single, intense event, while thinner, well-sorted layers can point to multiple, less violent events.
Analyzing the size and sorting patterns within lapilli beds helps volcanologists determine the eruption column height, the direction of prevailing winds, and the proximity to the volcanic vent. If the lapilli were deposited while still hot and semi-molten, the weight of the overlying material can cause them to flatten and fuse together, forming a dense rock called a welded tuff. This fossil record of explosive volcanism is used to assess regional volcanic hazards and reconstruct the geological evolution of landscapes.