lapilli

Lapilli are small volcanic rock fragments that range in size from 2 to 64 millimeters in diameter. The term originates from the Latin word for "little stones" and represents a specific size classification within volcanic pyroclastic materials. These fragments are larger than volcanic ash but smaller than volcanic bombs or blocks.

These pyroclastic particles are typically formed during explosive volcanic eruptions when magma is violently fragmented and ejected into the atmosphere. The rapid cooling of molten rock droplets in the air results in the formation of lapilli, which then fall to the ground due to gravity. The shape of lapilli can vary considerably, ranging from spherical and ellipsoidal forms to highly irregular shapes depending on the eruption conditions and cooling rates.

Lapilli can be composed of various volcanic materials including basalt, andesite, or rhyolite, depending on the composition of the parent magma. Some lapilli form as solid fragments during explosive fragmentation, while others begin as molten droplets that partially or completely solidify during flight. Accretionary lapilli represent a special type formed when ash particles stick together in the eruption column.

These volcanic fragments are commonly found in pyroclastic deposits around active and dormant volcanoes worldwide. They play an important role in volcanic hazard assessment as they can cause significant damage to structures and pose risks to human safety during eruptions. Lapilli deposits are also studied by volcanologists to understand eruption dynamics and volcanic history.

The accumulation of lapilli can form distinctive geological layers that provide valuable information about past volcanic activity. These deposits are often well-preserved and can be used to reconstruct ancient eruption sequences and determine the explosivity of historical volcanic events.