Lapilli vs. Tuff — What's the Difference?

Lapilli are classified based on their size, falling between ash and volcanic bombs in the spectrum of materials ejected by volcanoes. These granular materials are often angular or rounded, indicating their molten or semi-molten state upon ejection. On the other hand, tuff is characterized not by individual particles but by its composition as a rock. It forms when volcanic ash and smaller pyroclastic materials settle and then undergo compaction and cementation, transforming into a solid rock.

The formation process of lapilli involves the cooling and solidification of lava or magma once it is thrown into the air during an eruption. These particles can cool quickly, maintaining a glassy texture, or they can cool more slowly, developing a crystalline texture. Whereas, tuff's formation is a more prolonged process that involves the accumulation of volcanic ash layers that are subsequently buried, compacted, and solidified into rock over time.

Lapilli can vary widely in composition, reflecting the chemistry of the magma from which they originated. This variety can include basaltic, andesitic, or rhyolitic lapilli, each type indicating the nature of the volcanic activity and the magma chamber's characteristics. In contrast, tuff, while also variable in composition, primarily reflects the composition of the ash and materials from which it is formed, including the presence of lapilli-sized fragments within the ash matrix.

The environment of deposition also differs significantly between the two. Lapilli are typically found in proximity to their source volcano, deposited by fallout from explosive eruptions. They often form layers that can be traced back to specific eruptions. Tuff, however, can be found in more extensive deposits, as the fine ash can be carried over longer distances by wind or water before settling and eventually solidifying into rock.

Despite their differences, both lapilli and tuff play crucial roles in understanding volcanic processes and the history of volcanic eruptions. Studying lapilli can provide insights into the conditions of the eruption, such as temperature and magma viscosity, while examining tuff layers can offer a broader perspective on the eruption's extent and the distribution of volcanic ash.