In the sequence of pyroclastic materials, which is the smallest?

The smallest pyroclastic material in the sequence is ash or dust. Ash consists of fine particles that are ejected during volcanic eruptions and can be less than 2 mm in diameter, while dust refers to even finer particles. Due to their small size, ash and dust can be carried over long distances by wind and are often a significant component of volcanic plumes.

In contrast, the other options refer to larger pyroclastic materials. Lapilli or cinder are typically between 2 mm to 64 mm in diameter. Blocks and bombs are even larger, often exceeding 64 mm; blocks are solid fragments of erupted rock while bombs are molten or semi-molten material that solidifies into a streamlined shape during flight through the air. Hence, ash or dust is universally recognized as the smallest among these various types of pyroclastic materials.