What are the three types of tephra, listed from smallest to largest?

The correct sequence of tephra types, organized from smallest to largest, is indeed ash, lapilli, and blocks. Tephra refers to the fragments of volcanic rock and ash that are ejected during an explosive volcanic eruption.

Ash consists of fine particles that are less than 2 mm in diameter, allowing it to be carried by the wind over long distances. Lapilli, which means "little stones" in Italian, are larger than ash, ranging from 2 mm to 64 mm in diameter. They can resemble small pebbles and often fall to the ground closer to the volcanic vent. Blocks, on the other hand, are the largest type of tephra, typically more than 64 mm in size, and consist of solid rock that is expelled during eruptions.

Understanding the size classification of tephra helps in assessing the impact of volcanic eruptions on the environment, as well as their potential hazards. This classification also aids in the study of volcanic activity and eruption dynamics.