Figure 4. The rock shown in this photo also consists of layers of volcanic debris, but one of these layers contains peculiar round balls called accretionary lapilli. Close examination of the outcrop with a magnifying lens reveals that some of these little mud balls are internally layered like an onion. It is thought that accretionary lapilli form when an eruption spews a cloud of very finely pulverized dust into the atmosphere. Then, if rain clouds develop, the dust particles become stuck together and grow as they circulate within the cloud, much like a hailstone grows. When the wet mud balls grow big enough, they rain down to the ground, making a layer of mud balls like the one seen in this photo.
Last updated on October 6, 2005