Blocks Blocks Blocks are erupted fragments of solid rock that measure more than 256 mm in maximum dimension. There are two types of blocks. Accessory blocks are formed from rocks from previous eruptions of the same volcano. Accidental blocks are rocks torn from the subvolcanic crust or mantle.
Blocks are commonly angular and roughly eqidimensional.
In 1924, a lava lake in Halemaumau Crater drained to a very deep level. The removal of the lava allowed groundwater to seep toward the crater and contact hot rock. The 1924 blocks shown is this photo can still be seen along Crater Rim Drive near the crater. Photograph by T.J. Takahashi, U.S. Geological Survey, March 25, 1990. Accessory blocks composed of trachyte make part of the tephra at the Puu Waawaa cone on Hualalai volcano.