This block was ejected into the air by an explosion caused by the collapse of an active lava delta at Kilauea Volcano, Hawai'i. New land built by lava often slides into the ocean, which enables seawater to mix with lava and hot rocks. Such violent mixing may trigger steam explosions that can blast hot rocks 10 to 50 cm in diameter more than 50 m inland. Credit: C. Heliker / U.S. Geological Survey.
A volcanic block is a solid rock fragment greater than 64 millimeters in diameter that was ejected from a volcano during an explosive eruption. Blocks commonly consist of solidified pieces of old lava flows that were part of a volcano's cone.