Two harrowing incidents in the past two months have thrust the term “lava bomb” into the consciousness of casual volcano watchers around the state and across the country.

On May 19 a 57-year-old Puna man was struck by a lava bomb the size of a basketball as he stood on a third-floor balcony. The flaming projectile splintered his lower leg and knocked him onto a couch, which then caught fire.

Then, on July 16, another lava bomb, also estimated to be about the size of a basketball, smashed through the roof of a tour boat, injuring 23 people watching the lava flow from Kapoho Bay.

In reporting the incidents, local and national news organizations made liberal use of the inherently dramatic term for a mass of lava propelled from a volcano that hardens as it travels through the air: “Man’s leg shattered by ‘lava bomb’ spewed from Kilauea volcano stream” (Newsweek); “Lava bomb hits tourist boat in Hawaii, injuring 23” (New York Times); “23 injured after basketball-sized ‘lava bomb’ crashes through roof of Hawaiian tour boat” (Washington Post).

But what might sound like headline hyperbole is in fact part of the everyday lexicon of geologists and volcanologists.

What is a lava bomb?

By U.S. Geological Society definition, a lava bomb — more commonly called a “volcanic bomb” or simply “bomb,” is a semimolten blob of lava, measuring anywhere from 2-1/2 inches to 20 feet in diameter, projected from a volcano during active eruption. They are different from “blocks,” which are pieces of pre-existing rock propelled into the air by volcanic activity.

“Hawaii has a long history of lava bombs,” said Hawaiian Volcano Observatory geologist Janet Babb. “Basically, whenever you have molten lava that is propelled skyward, solidifies or partly solidifies, and comes back down, you can have lava bombs.”

Geologists distinguish different types of bombs by the shapes they form — spherical, ribbon, cowpie — during flight and upon impact. Spindle bombs take oblong, football-like form as they spin through the air. Cowpie bombs are formed when lava that is still relatively fluid splatters on the ground. Breadcrust bombs are so named for the cracked surface created as the lava hardens and dries.

Needless to say, lava bombs can be extremely dangerous. In 1993 six scientists and three other people were killed by falling bombs and other volcanic debris during an eruption at Galeras volcano in Colombia.

In the wake of the latest lava bomb incident on Hawaii island, the Coast Guard reinstituted the requirement that all boats remain at least 300 meters away from the lava flow at all time.

Previously, selected commercial and research vessels were allowed to approach within 50 meters.