A volcano erupts when hot magma from beneath the surface of the Earth breaks through the crust. Some of it comes out as lava, hot ash, gasses, pyroclastic flows, and even chunks of rock that rain down around the volcano. Any chunk of rock spewed out of a volcano that’s larger than 65 mm in diameter (2.5 inches) is considered to be a volcanic bomb, or lava bomb.
These volcanic bombs can be large, and they can be thrown tremendous distances away from the volcanic vent. In the 1935 eruption of Mount Asama in Japan, bombs measuring 5 meters in diameter were thrown 600 meters from the vent. And a volcano in Columbia killed 6 people near the summit with an eruption of volcanic bombs.
There are several different kinds of bombs that can occur, depending on the type of lava, and the force of the eruption:
- Bread crust bombs have a cracked surface. The outer shell of the bomb cools, but the hot gasses inside are still expanding and crack the outer layer of the rock.
- Core bombs is a chunk of lava that has cooled around some kind of solid core, like a piece of basalt. If you slice open the bomb, you’ll see the harder core, surrounded by porous pumice.
- Cow pie bombs look like a big cow patty. They aren’t fully hardened when they hit the ground, and flatten out on impact.
- Explosion bombs have a break in their side where hot gas inside the bomb blasted out.
- Fusiform bombs are elongated and streamlined, forming their shape as they harden in flight
- Ribbon bombs are long skinny strips of lava, thrown from the volcano, which then break when they impact the ground.
- Slag bombs look like smelter slag, and have a very glassy, porous exterior.
- Teardrop bombs look like rocky raindrops. They blast out of the volcano as liquid, and then solidify into a raindrop shape.
We have written many articles about volcanoes for Universe Today. Here’s an article about volcanoes for kids, and here’s an article about volcanic blocks; another type of rock that can be ejected from a volcano.
Here’s a great article that explains each of the different kinds of volcanic bombs in more detail. And here are some large volcanic bombs that landed around Mount Lassen.
We have recorded a whole episode of Astronomy Cast just about volcanoes. Listen to it here: Episode 141: Volcanoes, Hot and Cold.