Lava or volcanic bombs are rounded to ellipsoid masses of lava measuring at least 2.5 inches ejected during a volcanic eruption. They get their aerodynamic shapes in flight and most cool before landing, but some land before they fully solidify.
Recent examples of these lava bombs include the 2018 Kilauea eruption in Hawaii and the 2021 La Palma Volcano Eruption in Spain. There are many other examples.
Learn more about the meaning or definition of these bombs, their form, and common types. We will also give you the hazards they cause, how far they travel, and more.
What is a volcanic bomb?
Lava or volcanic bombs refer to the molten or semi-molten, rounded, ellipsoidal, or aerodynamically streamlined mass of lava more than 2.5 inches (64 mm) in diameter thrown into the air during an explosive volcanic eruption.
When these incandescent, plasticky lava clots or viscous lava fragments are less than an inch in size, they are known as achneliths. Unfortunately, the International Union of Geological Sciences, IUGS) doesn’t recognize the term yet.
Tephra (fragments produced during an explosive eruption) will be thrown into the air during a volcanic eruption. It may include ash (<2mm), lapilli (2-64 mm), angular and blocks (> 64 cm), mineral crystals, etc. However, you will have a bomb if pyroclasts (individual fragments) are at least 64 cm and rounded or ellipsoid.
If the bomb is so highly molten that it lands on the surface while still molten, you have a volcanic spatter. Spatters will weld together, forming spatter cones, solid rocks, or feed lava flows. They are common on fire fountains, especially of weak basaltic magma explosions, and get deposited or land near the vents.
Also, volcanic bombs form extrusive rocks. Why? Because solidification occurs after magma has left the volcano’s vent. An accumulation of mainly volcanic bombs forms an agglomerate.
Lastly, the difference between volcanic bombs and blocks is that blocks are angular and solids at the ejection, while bombs are rounded and molten to partially molten when blasted into the air.
What are the volcanic bombs made of?
Volcanic bombs are made mainly of molten to semi-solid juvenile clasts (cognate or essential clasts) formed from magma involved in a volcanic eruption. The exact composition depends on the kind of magma involved, but most form from basaltic to intermediate magma (andesitic). Nonetheless, you can have silicic ones.
Also, these bombs may have crystals or minerals (coarse-grained, plutonic) formed deep in the Earth’s crust as the magma rose in a finer-grained or glassy matrix. Sometimes, you can find phenocrysts in a finer-grained or glassy matrix. If the phenocryst is in a glassy matrix, you have vitrophyre, a porphyritic texture.
The other common composition of lava bombs is accidental clasts or xenoliths (or xenocrysts if individual crystals) torn from vent walls. These accidental clasts from country rock can be from metamorphic, plutonic, or sedimentary rocks.
More on lava bombs
How far do how far can volcanic bombs travel? Volcanic bombs don’t travel so far, i.e., they fall near the vent but can go up to 2000 feet or more. For instance, a conference paper (Galindo et al., 2013) observes that 1-ton lava bombs can go beyond 1341 feet (409 m), while 28-ton bombs have reached 931 feet (248 m) in Caldera Quemada de Arriba.
Also in Japan, an eruption in Mt. Asama threw bombs measuring 16-24 feet (5-6 m) in diameter up to 2000 feet (600 m) from the vent.
- Rock type: Pyroclastic, an extrusive, igneous rocks
- Size: Typical sizes are 1-2 feet, with some as large as 20 feet in diameter or larger.
- Texture or surface appearance: Smooth, cracked, fine-grained to glassy. Some may have embedded accidental clasts, phenocrysts, or mineral crystals.
- Colors: Their colors vary but are mostly brown, reddish, or gray. Weathering may make some turn brownish or yellowing.
- Shape: Rounded, ellipsoidal, spindle, ropy, ribbon, discus-shaped, spatter, ameboid, folded, ragged, cauliform, etc.
How do volcanic bombs form?
Volcanic bombs originate or form during a mildly explosive volcanic eruption where wholly or partially molted lava masses or clots more than 64 mm in diameter are through into the air.
The smooth surface and rounding or aerodynamic shapes form during flight since the blasted lava is still plastic. It confirms some deformation during flight. However, some rounding may occur later via sedimentary reworking.
Most land when they have solidified. The rapid cooling results in a glassy texture in some of these bombs. However, some, like the Cowdung bombs, land while still partially molten or plastic.
A less popular theory for their formation notes that the various shapes of lava bombs form during the ejection of magma from the volcano vent. It further notes that any change during the flight in the air is negligible.
Most of these bombs form basaltic (or mafic) to intermediate lava with lower viscosity, and cooling occurs during flight. However, some may form from silicic magma and hit the ground while still plastic or semi-plastic.
Lastly, the volcanoes that produce bombs are Strombolian, Vulcanian, and Hawaiian. Examples are Stromboli volcano, Italy (Vulcanian), Soufriere Hills in Montserrat (Hawaiian), Kilauea, and Mauna Loa, Hawaii (Strombolian). Another example is the Anak Krakatau eruption in Indonesia (2008), with bombs seen at night.
12 Types of volcanic bombs
There are several types of volcanic bombs, with the exact kind depending on magma or lava viscosity, flight duration, and cooling rate. Also, expanding gas vesicles and deformation impact contribute to the type formed.
The kind and intensity of the eruption may also influence the type of bomb formed. For instance, ribbon, Cowdung, and spindle bombs form during low-intensity or Strombolian eruptions that create cinder cones. Also, some peralkaline rhyolite and low-viscosity basalts may form spindle and ribbon bombs.
Lastly, the naming mainly tells you about their shape and sometimes appearance. Here are the common lava bomb types:
1. Bread crust bombs
These are the most common volcanic bombs. They have cracked or fractured surfaces resembling the French bread’s crust or baguette that form mostly after the bomb lands.
It forms when the outer shell or exterior surface of an ejected lava clot solidifies fast during flight while the inner part is still molten, expanding, or with expanding gases. The hot gases will cause cracks on the outer surface as they escape. Also, the impact on landing may cause a fractured appearance.
Bread crust bombs are common in silicic or andesitic to intermediate magma composition. Basaltic magma will have a glassy surface with little cracked surface or fine cracks due to stretching the thin glassy surface, while the interior is plastic upon impact. Also, the internal pressure of residual gas in basaltic magma is low, minimizing cracks’ formation.
These bombs have a fine-grained crust with coarse crystals or country rock fragments.
2. Core bombs
These bombs have a rind of lava surrounding a nucleus solid genetically related (previously solidified magma even from the same eruption) or accidental fragment (xenolith). Their composition is diverse and forms when lava cools around a solid core.
If you cut it, you will notice a unique solid core surrounded by lava. For instance, you could have a piece of basalt surrounded by pumice.
3. Armored bombs
They form when stem-rich, wet ash is plastered on a solid fragment, such as an accidental rock fragment or a previously created fragment from magma.
4. Cowdung, pancake, cow pie bombs
These volcanic bombs form when ejected lava hits the ground while still molten (not fully hardened), making it flatten or splash out on impact. They appear like round disks resembling a cow patty or down and form from very fluid lava.
5. Cylindrical or ribbon bombs
Ribbon bombs are elongated skinny or ropy-shaped bombs formed when a volcano ejects blobs or strings of moderately to highly fluid lava. During the flight, they stretch up to 12 inches and break into smaller ribbon-like fragments on impact with the ground.
These bombs have flattened to circular cross-sections and stretched or tubular vesicles. Also, they are fluted along their length.
3. Spherical bombs
They are spherical shaped and form from moderately to highly fluid magma. What pulls into the spherical shape is the surface tension of lava.
7. Spindle, fusiform, rotation, or almond bombs
They have an almond, football, or spindle-thread shape with twisted ends. They form like spherical bombs, but molten lava twisting, rotating, or spinning makes them stretch or elongate during flight.
Spindle bombs show longitudinal fluting, and their underside during the fall is smoother and broader than the upper side. Also, they commonly form in acidic to intermediate lava, rarely in silicic.
Lastly, while the formation or tearing of lava ribbons happens during the flight, they may form during impact.
8. Slag bombs
This type resembles slag from a smelting process; you could easily mistake them. However, you will know them because they have a glassy and porous surface.
9. Teardrop bombs
These kinds have a shape that resembles a teardrop or raindrop. They form when a volcano blasts a liquid lava clot. During the flight, this lava clot will form a raindrop and solidify.
10. Cannon bomb
It is a smooth, rounded lump common at the base of valleys. They form when bombs hit the ground and roll, bouncing down the valley with a whirring nose. Their corners become sheared and eroded making them rounder.
11. Explosion bombs
These types will have a broken side where hot gases blasted out during the flight. They form when ejected lava clots have hot gas inside.
12. Cauliflower bombs
They have shapes like cauliflower, hence the name. Their interior is dense to slightly vesicular. It forms via quick quenching in a hydrovolcanic eruption.
Hazards associated with volcanic bombs
Volcanic bombs can cause damage to man-made structures, cause fires, or cause human and life loss or injuries. However, since they don’t fall far away from the vent, they will not cause much damage or are not hazardous like pyroclastic flow.
In 1993, a volcanic bomb from Galeras volcano (Colombia) killed six people near the summit at the time of the unexpected eruption and injured several others.
Also, in July 2018, during the lower Puna eruption of Kīlauea volcano, a volcanic bomb about the size of a basketball hit a tour boat, injuring 23 people touring the nearby Kilauea volcano in Hawaii. Early in May, a man, Darryl Clinton, was hit by a lava bomb while trying to protect his home in the Puna district.
A recent example is the 2021 La Palma Volcano Eruption, Canary Islands in Spain, which fell from the summit of the La Palma cone in late October 2021, causing damage to man-made structures nearby. Here is the video of a lava bomb rolling from the la Palma volcano eruption.
Lastly, while some may explode while in flight, most of the damage will be the fires, which start on impact or as they roll and hop.
References
- Fisher, R. V., & Schmincke, H.-U. (1984). Pyroclastic rocks. Springer-Vlg.
- Selley, R. C., M., C. L. R., & Plimer, I. R. (2005). Pyroclastics. In Encyclopedia of geology (1st ed.). Elsevier.
- Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.
- Bonewitz, R. (2012). Rocks and minerals (1st ed.). DK Publishing.
- Volcanic bomb. (2022, October 27). In Wikipedia. https://en.wikipedia.org/w/index.php?title=Volcanic_bomb&oldid=1113766368
- Galindo, I., Romero, M., Sánchez, N., Dóniz, J., Temiño, J. Y., Morales, J. M., & Carretero, L. (2013, April). Morphology and distribution of volcanic bombs in Caldera Quemada de Arriba (Lanzarote, Canary Islands): Implications for volcanic hazard analysis: Semantic scholar. https://www.semanticscholar.org/paper/Morphology-and-distribution-of-volcanic-bombs-in-de-Galindo-Romero/c9c5f1d731c699a238c2e99ddc3131ddebc998b1