Lava bombs are dramatic fragments of molten rock ejected during explosive volcanic events. These projectiles represent a significant category of tephra, which is the general term for all fragmented materials thrown into the air by an eruption. The nature of these fragments, from their size and composition to their shape, offers volcanologists specific clues about the eruption’s intensity and the magma’s properties. Understanding the mechanics of a lava bomb’s formation and its subsequent trajectory is a key part of assessing volcanic hazards.
Defining the Lava Bomb
The defining characteristic of a lava bomb is that the material must be semi-molten or “plastic” when launched from the vent, solidifying into rock as it travels through the air. This distinguishes a bomb from a volcanic block, which is a fragment of solid, pre-existing rock also ejected by the explosion.
A lava bomb must be greater than 64 millimeters (about 2.5 inches) in diameter. Any fragments smaller than this limit are classified as lapilli, while even finer material is known as volcanic ash. Lava bombs can be extremely large, with some recorded specimens reaching up to six meters (20 feet) across. The composition of these bombs is typically basaltic or mafic, indicating a lower-viscosity magma source.
The Process of Formation and Ejection
Lava bombs are products of mildly to moderately explosive eruptions, most commonly associated with the Strombolian and Vulcanian types. In a Strombolian eruption, large gas bubbles, sometimes called “gas slugs,” rise through the magma column and burst near the surface, violently flinging clots of viscous lava into the air.
The physics of the ejection is ballistic, meaning the lava fragments follow a predictable parabolic path once launched. The height and distance traveled depend on the initial velocity and angle of the launch, with some bombs capable of being thrown for several kilometers. During this high-speed flight, the exterior of the molten blob begins to cool rapidly due to exposure to the cooler air.
Classifying Lava Bomb Shapes
The solidified shape of a lava bomb is directly influenced by the magma’s viscosity and the bomb’s rotation during its flight.
- Spindle or Fusiform bombs: Formed when highly fluid lava is ejected and spun, resulting in an elongated, almond-shaped projectile with twisted ends.
- Ribbon bombs: Result when highly fluid magma is ejected as irregular strings and blobs that break up into segments.
- Breadcrust bombs: Form when a thick outer crust solidifies quickly while the interior remains molten and expands. This expansion causes the rigid exterior to crack, creating a surface texture that resembles a loaf of bread.
- Cow-dung or Pancake bombs: Form when highly plastic or molten lava hits the ground and splatters or flattens upon impact.
Hazards and Impact Zones
Lava bombs pose a hazard due to their high velocity, mass, and intense heat. The primary danger comes from the immense kinetic energy upon impact, which can cause severe physical trauma or structural damage to buildings.
Beyond the physical impact, the fragments are incandescent when they land, posing a severe burn risk and the potential for igniting fires. The impact zones for lava bombs are localized to the areas immediately surrounding the volcanic vent, making them a short-range, localized threat.