Volcanic Bombs and Tephra: The Fragments Volcanoes Throw
When a volcano erupts explosively, it does not only produce flowing lava. It blasts fragments of rock and magma into the air, ranging from the finest dust to blocks the size of a car. These airborne fragments, known collectively as tephra, build cones, blanket the landscape, and pose serious hazards. Understanding the different sizes and types of tephra is key to understanding how explosive volcanoes work.
What tephra is
Tephra is the general term for all fragments of rock and solidified magma ejected into the air by a volcanic eruption, regardless of size. It is classified mainly by the size of its particles, from the finest ash through lapilli to large blocks and bombs. The mix of sizes a volcano produces reveals much about the violence and style of its eruption.
Volcanic ash
The finest tephra is volcanic ash, made of tiny fragments of rock, mineral, and volcanic glass less than two millimetres across. Despite its name, it is not soft like fire ash but abrasive and hard. Ash can be carried high into the atmosphere and drift for hundreds or thousands of kilometres, posing hazards to aviation, health, agriculture, and infrastructure far from the volcano.
Lapilli
Lapilli, from the Latin for little stones, are tephra fragments between about two and sixty-four millimetres across. They include small pumice and scoria pebbles thrown out during eruptions. Lapilli often fall close to the vent, accumulating to build the slopes of cones, and their rounded or angular shapes record how they formed and cooled in flight.
Volcanic bombs
The largest fragments ejected while still molten are volcanic bombs, blobs of lava larger than about sixty-four millimetres that take on distinctive shapes as they fly and cool. Spindle bombs, twisted by their flight through the air, and bread-crust bombs, cracked on the surface like a loaf, are among the recognisable forms. Bombs can be hurled hundreds of metres or more from the vent and are a serious hazard near active craters.
Blocks
Blocks are large, angular fragments of solid rock, distinguished from bombs by being already solid when ejected rather than molten. They are torn from the volcano's existing rock by the force of an explosion and hurled outward. Along with bombs, these ballistic projectiles are a major danger to anyone near an erupting vent, as the 2014 tragedy at Mount Ontake in Japan showed.
How tephra builds cones
The accumulation of tephra around a vent is what builds cinder cones and contributes to the slopes of larger volcanoes. Lapilli and bombs pile up at the angle of repose, forming the steep, symmetrical shapes characteristic of scoria cones. The layering of tephra deposits records successive eruptions, allowing geologists to reconstruct a volcano's history.
Reading the deposits
Layers of tephra are invaluable to scientists. Because a single eruption can spread a distinctive ash layer over a wide area, these deposits act as time markers that can be used to date events in geology and archaeology. The thickness, distribution, and composition of tephra layers also reveal the size, style, and direction of past eruptions.
Explore on the map
The cones, ash layers, and tephra deposits of explosive volcanoes are found throughout the world's volcanic regions. Explore these volcanoes on the interactive map — filter by type to see the scoria cones and explosive volcanoes whose eruptions have filled the air with tephra and shaped the land around them.