Tephra is any fragment of rock a volcano blasts into the air, regardless of size or composition. Geologists sort it into four classes by diameter — ash (under 2 mm), lapilli (2–64 mm), and bombs and blocks(over 64 mm). The word comes from the Greek for "ash," and the deposits it leaves behind are one of the most useful records we have of past eruptions.
Smallest (Ash)
< 2 mm
Largest (Bombs/Blocks)
> 64 mm
Ash Can Travel
Worldwide
Eruptions Tracked
11,079
By VolcanoDB Research Team. Data: Smithsonian Global Volcanism Program eruption database (11,079 eruptions), USGS Volcano Hazards Program.
What Is Tephra?
Tephra (pronounced TEF-ra) is the collective name for every solid fragment ejected during an explosive eruption and carried through the air, from microscopic ash to boulder-sized blocks. The key word is air: tephra is airborne. That's what separates it from a lava flow, which stays on the ground, and from a pyroclastic flow, which is a ground-hugging avalanche of gas and debris.
The term was popularized in the 1940s by Icelandic geologist Sigurður Þórarinsson, who realized that the ash layers stacked in Iceland's soil were a readable calendar of past eruptions. He borrowed the ancient Greek word téphra, meaning ash — the same root Aristotle used when he described volcanic fallout more than 2,000 years ago. Today "tephra" covers all sizes, while "ash" is reserved specifically for the fraction under 2 mm.
Tephra forms through fragmentation. As magma rises, dissolved gases — mostly water vapor and carbon dioxide — come out of solution and form bubbles. In sticky, silica-rich magma those bubbles can't escape fast enough, so pressure builds until the magma shatters into fragments and blasts skyward. Runny, gas-poor magma releases its pressure gently and tends to produce lava instead. That's why stratovolcanoes like Vesuvius and Pinatubo bury cities in tephra while shield volcanoes like Kīlauea mostly ooze lava.
The 4 Types of Tephra by Size
Volcanologists classify tephra purely by fragment diameter — not by what it's made of. The standard scale runs from ash through lapilli to bombs and blocks. Bombs and blocks share the same size bracket; the difference is whether the fragment was molten (bomb) or solid (block) when it left the vent.
Type
Size
Typical Travel Distance
Volcanic Ash
Less than 2 mm
Hundreds to thousands of km
Lapilli
2 – 64 mm
Kilometers to tens of km
Volcanic Bombs
Larger than 64 mm
Usually within 2–5 km of the vent
Volcanic Blocks
Larger than 64 mm
Usually within 2–5 km of the vent
Volcanic Ash
Less than 2 mm
The finest tephra — fragments smaller than a grain of rice. Ash is carried highest into the eruption column and drifts the farthest. The 1883 Krakatau ash circled the globe and reddened sunsets for years. It's the main aviation hazard because it stays airborne for days.
Lapilli
2 – 64 mm
From the Latin for 'little stones' — pebble-to-walnut-sized fragments. Lapilli build the classic cinder-cone shape and include rounded 'accretionary lapilli' that form when wet ash clumps in the plume. Most of what you crunch across on a scoria field is lapilli.
Volcanic Bombs
Larger than 64 mm
Ejected molten and still plastic, so they twist into aerodynamic shapes in flight — spindle, ribbon, and 'bread-crust' bombs that crack like a baguette as the outside cools faster than the gassy inside. A single bomb can weigh several tonnes and kill on impact.
Volcanic Blocks
Larger than 64 mm
The same size as bombs but ejected solid — chunks of old, cold rock ripped from the vent walls. Because they were already hard, they stay angular instead of streamlining. Blocks and bombs together make the deposit geologists call 'agglomerate.'
How Tephra Forms and Falls
In a big explosive eruption, tephra rides up inside the eruption column — a churning plume of hot gas and fragments that can punch more than 40 km into the stratosphere. The column sorts the tephra by weight as it rises and drifts. Heavy bombs and blocks fall out first, close to the vent. Lapilli rain down a bit farther. The featherweight ash rides the highest and travels with the wind, which is why an ashfall map is always stretched into a long plume downwind rather than a neat circle.
How much tephra an eruption makes comes down to two things: how much magma erupts, and how violently it fragments. Both are captured by the Volcanic Explosivity Index (VEI), a 0–8 scale based largely on erupted tephra volume. Each step up the VEI scale represents roughly a tenfold jump in volume. A VEI 3 eruption produces around 0.01 km³ of tephra; a VEI 7 like Tambora produces well over 100 km³.
Why Tephra Is Dangerous
Aviation:Fine ash melts inside jet engines and abrades windshields. It's the reason airspace closes during eruptions — see our volcanic ash guide for how the 9 ash advisory centers track it.
Roof collapse: Dry ash weighs a lot; wet ash can exceed 1,000 kg per cubic meter. Just a few centimeters of rain-soaked ash has collapsed roofs and killed people — this was the leading cause of death at Pinatubo in 1991.
Health: Ash irritates lungs and eyes, and fine particles with crystalline silica pose a long-term respiratory risk. It also contaminates open water supplies.
Ballistics: Bombs and blocks near the vent are simply lethal. Most fatalities on tourist volcanoes come from being struck by these, not from lava.
The Biggest Tephra Eruptions in Our Database
Our copy of the Smithsonian Global Volcanism Program catalog holds 11,079 eruptions. Of those, only 7 reached VEI 7 — the tephra-volume monsters — with another 52 at VEI 6 and 181 at VEI 5. The handful below are the ones whose tephra reshaped climate, history, or both. Click any name for its full eruption record.
The largest eruption in recorded history. Roughly 150 km³ of tephra — ash and pumice launched ~43 km into the stratosphere — triggered the global 'Year Without a Summer' in 1816. Crop failures reached New England and Europe.
The Minoan eruption blanketed the Aegean in tephra meters deep and may have crippled the Minoan civilization. Its ash layer is a keystone marker for dating the entire Late Bronze Age Mediterranean.
The Akahoya tephra from this submarine caldera south of Kyushu spread across Japan and is still used as a chronological marker. Kikai was confirmed to be recharging with fresh magma in 2026.
Ash rose 80 km and circled the Earth within two weeks, dimming sunlight and lowering global temperatures. The airborne tephra produced vivid red sunsets worldwide — possibly the sky in Munch's 'The Scream.'
The best-monitored large eruption in history. Ash mixed with Typhoon Yunya's rain, collapsing roofs across Luzon. Stratospheric aerosols cooled the planet ~0.5°C for two years.
The May 18 blast dropped measurable ash across 11 US states. Small by global standards, but it's the reference eruption Americans picture when they hear the word 'ashfall.'
See the full VEI record
Explore every tephra-producing eruption on our interactive map, or browse the largest by volume.
Here's the part most guides skip. A single large eruption drops a chemically unique ash layer over an enormous area in what, to geology, is an instant. Find that exact layer in a peat bog in Ireland, an ice core in Greenland, and a dig site in Turkey, and you've just linked all three to the same moment in time. That technique — tephrochronology — turns tephra into one of the most precise dating tools in earth science.
It's how archaeologists tie the collapse of Bronze Age settlements to the Santorini eruption, and how climate scientists sync ice cores from opposite poles. Þórarinsson invented the method to map Iceland's eruption history, and it now underpins the dating of everything from human migration to abrupt climate shifts.
Tephra vs Lava vs Magma: Quick Comparison
Term
Where it is
State
Magma
Underground
Molten rock still beneath the surface
Lava
On the ground
Molten rock flowing at the surface
Tephra
In the air
Fragmented rock ejected and airborne
The simplest way to remember it: magma is molten rock waiting underground, lava is that rock once it flows out, and tephra is that rock once it's blown into the sky. For the difference between the first two, see our magma vs lava explainer.
Frequently Asked Questions
What is the difference between tephra and lava?
Tephra is fragmented rock thrown into the air by an explosive eruption — it travels through the atmosphere before landing. Lava is molten rock that flows across the ground. The same magma can produce both: gas-rich, sticky magma shatters into tephra, while gas-poor, runny magma oozes out as lava. In short, tephra flies and lava flows.
How far can volcanic tephra travel?
It depends entirely on size. Volcanic ash (under 2 mm) can travel hundreds to thousands of kilometers and even circle the globe — Krakatau's 1883 ash reddened sunsets worldwide for years. Lapilli (2–64 mm) fall within tens of kilometers. Bombs and blocks (over 64 mm) are heavy and usually land within 2–5 km of the vent, though large explosions can throw them farther.
What is the most dangerous type of tephra?
Fine volcanic ash causes the most widespread harm. It abrades and stalls jet engines (the reason airspace closes during eruptions), collapses roofs when it piles up wet, contaminates water, and irritates lungs. Bombs and blocks are more lethal up close — a single one can weigh tonnes — but they affect only the immediate area near the vent.
Can tephra cause climate change?
Yes, when the eruption is big enough to inject sulfur and fine ash into the stratosphere. Tambora's 1815 tephra caused the 1816 'Year Without a Summer,' with June snow in New England and failed harvests across Europe. Pinatubo's 1991 eruption cooled global temperatures roughly 0.5°C for two years. The cooling comes mainly from sulfate aerosols, with fine ash adding a short-term dimming effect.
What is tephrochronology?
Tephrochronology is dating rock and sediment layers using tephra. Each large eruption lays down a chemically unique ash layer over a huge area in a geological instant, so finding that layer in a bog, ice core, or archaeological dig pins the surrounding material to a known date. The technique was pioneered by Icelandic geologist Sigurður Þórarinsson, who also popularized the word 'tephra' itself.