Why do river stones sometimes crack and shoot pieces out of a campfire ring?
― Water inside the stone turns to steam and blows it apart from within
When you build a campfire, it's tempting to gather stones from nearby to make a ring around it. But if you use stones from near the water, they sometimes crack with a loud bang and send fragments flying. Stone doesn't burn, so why does it break? The culprit is invisible water soaked into the stone, combined with the temperature gap between its outside and inside.
It's an autumn camping trip. You gather some round river stones, arrange them in a ring, and light a fire in the middle. It even doubles as a stand for your cooking pot — not a bad setup.
After a while, you hear small "crack" and "hiss" sounds from the stones near the fire. Sometimes you can see a faint wisp of steam on the surface.
Then, suddenly: BANG. One of the stones splits, and a thumb-sized fragment flies out and lands at your feet. Stone isn't supposed to burn — so what just happened?
There are two main reasons stones crack
Stones contain gaps and cracks too fine to see. Stones from near water soak up plenty of it in those gaps. When heated by fire, that water turns to steam, trying to expand to more than a thousand times its original volume.
Stone conducts heat slowly. The face touching the fire heats up and expands first, while the inside stays cool. The part that wants to expand and the part that doesn't pull against each other, creating a force that tears the stone apart.
Both mechanisms break the stone "from the inside." And stone has a key weakness: it's strong under compression but very weak under tension. Both forces attack that same weak point at once.
Reason 1: water trapped inside becomes steam with nowhere to go
River stones have spent years soaking in water or getting rained on. Stones made of packed grains, like sandstone, are full of gaps and soak up water like a sponge. Even hard-looking stones like granite have fine cracks running along the boundaries between grains.
Water boils into steam at 100°C. At that point, the same amount of water expands to occupy roughly 1700 times its volume. Picture a cup of water becoming 1700 cupfuls of steam. In a pot, that steam simply escapes upward. But water deep inside a stone is trapped in narrow gaps.
Steam with no way out pushes on the surrounding stone from within. The higher the temperature climbs, the greater the pressure. At 200°C, it's said to reach around 15 times atmospheric pressure. Figure 1 (left) shows this. When the gap's exit is blocked, eventually the stone gives way and cracks. Those small "crack" and "hiss" sounds are thought to be steam jetting out through fine gaps.
Reason 2: stone breaks all too easily under tension
Almost everything expands slightly when heated. Stone is no exception. But stone conducts heat very slowly, so even once the fire-facing surface has heated up, a spot just a few centimetres in stays cool for a while.
The hot surface wants to expand. The cool core holds it back. The surface gets compressed, while just beneath it a tensile force builds up. As in the right side of Figure 1, a crack forms running parallel to the surface, and a slab-like layer can peel off and fly away.
Stone is very strong under compression — that's why it's used for building foundations and stone walls. But its strength under tension is said to be only about a tenth of its compressive strength. So even a temperature gap of around 100°C can generate enough force to crack it. It's the same mechanism as pouring cold water into a hot glass and having it shatter.
Reasons 1 and 2 also happen together. The more water a stone holds, the more fine cracks it tends to have too — and those cracks become the "crack tips" where tensile force concentrates. That's why stones from near water are especially risky.
The stones placed on a sauna stove, which get doused with water, are said to be a type that resists cracking even under sudden heating and cooling — typically fine-grained volcanic stone with few internal gaps. Even so, they gradually crack into small pieces with continued use, so they're routinely replaced.
So what should you do?
- Don't put stones from in or near water next to the fireThe wetter a stone was, or the more it was buried in damp sand, the more water it holds.
- Don't put stones into the fire or use them directly as a hearthA camp fire stand or a purpose-built fire pit is by far the safest option.
- If you hear a "crack" or "hiss," step back from the stonesDon't lean in close. Fragments fly fast and hot.
When a stone cracks, hot fragments can fly up to face height. Around a fire, avoid crouching down with your face close to it. If a fragment gets in your eye or causes a deep burn, don't rub it — cool it under running water and call emergency services or a medical facility right away. If a fire looks like it's spreading, don't try to fight it — prioritize getting away and evacuating. Also note that campfires on riverbanks are banned outright in many places, so check local rules first.
Summary
River stones crack in a fire because water trapped in their gaps turns into steam with nowhere to go and pushes outward from within, at the same time as the outside alone heats and expands, generating a force that tears the stone apart. Stone is strong under compression but weak under tension. Even if it looks dry on the outside, a stone from near water may be holding water within.
Even stone that can't burn can crack from fire.
The danger is the water and temperature gap hidden inside.
For how the heat of a campfire actually reaches your body, see Why does your face feel hot by a campfire while your back stays cold?. For why river stones are round in the first place, see this article.
- Pick up two or three different types of stone from a riverbank or garden, dry them thoroughly, and weigh them (a kitchen scale is fine).
- Submerge the stones in water for a full day. Take them out, wipe off only the surface water, and weigh them again.
- The weight gained is the water that soaked into the stone's gaps. Rougher-looking stones should absorb more.
If the absorbed water weighs 1 gram, that's equivalent to about 1.7 litres once turned to steam. Do not put these stones over a fire for this experiment.
Want to go deeper? ― Terms, formulas, and textbook linksEach section is labeled by level, from middle-school science to university specialist courses
- MSCovered in middle-school science
- HSCovered in high-school physics or earth science
- HS+Advanced high-school content, or textbook sidebar material
- UnivUniversity-level specialist content (materials science, rock mechanics) not taught in high school
- ResearchNot yet settled as textbook fact — an active research question
MSTerms: this phenomenon has names
- Phase change: a substance changing between solid, liquid, and gas — like water turning to steam. Volume changes dramatically.
- Thermal expansion: matter swelling slightly as temperature rises. The rate of swelling is called the "linear expansion coefficient."
- Thermal stress: a force generated inside an object because different parts expand by different amounts at different temperatures. Cracking from a sudden temperature change is called "thermal shock."
MSHSCheck with a formula: how much force builds up in a stone from a 100°C gap
Let's estimate the force (thermal stress) generated inside a stone when the surface's attempt to expand is fully constrained. We'll use a hard stone like granite as an example.
| In symbols | σ ＝ E × α × ΔT |
| In words | Thermal stress ＝ stone's stiffness (Young's modulus) × expansion per 1°C (linear expansion coefficient) × temperature gap |
| Where it comes from | The fraction of length the stone "wants" to expand from the temperature gap (α × ΔT), converted into a force via Hooke's law (force ＝ stiffness × fractional stretch) |
| Symbol | Meaning and unit |
| σ | Thermal stress. Force per square metre (unit: pascals; here, megapascals ＝ 1 million pascals) |
| E | Young's modulus. A measure of stiffness (unit: megapascals) |
| α | Linear expansion coefficient. Fractional length increase per 1°C rise (unit: 1/°C) |
| ΔT | Temperature gap between surface and core (unit: °C) |
| Young's modulus of granite (approx.) | said to be around 50000 MPa |
| Linear expansion coefficient of granite (approx.) | said to be around 0.000008 /°C |
| Temperature gap between surface and a few cm deep (example) | 100 °C |
| Tensile strength of stone (approx.) | said to be around 10 MPa |
| Force generated per 1°C gap | 50000 × 0.000008 ＝ 0.4 MPa |
| Force generated at a 100°C gap (thermal stress) | 0.4 × 100 ＝ 40 MPa |
| How many times the tensile strength | 40 ÷ 10 ＝ 4 times |
| Rough temperature gap where cracking begins | 10 ÷ 0.4 ＝ 25 °C |
This is an upper-bound estimate assuming full constraint, but the force from a 100°C gap is several times the stone's tensile strength. A real stone's shape and existing cracks let some force escape, so it won't necessarily snap right at a 25°C gap. Still, exposed to the several-hundred-degree heat of a campfire, the conditions for cracking are more than met.
| Volume of 1 gram of water | 1 cubic centimetre |
| Volume of 1 gram of steam at 100°C, 1 atm | said to be about 1700 cubic centimetres |
| Expansion factor | 1700 ÷ 1 ＝ 1700 times |
HSHS+The pressure of trapped steam
HSThat 1700-fold figure comes from plugging 1 gram of water (about 1/18th of a mole) and 100°C into the ideal gas law (pressure × volume ＝ moles × gas constant × temperature). When it's trapped in a gap and can't change volume, the pressure rises instead.
HS+In a closed space containing both water and steam, the pressure is set by that temperature's "saturated vapor pressure." Saturated vapor pressure rises sharply with temperature, a relationship described by the Clausius–Clapeyron equation. It's said to reach about 5 times atmospheric pressure at 150°C and about 15 times at 200°C.
UnivWhy is rock so weak under tension?
In materials science, force from a temperature gap is called "thermal stress," and fracture from a sudden temperature change is called "thermal shock fracture." Rock's tensile strength is far lower than its compressive strength because force concentrates at the tips of the countless microscopic cracks inside it — explained by Griffith's fracture theory. The effect of water pressure in gaps pushing rock apart is treated in rock mechanics as "pore water pressure," organized through the concept of effective stress. A common index of thermal-shock resistance is tensile strength divided by (Young's modulus × linear expansion coefficient) — the last row of section ② above (10 ÷ 0.4) is exactly this index.
📖 For the derivation and further reading: Thermal expansion and thermal stress (Japanese Wikipedia) ／ Vapor pressure (Japanese Wikipedia)
ResearchWhat's still not fully understood
- Which stones crack at which temperature. Even within the same type of rock, results vary widely depending on how cracks run and how much water is held, so predicting a given stone's cracking temperature in advance is said to be difficult.
- Which matters more, steam or the temperature gap. In a real rupture both act at once, so experiments to separate out how much each contributes are still an ongoing effort.
- The link to natural rock weathering. How far the same mechanism applies to rock spalling from wildfires or from day–night temperature swings in deserts is a topic still being studied in geomorphology.
In other words, this article too describes things "as currently understood." Judging which stones are safe by appearance alone remains difficult — that's the honest conclusion for now.
Links to textbook units (by level)
| Level | Subject / unit | Where it appears in this article |
|---|---|---|
| MS | Science, Year 1, "phase change" | Water turning to steam and greatly increasing in volume |
| HS | Physics "heat and gases," earth science "rocks" | Ideal gas law, thermal expansion, rock structure |
| HS+ | Physics (advanced) "saturated vapor pressure" | Pressure of trapped steam |
| Univ | Materials science / rock mechanics | Thermal stress, Griffith's fracture theory, pore water pressure |
| Research | Rock mechanics / geomorphology | Variability in cracking conditions, link to natural weathering |
| ― | Everyday connections | Choosing campfire stones, sudden temperature change in glassware |
- National Astronomical Observatory of Japan (ed.), *Rika Nenpyo* (Chronological Scientific Tables), Maruzen Publishing (rock properties, saturated vapor pressure of water)
- Umetaro Yamaguchi and Yuichi Nishimatsu, *Introduction to Rock Mechanics* (岩石力学入門), University of Tokyo Press
- Jaeger, J. C., Cook, N. G. W., Zimmerman, R. W. "Fundamentals of Rock Mechanics", 4th ed., Blackwell
- Wikipedia (Japanese), "Coefficient of thermal expansion" (thermal stress section)
※This article is a general-audience science explainer. The figures given are approximate estimates meant to aid understanding of the mechanism. Follow the instructions of local land managers, municipal authorities, and fire services regarding whether and how campfires are permitted.