Why does lightning turn beach sand into glass?
― A tube shaped like a bolt of lightning is left under the ground
When lightning hits sandy ground, it can leave a long, thin glass tube underneath. It is called a "fulgurite". The bolt's path is hotter than the surface of the Sun, and it melts sand in an instant. Then it cools so fast that the sand cannot settle back into grains, and it sets as glass.
Have you ever watched lightning flash far off over the sea on a summer evening? After the flash, there is a pause, and then the rumble reaches you.
Now suppose that bolt strikes an empty beach. Dig there the next morning, and you may find a thin grey tube that branches like a tree root. The outside is rough. The inside is smooth and shiny.
This is not stone. It is glass. The raw material of window glass is also sand. In less time than a blink, lightning has done the job of a factory furnace.
There are just two reasons
A lightning channel is said to reach about 30,000 °C. Quartz, the main ingredient of sand, melts at about 1700 °C. When electricity passes through sand, the sand along its path melts all at once.
Molten sand quickly loses its heat to the cold sand around it. If it cools slowly, its atoms can line up in neat order again. If it cools suddenly, it sets before they can. That is glass.
So a fulgurite is a natural creation that appears only when two things come together: "extremely hot" and "extremely fast". Let's take them in turn.
Lightning's electricity turns into heat as it travels through sand
Dry sand conducts electricity very poorly. Force a huge current through something that conducts poorly, and its path heats up. It is the same principle that makes the nichrome wire in an electric heater glow red.
A lightning current is said to reach tens of thousands of amperes. That is over a thousand times the current that trips a household breaker. This current branches as it moves through the sand, and the sand melts along its route. That is why a fulgurite looks like a bolt of lightning printed into the ground.
The tube is hollow because air and moisture in the gaps between the grains are heated suddenly and expand. The expanding gas presses the molten sand outward, and it sets in that shape. The inside is smooth because it is the molten surface that cooled and hardened. On the outside, partly melted grains stick on, so it is rough.
In Figure 1, the left side is a cross-section of the ground. The right side is the tube cut across. Use the slider to make the melted region wider.
Glass and quartz have the same ingredients. So what is different?
Most sand grains are a crystal called quartz. A crystal is a solid whose atoms are lined up in regular order, like tiles. When it melts, that order breaks down and the material flows.
If it then cools slowly, the atoms gradually find their old places and line up neatly again. But in a fulgurite, cold sand lies right next to the molten sand. The heat is stripped away at once, and the atoms are frozen in place before they can re-order. A solid that sets in a jumble like this is glass (Figure 2).
In fact, window glass is made on the same principle. Sand is melted in a furnace and cooled fast enough that it cannot return to a crystal. Lightning does the same thing in the wild, with no preparation at all.
Charles Darwin, known for the theory of evolution, recorded seeing fulgurites in sand dunes in Uruguay, South America, during the voyage of the Beagle. Several glass tubes ran vertically through the sand. They look plain, but to people long ago they were strange objects, a sort of "fossil of lightning".
Fulgurites contain phosphorus compounds that are almost never found in ordinary rock. Some research suggests that on the early Earth, lightning may have turned phosphorus, an ingredient of living things, into a more usable form.
The kind of open, sandy ground where fulgurites form is a place where a standing person can easily become the bolt's path. If you hear a rumble, get inside a building or a car at once. Stay away from tall trees and poles too. If someone is struck, call emergency services (119 in Japan) right away. A person struck by lightning does not carry an electric charge, so check that the surroundings are safe and then give first aid.
Summary
A lightning channel is about 30,000 °C. Quartz, the main ingredient of sand, melts at about 1700 °C, so the sand along the path melts in an instant. The cold sand around it then cools it so fast that the atoms cannot re-order, and it sets as glass. The hollow inside is the mark left by expanding air and steam pushing outward.
A fulgurite is a "moment recorded" by lightning in sand.
Being too hot and too fast, both together, is what makes glass.
To find out how much energy one lightning bolt holds, read "How much household electricity could one lightning bolt supply?". For how lightning turns air into fertiliser, read "Is it true that years with more lightning mean a better rice harvest?".
- Take a little sand from a beach or park and look at it with a magnifying glass. The clear, angular grains are quartz crystals.
- With an adult, boil down 3 tablespoons of sugar and 1 teaspoon of water in a small pan. When it turns golden, drip a little at a time onto aluminium foil to cool it suddenly. It becomes clear, hard toffee. This is sugar glass.
- For comparison, put sugar water on a small dish and let it dry over a few days. You will see small white grains (crystals) appear. Even with the same sugar, how it cools and sets changes its look completely.
Boiled sugar is far above 100 °C. To avoid burns, let an adult handle the pan, and do not touch anything until it has cooled.
For those who want more ― terms, formulas and links to textbooksEach part is labelled by level, from middle school science to university courses
- Middle schoolCovered in middle school science
- High schoolCovered in high school physics or chemistry
- High school+Advanced high school material, or a textbook sidebar
- UniversityNot taught in high school; university courses (materials science, earth science)
- ResearchNot yet settled even at university level; what researchers are studying now
Middle schoolTerms: this phenomenon has names
- Fulgurite: a glass tube or lump formed when lightning strikes sand or rock and melts it along the path, which then sets. It is sometimes called a "lightning tube stone".
- Quartz: a crystal of silicon dioxide. It makes up most sand grains, and large clear crystals are called rock crystal.
- Glass (amorphous): a solid that set without its atoms lining up in order. Unlike a crystal, it does not form grains of a fixed shape.
- Joule heating: heat produced when an electric current flows through something that conducts poorly.
Middle schoolHigh schoolChecking with a formula: how much heat does 1 m of fulgurite need?
Treat the melted region as a cylinder of radius 1 cm and length 1 m, and calculate the heat needed to melt that sand. The symbols mean the following.
| Q | Heat needed (unit: kilojoules, kJ) |
| m | Mass of sand melted (unit: kg) |
| c | Specific heat: heat needed to raise 1 kg by 1 °C (kJ/(kg・°C)) |
| ΔT | Temperature rise (°C) |
| L | Latent heat of fusion: heat needed to turn a solid into a liquid at its melting temperature (kJ/kg) |
| In symbols | Q ＝ m × ( c × ΔT ＋ L )、 m ＝ π × r² × length × density |
| In words | Heat needed ＝ mass of sand ×（heat to warm it to melting ＋ heat to melt it） |
| Where it comes from | Conservation of energy. The lightning's electricity turns into heat in the sand, and that heat is spent on "raising the temperature" and on "turning solid into liquid". It is a heat balance. |
| Radius 1 cm, squared (r²) | 0.0001 m² |
| Density of dry sand | about 1600 kg/m³ |
| Specific heat of quartz (rough average at high temperature) | about 0.8 kJ/(kg・°C) |
| Temperature rise (about 20 °C to about 1720 °C) | about 1700 °C |
| Latent heat of fusion of quartz (rough guide) | said to be about 150 kJ/kg |
| Energy of one lightning bolt (rough guide) | said to be about 1 billion joules ＝ 1000000 kJ |
| Cross-section area (π × r²) | 3.14 × 0.0001 ＝ 0.000314 m² |
| Mass of sand in 1 m length | 0.000314 × 1600 ≒ 0.5 kg |
| Heat to warm 1 kg to 1720 °C (c × ΔT) | 0.8 × 1700 ＝ 1360 kJ |
| Add the heat to melt it (＋ L) | 1360 ＋ 150 ＝ 1510 kJ |
| Heat needed for 1 m length, Q | 1510 × 0.5 ＝ 755 kJ |
| How many metres one bolt could melt, in theory | 1000000 ÷ 755 ≒ 1325 m |
The heat needed for 1 m of fulgurite is about 760,000 joules. That is about what a 600 W microwave uses in a little over 20 minutes. The energy of one bolt is, on paper, enough to melt more than 1 km. Real fulgurites are still only tens of centimetres to a few metres long, because most of a bolt's energy heats the air and escapes as light and sound. Only a small part is thought to reach the ground and melt sand.
High schoolHigh school+How current becomes heat, and why fast cooling gives no crystal
High schoolWhen a current I flows through something with resistance R, it gives off I² × R of heat every second (Joule's law). Because the effect goes with the square of the current, a lightning current of tens of thousands of amperes produces enormous heat even in a tiny time. Dry sand has high resistance and heats more easily than wet sand. Even so, the current picks its way through damp patches and gaps between grains, so the tube branches.
High school+Molten silicon dioxide is very sticky, so its atoms take a long time to move and re-order. It therefore does not crystallise easily even if cooled less quickly than window glass, which makes it a substance that readily stays as glass in nature too. The glass in a fulgurite is almost pure silicon dioxide glass, called lechatelierite.
UniversityThe glass transition, and reduction by lightning
As a liquid cools, its stickiness rises sharply near a certain temperature and the arrangement of atoms is frozen in. This is called the glass transition, and that temperature is the glass transition point. If the cooling rate beats the speed of crystallisation, the liquid becomes an amorphous solid instead of a crystal. Also, a lightning path is at several thousand °C or more, an environment where reactions that strip away oxygen (reduction) happen easily. This is thought to be why grains of metallic iron and phosphides (such as schreibersite), both rare at the surface, are found in fulgurites. The heat flows can be estimated with Joule's law and the latent heat of fusion.
📖 Derivations and more: Joule heating (Wikipedia, in Japanese) ／ Latent heat of fusion (Wikipedia, in Japanese)
ResearchWhat is still not clear
- Did lightning help life begin? In 2021, an estimate was published that on the early Earth, lightning turned a considerable amount of phosphorus into a "usable form" each year, based on phosphides in fulgurites. How important this was is still debated.
- How long, and how hot, was it inside the sand? Some studies work backward from fulgurite minerals to the peak temperature, but results vary greatly from place to place, and how conditions changed during a single strike is poorly understood.
- Can old fulgurites tell us how much lightning there was long ago? There are attempts to count fulgurites preserved in rock layers to explore the link between past climate and lightning.
In other words, this article too is "an explanation within what is known today". Read the numbers as rough guides that vary greatly with conditions.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science Year 1 "Changes of state", Year 2 "Current and heat" | Sand melts and sets / current gives off heat |
| High school | Physics "Conservation of heat, Joule's law" | Checking with a formula / how heating works |
| High school+ | Chemistry "Crystalline and amorphous solids" | The difference between quartz and glass |
| University | Materials science, geochemistry | Glass transition / reduction reactions |
| Research | Planetary science, origin of life | Lightning changing phosphorus |
| ― | Links to daily life | How window glass is made / shelter in a storm |
- Fulgurite - Wikipedia (閃電岩, in Japanese)
- Hess, B. L., Piazolo, S., Harvey, J. (2021) Lightning strikes as a major facilitator of prebiotic phosphorus reduction on early Earth. Nature Communications 12, 1535.
- Charles Darwin, The Voyage of the Beagle (record of fulgurite observations)
- Joule heating - Wikipedia (ジュール熱, in Japanese)
- Latent heat of fusion - Wikipedia (融解熱, in Japanese)
*This article is a general-audience science explainer. The numbers given are rough guides and estimates to help you understand how things work. In a thunderstorm, follow weather information and instructions from the fire service and local authorities.