Impact Craters in Flour: Why the Moon Looks Like That

Dust a tray of flour with cocoa powder, drop a marble into it from a metre up, and what you pull out of the tray is startlingly moon-like: a round bowl, a raised rim, and pale rays spraying outward across the dark surface. The rays are the part that makes people stop. Then you do the measuring, and this impact crater experiment delivers something genuinely counterintuitive. Drop the marble from twice as high, doubling the energy, and the crater gets only about a fifth wider. Energy goes up a lot and the hole barely notices.

Key Takeaways

• You’ll drop marbles into deep flour dusted with cocoa from four measured heights and record crater diameters.
• It teaches that crater size scales as roughly the fourth root of impact energy, and that ejecta rays record the impact direction.
• Best for ages 6 to 12, outdoors or on a sheet.
• The usual failure is flour that is too shallow or too packed. You need at least 4 cm, loose and sifted.
• Marbles are a choking hazard for under-3s, and airborne flour dust is flammable near a flame.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Deep tray or roasting dish	1, at least 25 cm across and 6 cm deep	Kitchen	A plastic storage box lid, or a cardboard box lined with a bin bag
Plain flour	about 1 kg	Supermarket. Contains wheat, an allergen	Cornflour works and gives sharper rims. Fine dry sand works outdoors
Cocoa powder or drinking chocolate	3 tablespoons	Baking cupboard	Ground cinnamon, paprika, or powdered drink mix. Anything dark and fine
Marbles or ball bearings	3 of different sizes	Toy box or hardware shop	Dried chickpeas, small pebbles, a steel nut
Tape measure and a ruler	1 each	—	A metre of string with marks at 25, 50, 100 and 200 cm
Sieve or tea strainer	1	Kitchen	Sprinkle the cocoa through your fingers, slowly
Old sheet or newspaper	1	—	Do it on grass outside and skip the sheet

Time: prep 10 min, experiment 25 min, cleanup 10 min Cost: a bag of flour Mess level: do this outside

Do the Experiment

Step 1 — Build the target properly

Pour flour into the tray to a depth of at least 4 cm and level it with a ruler held flat, without pressing it down. Loose, deep flour is what lets a crater form. A thin layer bottoms out and gives you a dent, not a crater. Sift a thin, even layer of cocoa over the top until the surface is uniformly dark.

Step 2 — Take the prediction

Ask: “If I drop this marble from 1 metre and then from 2 metres, how much bigger will the second crater be? Twice as wide? Half as wide again?” Write the answer down. Almost everyone guesses far too much.

Step 3 — Drop from 25 cm and measure

Hold the marble at exactly 25 cm above the surface and let it fall. Do not throw it. Lift it out carefully with a spoon, then measure the crater’s diameter rim to rim with the ruler.

What you should see: a circular bowl with a raised rim, and thin pale rays of flour thrown out across the cocoa in a radial pattern. Those are ejecta rays, and they are the same feature you see around the lunar crater Tycho through binoculars.

Step 4 — Repeat at 50, 100 and 200 cm

Re-level and re-dust the surface between every drop, in a fresh part of the tray if there is room. Record every diameter. Do each height twice.

What you should see: the craters grow, but far less than the heights do. A typical run might give 3.0 cm at 25 cm, 3.6 cm at 50 cm, 4.3 cm at 100 cm and 5.1 cm at 200 cm. Each doubling of height adds roughly a fifth to the width.

Step 5 — Change the marble, not the height

From a fixed 100 cm, drop a small marble, a large marble and a steel nut of similar size but much greater mass.

What you should see: mass matters as much as height, because both feed into kinetic energy. A heavy projectile from 100 cm beats a light one from 200 cm. The crater does not care what hit it, only how much energy arrived.

Step 6 — Throw one in at a low angle

From the side, roll or flick a marble in so it strikes at perhaps 20 degrees to the surface.

What you should see: the crater is still essentially round, but the ejecta is lopsided, with most of the flour thrown downrange. This is the key observation. Real craters stay circular even for very oblique impacts, and it is the ejecta pattern, not the crater shape, that records the direction of arrival.

What’s Actually Going On in the Impact Crater Experiment

An impact is an energy-delivery event. A falling marble arrives with kinetic energy equal to its mass times gravity times the drop height, and almost all of that energy is dumped into the flour in a few thousandths of a second. The flour cannot get out of the way fast enough, so it is pushed down and outward in a roughly spherical pulse, which is why the hole is round even when the marble came in sideways.

The size of the hole is set by a competition. The impact energy is trying to excavate material; gravity and the target’s own strength are trying to hold it in place. For craters big enough that gravity dominates, decades of laboratory and field work summarised by Holsapple in the Annual Review of Earth and Planetary Sciences show that crater dimensions follow power-law scaling against impact energy with a small exponent, close to one quarter (Holsapple, 1993). A quarter-power law is why your numbers behave the way they do: double the energy and the diameter goes up by the fourth root of 2, which is 1.19, or about 19%. To double the crater width you need roughly sixteen times the energy.

That one relationship reframes the Moon. A crater ten times wider than another was not made by something ten times bigger. It was made by something carrying thousands of times more energy.

The ejecta rays are the second payoff, and the cocoa is what makes them visible. Material thrown out of the growing cavity lands in a radial pattern, and because your surface has a dark top layer over pale flour, every ray shows up as a bright streak. The Lunar and Planetary Institute’s activities on impact cratering use exactly this two-layer trick, and the real Moon shows the same thing: bright rays around young craters, fading as space weathering darkens them over time.

So why is the Moon covered in craters and the Earth not? Not because the Earth gets hit less. Three things erase terrestrial craters. The atmosphere destroys small impactors before they land. Water, wind and ice grind down the ones that do. And plate tectonics recycles ocean floor entirely, which the National Park Service’s geology pages describe as a continuous resurfacing of the planet. The Moon has none of these, so a crater there can sit essentially unchanged for billions of years. Earth’s biggest known impact structure, Chicxulub beneath Mexico’s Yucatán Peninsula, is roughly 180 km across and dates to the Cretaceous-Paleogene boundary 66 million years ago, and you cannot see it at all from the surface. It was found from gravity and magnetic surveys, and the Lunar and Planetary Institute maintains a detailed account of the event.

Two honest limits. Flour is far weaker than rock, so the crater-to-marble size ratio here is much larger than reality. And large real craters collapse under their own gravity to form central peaks and terraced walls, which flour cannot reproduce. You are modelling the excavation stage only.

How to Explain It to Your Kid

Ages 5–8: The splash leaves a mark

Compare it to dropping a stone in a puddle. The water splashes out in a ring and then settles, but the flour cannot settle, so the splash stays. Have them drop marbles and point out the rim and the rays. Then show them a photo of the Moon, or look at it through binoculars, and let them find the rays themselves. Do not introduce energy yet.

Ages 9–12: Sixteen times the energy for twice the hole

Build the table with them: height, diameter, two trials each. Then do the arithmetic on ratios rather than differences. Doubling the height gives about 1.2 times the width, and 1.2 four times over is about 2. That is the fourth-root rule, discovered from their own measurements rather than told to them. This is also where the oblique-impact result lands best, because it is surprising and cheap to test.

Ages 13+: Scaling laws and what they really say

Give them E = mgh and the idea that D is proportional to E to some power n, then have them find n from their own data by plotting log D against log E and taking the slope. Real measurements on loose granular targets give slopes in the region of a quarter, and theirs will land somewhere nearby with real scatter. Then ask the harder question: which variables are missing from this model, and why does flour not form a central peak?

The question to ask: “A crater on the Moon is twice as wide as the one next to it. Was the rock that made it twice as big, and if not, roughly how much more energy did it carry?”

Why Teaching This Early Pays Off

Power-law scaling is everywhere in science and almost absent from school until late. Earthquake magnitude, sound intensity, drug dose-response, the strength of materials at different sizes: all of them behave like this, and all of them are counterintuitive in the same way. A child who has measured a quarter-power law in flour has a concrete case to reason from, and that is a better foundation than a definition.

The second payoff is about reading a landscape as a record. Vosniadou and Brewer showed that children build coherent but wrong models of the Earth and patch them rather than replace them (Vosniadou & Brewer, 1992). The common model of the Moon is a decorative object with spots on it. Making the spots yourself, with directional ejecta that records where the projectile came from, turns the Moon into a surface with a history that can be read. That shift is what planetary science actually is.

Modest claim, as always. One tray of flour will not make a geologist, and no study shows it moves test scores. It does give a child a reason to look up.

Before You Start: What Parents Should Know

Best age	6–12. Under 6 the measuring is beyond them but the dropping is not
Supervision	Adult present throughout, mainly to manage the marbles and the dust
Real hazards	Choking. Marbles and ball bearings are a choking hazard for children under three, so count them out and count them back in. Flour dust irritates airways: sift gently, keep faces away from the tray, and consider a dust mask for anyone with asthma. Airborne flour is combustible, so keep this well away from any flame, gas hob or candle
Don’t	Do not throw the marbles hard. A hard throw bounces the marble back out at unpredictable angles and risks an eye injury. Do not do this on a hard floor indoors, where the dust travels and the marbles roll

When It Doesn’t Work

• The crater has a flat bottom. The flour is too shallow and the marble hit the tray. Go to at least 4 cm, deeper for the 2 m drops.
• No rim, just a dimple. The flour is packed down. Fluff it with a fork and re-level without pressing.
• No ejecta rays. The cocoa layer is too thick, so the dark powder gets thrown out with the flour and the contrast disappears. Sift a layer so thin you can almost see white through it.
• Diameters vary wildly between repeats. You are measuring the loose outer debris rather than the rim. Decide on one definition, rim crest to rim crest, and stick to it. Also re-level completely between drops.
• The marble is buried and you cannot find it. Expected at 2 m. Lift it out with a spoon from the side rather than digging, so you do not destroy the crater before measuring.

Take It Further

1. Find the exponent. Six heights, three repeats each, log-log plot, read the slope. This is a real measurement of a scaling law using a tray of flour and is more than most school practicals ever reach.
2. Change the target, not the projectile. Run the same drop into flour, into cornflour, into dry sand and into damp sand. Harder targets give smaller craters for the same energy, which is the strength-dominated regime rather than the gravity-dominated one.
3. Match it to the real Moon. Photograph the Moon with binoculars or a phone through an eyepiece, pick a crater with visible rays, and compare the ray pattern with yours. Mapping the Moon from your own backyard is the project that follows, and the ball-and-lamp model of phases explains why you can only see the rays well near the terminator.

What to Watch For Over the Next 3 Months

• Week 4: they point out craters on the Moon and mention rays or rims rather than just spots.
• Month 2: they resist the assumption that a bigger hole means a proportionally bigger rock, which is a real piece of quantitative reasoning.
• Month 3: the scaling idea surfaces elsewhere, often around earthquakes or explosions. The question worth chasing is “how big would an asteroid have to be to matter?”, and the honest answer involves exactly this arithmetic.

Frequently Asked Questions

Why are craters round if the asteroid came in at an angle?

Because the projectile’s energy is released almost instantly at a point, and the resulting pressure pulse spreads out roughly spherically. The target material is excavated symmetrically around that point. Only the ejecta pattern becomes lopsided, which is what Step 6 shows.

Can we use sand instead of flour?

Yes, and it is a better choice outdoors. Dry sand gives a slightly sharper rim and is far less messy to clean up. You lose a little of the dramatic ray pattern, because the colour contrast with a dark dusting is weaker.

Does Earth really get hit as often as the Moon?

Per unit area, yes, broadly. The difference is that Earth’s atmosphere destroys small impactors, and erosion plus plate tectonics erases the craters that do form. The Moon has no air, no water and no tectonics, so its record survives.

How much energy is in a 1 metre marble drop?

A 10 gram marble falling 1 metre arrives with about 0.1 joules, which is roughly the energy of a bee flying into a window. The Chicxulub impact released something on the order of a hundred million million times the energy of the largest nuclear weapon ever tested. The quarter-power law is why both still make a round hole.

What school topic does this prepare?

Earth and space science, energy transfer, and the handling of experimental data. The National Research Council’s framework for K–12 science names “analysing and interpreting data” and “using mathematics” as core practices, and the log-log plot in Take It Further is both at once (NRC, 2012).

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. Holsapple, K. A. (1993). “The Scaling of Impact Processes in Planetary Sciences.” Annual Review of Earth and Planetary Sciences, 21, 333–373. https://doi.org/10.1146/annurev.ea.21.050193.002001
2. Lunar and Planetary Institute. “Impacts.” Explore! Shaping the Planets. https://www.lpi.usra.edu/education/explore/shaping_the_planets/impacts/
3. Kring, D. A. “Chicxulub Impact Event.” Lunar and Planetary Institute. https://www.lpi.usra.edu/science/kring/Chicxulub/
4. National Park Service. “Plate Tectonics and Our National Parks.” https://www.nps.gov/subjects/geology/plate-tectonics.htm
5. Vosniadou, S., & Brewer, W. F. (1992). “Mental models of the earth: A study of conceptual change in childhood.” Cognitive Psychology, 24(4), 535–585. https://doi.org/10.1016/0010-0285(92)90018-W
6. National Research Council. (2012). A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas. National Academies Press. https://doi.org/10.17226/13165