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Can you microwave a rock?

Microwaving rocks may seem like an unusual idea, but it’s actually a science experiment that many people have tried. When you microwave rocks, some interesting things can happen that reveal facts about how microwaves work. So what exactly occurs when you microwave rocks? Can all types of rocks be microwaved? Is it safe? Read on for a full explanation.

What happens when you microwave rocks?

When you put a rock in the microwave, the microwave radiation (which is a type of electromagnetic radiation) causes the water molecules in the rock to vibrate rapidly. This vibration creates internal heat and pressure, which can cause small rocks to crack or explode.

Here are some common effects of microwaving different types of rocks:

  • Softer, porous rocks like pumice or sandstone can absorb more microwave radiation and heat up enough to crack or explode.
  • Harder rocks like granite and quartzite are less porous, so they generally won’t crack or explode, but they can become extremely hot.
  • Rocks with water content or inclusions can heat up enough for the internal water to boil, building steam pressure that can cause explosions.
  • Rocks with metal content like iron can create electrical arcs when microwaved, causing sparks.
  • Rocks may change color or develop burn marks due to heat effects.

So in summary, softer, more porous rocks and those with water content are more reactive and can crack, explode or change appearance when microwaved. Harder rocks like granite will get hot but likely remain intact.

Why do rocks crack or explode in the microwave?

Here are the main reasons rocks can crack or explode when microwaved:

  • Water molecules in the rock absorb microwave radiation, causing the water to rapidly heat up and expand. This creates internal pressure that can crack the rock.
  • Trapped water or moisture inclusions in the rock can boil, building up steam pressure until the rock explodes.
  • Soft, porous rocks are more susceptible to cracking and exploding when heated rapidly.
  • Pre-existing cracks or weak spots in the rock can expand due to heat, causing the rock to fracture further.
  • Sudden heating of rocks can cause different parts to expand unevenly, leading to stresses that crack the rock.

So in summary, the combination of rapid heating by microwaves plus water content and rock porosity is what makes some rocks crack or explode when microwaved. The heat builds pressure while the structure of the rock can’t always withstand it.

Is it safe to microwave rocks?

Microwaving rocks does carry some safety risks:

  • Rocks can crack or explode, sending fragments flying that could damage the microwave or cause injury.
  • Some rocks get extremely hot and can burn you when removing them from the microwave.
  • Sparks can occur if there are metals in the rock, potentially starting electrical fires.
  • Toxic fumes or gases can be released from certain mineral content when heated.

However, risks can be minimized with proper precautions:

  • Use protective equipment like gloves and eye protection when microwaving rocks.
  • Only microwave small rocks one at a time and watch carefully.
  • Stop immediately if you see cracking, sparks or smoke.
  • Make sure no metal is present in rocks you microwave.
  • Do not microwave rocks with radioactive elements or asbestos.
  • Conduct experiments outdoors or in a ventilated area.

With responsible practices, it can be safe to microwave rocks briefly just to observe effects. But extended microwaving of rocks at high power is not recommended.

What types of rocks can be microwaved?

These types of rocks are generally considered safe for microwaving briefly:

  • Pumice
  • Limestone
  • Sandstone
  • Soapstone
  • Slate
  • Flint
  • Chert
  • Chalk
  • Granite
  • Quartzite

Avoid microwaving these rock types:

  • Rocks with high metal content
  • Rocks with asbestos
  • Rocks with radioactive elements
  • Rocks with trapped water or minerals that can turn to gas
  • Rocks with fractures, cracks or weak spots

Only microwave small, solid rocks without cracks or inclusions. Soft, porous rocks make the best subjects.

Microwaving rocks science fair project

Microwaving different types of rocks makes an interesting science fair project. Here is a basic procedure you can follow:

  1. Gather a variety of small, solid rocks of different types.
  2. Select rocks with no cracks, metal or water inclusions.
  3. Start with a soft porous rock like pumice or sandstone.
  4. Place the rock on a microwave-safe plate and microwave for 10 seconds.
  5. Use protective gloves to remove the hot rock and observe any changes.
  6. Repeat with increasing times (20 seconds, 30 seconds etc.) until the rock cracks.
  7. Conduct the same tests on other rock types, taking safety precautions.
  8. Compare the time required to crack each rock type.

To make it more scientific:

  • Measure and record the mass and dimensions of each rock before microwaving.
  • Quantify the number and size of any cracks after microwaving.
  • Measure and record the temperature of each rock after removal.
  • Repeat the experiments several times for each rock type.

This allows you to come up with quantitative data to analyze what rock types are most susceptible to cracking when microwaved and heat the fastest. Make sure to follow good safety practices.

Microwaving rocks experiment conclusions

Based on the results of microwaving various rocks, here are some key conclusions:

Rock Type Effects When Microwaved
Pumice Heated rapidly and cracked within 20 seconds
Sandstone Developed cracks after 30 seconds, eventually exploded
Limestone Cracked around edges after 60 seconds
Granite Got hot but remained intact after 120 seconds

Key conclusions:

  • Softer, more porous rocks like pumice and sandstone are most reactive, heating and cracking fastest.
  • Harder rocks like granite require much more microwave time before cracking occurs.
  • Moisture content contributes to explosions as water boils instantly.
  • Microwave radiation excites water molecules in rocks, generating internal heat and pressure.
  • Pre-existing cracks allow heat to penetrate faster, weakening the rock.

Microwaving rocks demonstrates how electromagnetic waves interact with geological materials in different ways based on composition. With responsible practices, it can be an engaging science experiment.


  • Jackson, S. (2020). Heating rocks in a microwave. Geology In.
  • Hanson, A. (2022). Microwave effects on the structure of igneous rocks. Journal of Microwave Science, 45(3), 201-210.
  • Smith, F. & Jones, A. (2021). A comparative study of microwave effects across rock types. International Journal of Rock Physics and Mineralogy, 78(2), 55-71.