Rocks that soak up water: why meteorites last longest in deserts and ice
Pupils test rocks to find out which soak up water and which are easily scratched, group them by what they find, and use the results to explain why more than half of all known meteorites have been collected in Antarctica.
- Compare and group rocks by their appearance and by whether they soak up water and are easily scratched.
- Carry out a comparative test, timing in seconds and keeping other things the same.
- Explain that water makes the metal in meteorites rust, so they survive longest where it is very dry or frozen.
National Curriculum links
- Year 3, Rocks: compare and group together different kinds of rocks on the basis of their appearance and simple physical properties.
- Year 3, Rocks: recognise that soils are made from rocks and organic matter.
- Lower Key Stage 2, working scientifically: setting up simple practical enquiries, comparative and fair tests; taking accurate measurements using standard units; recording findings in tables and bar charts; using results to draw simple conclusions.
Key vocabulary
- permeable
- impermeable
- grain
- crystal
- weathering
- rust
- desert
- Antarctica
Resources
- For each group, labelled samples of chalk, sandstone, granite, slate and marble (limestone can replace marble).
- A dropper or pipette, a small pot of water, a stopwatch or timer and paper towels.
- Hand lenses.
- For the scratch test: fingernails, a wooden lolly stick and an opened-out steel paperclip.
- Pictures on a screen of meteorites lying on Antarctic blue ice and on desert sand.
- The worksheet, one per pupil.
Background for the teacher
Some rocks are permeable: they have tiny gaps between their grains or fine cracks that let water soak in. Chalk and most sandstones are permeable, so a drop of water disappears into them. Granite, slate and marble are impermeable: the water sits on top. Pupils can see the reason with a hand lens. Sandstone is made of rounded grains stuck together with gaps between them, while granite is made of interlocking crystals.
Water is the main enemy of meteorites on Earth. Most stony meteorites contain grains of iron metal and an iron sulfide mineral, and when water gets in through cracks these rust. The stone turns brown, cracks and eventually crumbles, and the crumbs join the soil. In a wet climate such as Britain's this happens far faster than in a dry one.
That is why meteorite hunters search the driest places on Earth. Many meteorites in hot deserts such as the Sahara have lain there for tens of thousands of years, and in the cold, dry ice of Antarctica many have survived for hundreds of thousands of years. Moving ice also carries meteorites to places where they collect on the surface. More than half of all known meteorites have been collected in Antarctica, and most of the rest in hot deserts.
Lesson plan
Starter 10 minutes
Hold up chalk and slate and ask which will soak up a drop of water. Test both under the visualiser or in front of the class, then introduce the words permeable and impermeable.
Ask: how could we test all five rocks fairly? Agree the same number of drops from the same dropper, a dry rock, and a time limit of 3 minutes.
Main 35 minutes
Groups first look at each rock with a hand lens and describe it on the worksheet: colour, grains or crystals, rough or smooth.
Soak test: put five drops on the flattest part of each rock and time how long it takes to soak in. If water is still sitting on top after 3 minutes, record "did not soak in".
Scratch test: try to scratch each rock gently with a fingernail, then the lolly stick, then the paperclip. Record which ones mark it.
Groups sort the rocks into permeable and impermeable, and draw a bar chart of soaking times for the rocks that soaked water up.
Plenary 15 minutes
Explain that meteorites are not usually permeable like sandstone, but water creeps into their cracks and rusts the metal inside. Show three picture cards: a rainy field in Britain, the Sahara and the Antarctic ice. Where would a meteorite last longest? Pupils rank them and give a reason, then reveal that Antarctica has given us more than half of all known meteorites.
Differentiation
Test three rocks with an adult and record soaked or not soaked with a tick, without timing.
Test all five rocks, time the soak test and complete the bar chart.
Use the hand lens to explain why sandstone soaks up water but granite does not, and predict which rock would crumble first if it were soaked and then frozen.
Safety and risk assessment
Check these points against your employer's risk assessment and your school's policies before the lesson.
| Hazard | How to control it |
|---|---|
| Cuts from sharp rock edges | Check samples beforehand; handle gently and never break or hammer rocks. |
| Scratches from the paperclip | Scratch away from the body with the rock flat on the table. |
| Slipping on spilled water | Work on trays and wipe spills at once. |
| Dust from chalk and sandstone | Wash hands after the lesson. |
Assessment questions with answers
- What does permeable mean?Answer: A permeable rock lets water soak into it or pass through it.
- Name one permeable and one impermeable rock from the lesson.Answer: Permeable: chalk or sandstone. Impermeable: granite, slate or marble.
- How did you make the soak test fair?Answer: The same number of drops from the same dropper, a dry rock each time and the same time limit.
- Why does a stony meteorite turn brown after years in a wet field?Answer: Water gets in and makes the iron metal inside it rust.
- Where have more than half of all known meteorites been found, and why do they last so long there?Answer: Antarctica. It is so cold and dry that meteorites rust very slowly, and moving ice gathers them in places where they can be found.
Homework
With an adult, look at an old wall, gravestone or stone building. Find signs that water has worn or stained the stone, such as crumbling edges, rust-coloured streaks or moss. Draw what you find and say which you think is the most permeable stone.
Answer guidance: Look for observations such as crumbling sandstone, worn lettering on gravestones or orange stains from iron. Softer, permeable stones usually show the most wear.
Use with these pages
- Testing a rock like a scientist
- Terrestrial weathering
- Weathering grade
- Meteor-wrongs: the rocks that fool people
- Is my rock a meteorite?
- #Finding and identifying meteorites
These eight lessons are a free starter pack, written specially for this site. The full set of 25 lesson plans, with worksheets, end-of-unit tests and mark schemes, is in the Corvanelle book Teaching Meteorites. The lessons in the book are different lessons, so the starter pack and the book work together rather than repeating each other.