Mochi Isle
Cave formations · Crystal MineScience for kidsHow do stalactites and stalagmites form?
In short: Rainwater seeping through limestone dissolves a little rock. When it drips into a cave, the mineral slowly comes out of the water and builds up: stalactites hang from the ceiling and stalagmites grow up from the floor.
How it works
As rain falls through the air and soaks through soil, it picks up carbon dioxide and becomes a very weak acid. This can slowly dissolve limestone, a rock made mostly of a mineral called calcite. The mineral-rich water trickles down through cracks until it reaches the roof of a cave.
When a drop hangs from the cave roof, it releases some carbon dioxide into the cave air. The water can no longer hold all its dissolved mineral, so a thin ring of calcite is left behind. Drop after drop, ring after ring, an icicle-shaped stalactite grows downwards.
Drops that land on the floor leave mineral too, building a stalagmite upwards. If the two meet, they form a column. They usually grow very slowly, so large ones can take thousands of years. A memory trick: stalaCtites hold tight to the Ceiling, stalaGmites grow from the Ground.
Where to see it in New Zealand
Waitomo Caves in the King Country is one of New Zealand's best-known limestone cave areas, full of stalactites, stalagmites and columns. Visitors are asked not to touch them, because they grow very slowly and hands can easily break, stain or damage them.
Try it together
Ask an adult to help you stir baking soda into two jars of warm water until no more will dissolve. Place the jars apart with a saucer between them and drape a wool string from one jar to the other. Over several days, watch for a tiny 'stalactite' under the sagging middle. Do not drink the mixture.
Kids also ask
How can I tell stalactites from stalagmites?
Stalactites hang down from the cave ceiling like icicles. Stalagmites rise up from the floor, often right underneath, because they are built by the drips falling from above. When the two join, they make a column.
Why shouldn't you touch cave formations?
They grow so slowly that a broken piece may take thousands of years to regrow. Oils and dirt from hands can also stain the surface and get in the way of new mineral building up. Admire them with your eyes only.
In the game, “Cave formations” is a nature card on Crystal Mine.
Your child finds this card by reaching the right place or meeting the right weather on the island, and lights it up by answering the question on the back. Tap the button below and the game shows your child which island it is on and how to find it. Mochi Isle is a browser learning game for ages 5 to 18; the main island and the daily lesson are free.
Find “Cave formations” on the island ↗ All cards from Crystal Mine