Basalt & black sand · Chimney VolcanoScience for kids

Why is the sand on some beaches black?

In short: Black sand comes from volcanic rock. Lava cools into dark rock such as basalt, and waves pound and tumble it year after year until it breaks into tiny black grains. That's why black sand beaches are often near volcanoes.

Basalt & black sand: a nature card illustration

How it works

When lava cools and hardens, it often becomes a rock called basalt. Basalt is full of minerals containing iron and magnesium, which make it very dark, almost black. On volcanic islands such as Hawaiʻi and Iceland, black basalt is everywhere you look.

Waves crash against the coast thousands of times a day, cracking the rock and knocking off pieces. The pieces roll around in the surf, bashing and rubbing against each other and becoming smaller and rounder, until they're tiny black grains that the waves pile up into a beach.

Ordinary golden or white beaches are mostly made of pale quartz or broken-up shells and coral. So the colour of a beach is a clue to the kind of rock found nearby, or upstream along the rivers that carry sand down to the sea.

Where to see it in New Zealand

Piha, Karekare and Muriwai on Auckland's west coast are famous black sand beaches. Much of their sand comes from volcanic rock in Taranaki and the central North Island, washed to the coast by rivers and carried north by ocean currents. It's packed with ironsand, which sticks to a magnet.

Try it together

With an adult, visit a black sand beach and stay well back from the waves. Put a magnet inside a plastic bag and drag it gently across dry sand. How much black ironsand sticks? Hold the bag over a box and pull the magnet out to drop the sand. Wear shoes, as black sand gets very hot.

Kids also ask

Why does black sand get so hot?

Dark colours soak up more sunlight and reflect less of it, so black sand heats up much more than pale sand on a sunny day. Wear jandals or shoes rather than walking far barefoot.

Is New Zealand's black sand useful?

Yes. The black sand on the North Island's west coast contains lots of iron-rich minerals. For many years, New Zealand has mined this ironsand to make steel.

In the game, “Basalt & black sand” is a nature card on Chimney Volcano.

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 “Basalt & black sand” on the island ↗ All cards from Chimney Volcano