Waves and the formation of beaches

by Rebecca Priestley / 20 December, 2012
Much of the sand on our beaches dates back 12,000-30,000 years.
Waves and the formation of beaches

I’ve never been much of a beach person, but I plan to take a refreshing late-afternoon swim on Christmas Day. If I’ve had a glass or two of festive bubbly with lunch, I’ll be walking to my nearest beach, a small Wellington bay with only a few metres of coarse dark sand between the road and the waves. If I drive, I can choose between the expansive grey sands of Lyall Bay, the glorious white sands of Scorching Bay or the imported golden sands of Oriental Bay. Why are all these beaches so different? And where did they come from?

Much of the sand on our beaches dates back to the last glacial period, 12,000-30,000 years ago, says Auckland marine geologist Bruce Hayward, when reduced forest cover greatly increased the amount of erosion and the supply of sediment to the sea. As global temperatures warmed and the sea level rose, he says, “sand that had built up along the coast swept shoreward”. The sea reached its current level about 7000 years ago, and since then “vast quantities of sand” have been “thrown up against the land to form beaches, barriers and dunes”.

The size and composition of beach material – heavy black (often magnetic) sand, fine white sand, coarse orange sand or even small boulders – depends on the terrestrial rocks that have eroded to form the sediment. In the North Island, rivers such as the Waikato and Whanganui transport eroded sediment from the middle of the island out to sea and then back to shore with the waves. As offshore currents carry the particles along the coast, the material is sorted, with heavy material, such as black iron sands, deposited closer to the river mouths and fine light material, such as grains of mica, silica and pumice, carried further away.

On beaches with very high wave energy, such as Wellington’s Makara Beach, material is being removed rather than deposited and all the fine material is eroded away, leaving only cobbles or small rocks.

In places where there is less influence from large rivers, beaches are more likely to be made from local material. In Abel Tasman National Park, erosion of granite rocks creates a coarse orangy sand made mostly from grains of feldspar. The South Island’s West Coast has a green beach, coloured by olivine minerals eroded from the Olivine Range.

But there’s more to beaches than rocks, says Niwa marine geologist Helen Bostock, whose favourite beach is Wharariki, just south of Farewell Spit. Local marine organisms also play a part. Geologists refer to the calcareous material that erodes and washes up on beaches as shell hash or coral rubble.

“It’s usually calcareous,” says Bostock, “but it can be made up of foraminifera, bryozoan, your standard shellfish such as bivalves and gastropods, or coral or coralline algae.” Many orange-hued beaches are made from broken-up shells – Hayward’s favourite is a shell beach at Northland’s Cable Bay – and several pink beaches around the country are made from the plates of offshore barnacles.

Reduced forest cover since humans arrived in New Zealand has led to an increase in erosion similar to that which occurred during the last glacial period. But sea level is rising now, and in many places the supply of sediment to beaches is not keeping up with sea-level rise. So enjoy your favourite beach while you can. And if you want to find out what it is made of, remember to take a magnifying glass and a magnet to investigate the sand.

2013 is the International Year of Mathematics of Planet Earth: mpe2013.org. Send your mathematically themed questions and regular science questions to: science@listener.co.nz.
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