J Rekker of Macandrew Bay asks :-

Why does sand, which is heavier than sea water, accumulate on our seashores?

Gary Wilson, a geologist at the University of Otago, responded.

While it is true that sand grains are heavier than water, settling grains will only remain where they fall if the water they fall in does not move. The ocean, however, is in constant motion, particularly at its edges where waves break on the shore and currents travel around landmasses.

Ocean currents are water masses that flow in one direction, a bit like wind blowing across the surface of the earth. As the current flows across a sand grain, it is like air over the wing of an aeroplane and the grain is lifted into the current and moved along with the current until it falls back to the sea floor. This process can be repeated over and over again and we call this saltation. Once you add many more grains of sand, the bouncing grains will interact and help to keep each other from settling and the sand will travel along at the base of the ocean current in waves.

Ocean waves generally form from on-shore winds lifting the ocean surface and letting it fall again, but in an oscillatory motion (up - across - down - and back). This oscillatory motion is transferred down into the water column and as the wave moves towards the land, the oscillating water hits the sea floor and picks up the sand grains and drives them towards the beach. The coarser sand grains are only moved by bigger waves and the smaller the waves, or the further off-shore one goes, the smaller the grains of sand that are able to be moved. If the waves strike the beach at an angle, the sand will also gradually be moved from one end of the beach to the other.

The sand grains themselves begin life by being eroded from land either by rivers or ocean waves at the coast and washed off-shore. They may then just be reworked into a beach by the ocean waves or transported along the coast by waves hitting the beach at an angle or in ocean currents. This is the case, for example, in coastal Otago where a lot of sand is deposited into the ocean by the Clutha River and then transported north by currents and reworked into beaches along the Otago Peninsula by ocean waves.