Land Meets Sea

In the grand scheme of things—the geologic cycle—minerals are combined into rock, the rock is lifted up as mountains, the mountains are eroded, and rivers carry the sediment back to the sea.

Where the rivers meet the sea some of the sand is deposited, while the finer particles are carried further in suspension. In this air photo, the waves can be seen breaking on the sand bar across the mouth of the river.

Central America is sufficiently mountainous for there to be many short rivers carrying coarse material to the coast, why sand beaches are common.

In the hot, humid, and lowland tropics, clay dominates over sand in the river load. For that reason, mudbanks and mangrove tend to be more common than sand beaches.

Sand beaches are most common in desert climates and in temperate climates. In previously glaciated seas the downstream shore tend to contain vast amounts of glaciofluvial sand (i.e., sand transported and sorted by glacial meltwater).

Other sand is created by fragments of coral or mollusc shells, or by chemical precipitation as "ooids". Coral reefs require very clear water with no sediment input from rivers. Also ooids occur only far from rivers, since they cannot form where the water is diluted. A lack of rivers is found both on tropical islands, and in desert climates such as the Red Sea.

Regardless of the origin of the sand, all beaches one more factor, without which they cannot exist: Waves. Waves prevent vegetation from taking over, by stirring up the sand. However, waves can also erode the beach.

The beach exists in a dynamic equilibrium with the waves

Some waves tend to move sand out to deeper water, others to move it back in. Some beaches have sand that is trapped between headlands, while others receive sand in one end and loose it in the other. The beach exists when these various processes balance each other over some time.

The waves thus play a crucial role for all beaches, which we will look closer at on the next page.