DK Science: Coasts
Coasts, which form the boundary between land and ocean, receive a constant battering from the wind and waves. In calm weather, the water merely laps at the shore, but on windy days, towering, foam-capped breakers smash onto coasts. It’s no wonder that the shapes and even location of coasts are constantly shifting, as waves erode the land and as SEA LEVELS CHANGE. In some places, coasts are retreating inland by several metres each year.
Coastal features, such as cliffs and arches, are formed by wave erosion. As the sea beats on rocky headlands, softer rocks are eroded (worn away) to form hollow caves. Twin caves on either side of a headland may eventually wear right through to form an arch. As the battering continues, the top of the arch collapses to leave an isolated pillar.
In the last few million years, sea levels have risen and fallen by up to 200 m (660 ft). Scientists believe these are caused by temperature changes, as Ice Ages come and go. During Ice Ages, sea levels are low because large amounts of water are frozen. When the climate warms, the ice melts and sea levels rise. Today, sea levels look set to rise because of global warming. This will bring a risk of flooding to coasts.
During an Ice Age, the weight of the ice depresses (pushes down) the land. Sea levels are low, so the crust beneath the ocean is not depressed. When the weather warms, melting ice causes sea levels to rise. This effect is partly offset by the land springing up when released from the ice’s weight, while the ocean bed sinks beneath the weight of water.