A pattern of islands

Even the broadest sea is not a bare expanse of blue. If it were, where would treasure be hidden, and where would the castaways be cast?

Islands come in several flavours and we have met a few already. Some are simply accumulations of sediment that happen not to join onto the land. At major river outfalls such as the Bay of Bengal, new islands can arise and vanish in the timescale of human memory. The Bay sees the deposition of vast amounts of material but is also subject to severe storm surges that can remove it rapidly. Islands formed by such sediment tend to be long and narrow, with their long axis parallel to the flow of water. They are laid down at the point where the current carrying the sediment slowed.

But if the map shows you an island just offshore with a less regular shape, it is a fair bet that it is rockier and older. It is also likely to rise more imposingly from the sea. Rocky offshore islands are usually bits of fossil land that have not been eroded away yet. They tend to match the geology of the nearest piece of mainland. Limestone, sandstone and other hard rocks dominate their make-up and they form handy niches for seabirds and other hardy species. The prominent rock arches that are a feature of many sea coasts can be thought of as future islands, just as soon as the connecting rock has collapsed. But as the rocks in question are hard, the process could take thousands of years to show perceptible effects. Softer rock is removed faster but leaves bays rather than promontories.

An island found far out to sea is bound to have a different explanation. As we have seen, many are volcanic in origin, including some prominent island chains such as Hawaii, and big islands such as Iceland.

Such islands manage to break the ocean surface – and, in Hawaii's case, rise kilometres above it – because of the immense volumes of magma that the volcanoes that produce them generate. But not all subsea volcanism is so successful. It is now known that ocean floors, especially that of the Pacific, are strewn with thousands of “seamounts”, underwater mountains defined as being more than 1000m high, measured from their local sea floor, but not reaching the sea surface.

It is common for new seamounts to be discovered. Many are in areas of intense seismic activity and are still live volcanoes, suggesting that they might accumulate enough material at some point to break the sea surface as islands. Others have been above sea level in the past and have now been eroded back, as with the older members of the Hawaiian chain.

Artwork depicting seamounts often has the vertical and horizontal scales fixed so that they look as steep as medieval towers. In fact, they slope at up to 25° but even these angles are not stable. Landslides on seamounts, perhaps set off by earthquakes, can cause tsunamis. Other seamounts can occur near land and can have sedimentary or volcanic origins. Seamounts tend to end far enough below the sea surface to be of interest to scientists rather than sailors. Reefs, by contrast, are a byword for hazards to shipping.

They come in two sorts – coral and not coral – that have one thing in common. They both coincide almost exactly with the sea surface, allowing them to snare unwary vessels. If no coral is involved, a reef is simply a piece of rock, especially likely to appear on a coast which exhibits rapid erosion. Some are exposed at low tide. Around the world, many have been tamed – comparatively – by the addition of bells, lights and other warning devices.

A coral reef is equally dangerous to navigation, but is also one of the wonders of the world. A coral is a combination of alga and animal, and can only cope with shallow water, down to perhaps 30m, because the alga needs sunlight to carry out photosynthesis. Some corals are soft but others produce solid calcium carbonate that builds to a reef. In addition, other species such as sponges can secrete calcium carbonate, so coral should not take all the credit for reef-building.

Coral reefs can take a number of different forms. Barrier reefs are long reefs parallel to the shore that have formed in shallow water. They have a vital role in protecting the shore from erosion. An atoll is a ring of coral reef surrounding a lagoon. They are usually on the site of a sunken volcanic island. The atoll marks the initial coastline, and will remain only so long as coral growth keeps pace with the subsidence of the island.

Reef Atoll Belize, surrounding the Blue Hole, a favourite diving spot

Coral-bearing limestone is known from hundreds of millions of years ago. But it has been in a conspicuous golden age of late. As the last ice age ended and sea levels rose, corals grew to keep pace with the rise in water levels by building higher on the limestone laid down by their predecessors. Oceanographers now use coral as a ruler for measuring sea level change.

In the modern era, however, the picture is less favourable. As well as sea level rise, global warming causes sea temperatures to rise. Corals find it hard to cope with warmer water, which is why some of the worlds hottest ocean waters, for example in the Arabian Gulf, do not have coral. In 1998 and 2004, when the El Nino effect caused sea temperatures to rise well above normal, many tropical coral reefs suffered from “coral bleaching” or were killed. If human-induced climate change causes sea temperatures to rise in the future, more coral will die. In addition, many reefs are subject to human-induced stress at the local level. Some are mined for rock, built on by hotel owners, or polluted. As they can only exist in shallow water and are usually near to land, they are often in places where pollution disperses only slowly.