GEOLOGICAL ORIGINS OF ISLANDS
Volcanoes are places where the hot molten rock from inside the earth comes to the surface and piles up to make a mountain. Many volcanoes occur at places where the oceanic plates are rising up or sinking down under another plate, making what is sometimes called the "ring of fire" around the Pacific. There can also be volcanoes in the middle of a plate, sometimes at hot spot where the plate is moving over a very hot place in the earth's interior. As the ocean bottom or plate moves over the hot spot, this can produce a chain of volcanoes in a line with the oldest at one end and the youngest (still active) at the other.
It is these basic geological processes that produce the different types of islands.
Continental islands are pieces of continental crust that have broken off from a big continent and drifted out into the ocean. Because the rock of which they are made has come from a continental area, they can have many kinds of rock of different ages in a very complicated structure. As a result, they have many kinds of soils and may be rich in minerals. Because they have broken off from a continent, they may have carried with them plants and animals that lived on the continent at the time of their separation. New Caledonia and Cuba area good examples of continental islands.
Some low islands on continental shelves have been built up from coral reefs on the continental margins. They may be similar in structure to the other coral islands described below.
The other types of islands all originated from the ocean bottom and were never part of a continent. They all started as volcanoes, even if the island as we see it today has no volcanic rock left. When volcanic activity starts, it first builds a submarine volcano, erupting underneath the ocean surface. Such eruptions may produce large amounts of pumice, a light volcanic stone which floats and washes up on beaches far away. When the volcano reaches the surface, it becomes a volcanic island. These islands are built of lava, which is liquid rock which spills out of the volcano and runs down its sides until if finally cools to make a hard black rock, often with bubbles in it. Volcanoes also produce ash, which is more like sand or dust which builds up in layers around the volcano. Some volcanic ash can be shot high in the air and carried long distances by the wind, where it may fall on other islands.
Volcanic islands can have different shapes depending on their age. A young volcanic island with an active volcano and frequent eruptions will be shaped like a broad cone or shield with black lava rock that goes to the water's edge and little or no lagoon or reef. As the island gets older and volcanic activity slows and then stops, the wind, rain and waves will start to erode the island, cutting into the broad shield and carrying away the softer ash deposits. The best soil will collect in the valley bottoms and along the coast. An older volcanic island will thus have jagged mountains with many valleys and steep slopes in the centre. At the same time a coral reef will build around the island, making a lagoon. The island may slowly sink as well. The old volcanic island will get smaller and smaller until it finally disappears.
However, all is not lost for an old volcano in tropical waters, for while the volcanic island is sinking and washing away, the coral reef is growing, turning the volcano into a coral island. The more-or-less circular reef with a lagoon in the middle left by a sinking volcano is called an atoll. The coral reef (see unit C8) will keep adding material to the island, which may be piled up on the reef by storms to make areas of land on the reef. Coral islands are made of sand and coral rock that comes from the skeletons of corals, shells and some marine plants (algae) and is composed almost entirely of limestone or calcium carbonate. Such rock is very poor in minerals, and will even dissolve slowly in fresh water.
If the sea bottom rises again, a coral reef or atoll may be lifted out of the water, making a raised coral island, with a platform (the old reef and lagoon) several tens of meters above sea level. Where the rain falls on the raised coral rock, it will dissolve some of it leaving sharp jagged shapes and even making caves deep in the rock.
The islands as we know them today may be classic types like one of the above, but more often they are mixtures of different geological structures. There are often raised reef areas on continental or volcanic islands. A lava flow may cover up a former lagoon or reef. A raised coral island may be covered with rich soil from volcanic ash deposited on the island by nearby volcanoes.
The history of many islands has also been marked by changes in sea level and the rising and falling of the ocean bottom. These may cause islands to be partly covered up or even drowned entirely, then uncovered again. When the sea level fell during the ice ages, many islands were more uncovered than they are today. Sometimes terraces cut by waves in the shoreline above sea level or in the reef below sea level show where the water level was at different times in the past.
The knowledge of the geological history and structure of an island is
important to understanding its environment and natural resources. It is
the geology that determines the presence of minerals and the possibilities
of mining. The geological structure has an important influence on ground-water
resources and their behaviour. It is the island rocks that determine the
nature and composition of the soils. All of this is important to knowing
what kind of environmental impacts to expect from a development project.
QUESTIONS
What kind of an island do you live on?
Is it a mixture of several kinds of geological structures?
What kind of rock did your island soils come from?
Can you tell the story of the origin of your island?
What do you think will happen to your island in the distant future?