TRAINING MATERIALS IN RURAL ENVIRONMENTAL MANAGEMENT
GEOLOGICAL ORIGINS OF THE LAND
The earth is a planet in space turning around the sun since it was born from the sun's fiery heat millions of years ago. The middle of the earth is still hot liquid rock, although the crust at the surface has cooled down enough for oceans to form and for life to develop. Even though the crust has cooled, it is not fixed in place; it is made up of plates that slowly move around. Some of the plates are of lighter continental rocks that drift around and can collide with each other. There are also ocean plates made by rock which rises up from the interior of the earth and cools, adding on to one edge, and dips down under another plate at the other edge to melt again in the interior, rather like a conveyor belt. These movements are very slow in human terms, but they do lead to important changes over geological time. For instance, North and South America are moving westward at a few centimetres per year, but this has been enough to create the whole Atlantic Ocean since they broke off from Europe and Africa.
Mountain chains are formed by these processes. As North and South America move westward, their western edges have crumpled up to form the Andes in South America and the mountains of the western United States and Canada. The great southern continent broke up, with India crossing the Indian Ocean to collide with Asia and produce the Himalayas, and Africa colliding with Europe to make the Alps. Earlier geological processes produced mountains that are much older and have now worn down through erosion. Other land areas were once the bottom of shallow seas.
Volcanoes are places where the crust has split and 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 a "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.
The result of all these geological processes is that each land area is different. Each kind of rock produces its own kind of soil, which may be thick or thin, rich or poor. The land surface may be flat, hilly or mountainous, growing or eroding. These processes are still continuing today. Understanding them can be important to managing your own local environment.
There are too many different geological situations to describe them all here, but it should be possible to find information on your own local environment and its geological history. Governments often produce soil maps, for instance. Knowing the geology of your area can help to know what risks there may be of earthquakes, landslides, tsunamis, volcanic eruptions or other geological hazards. The risk of flooding is also often related to the landforms and geology of an area. The geological history can also be important in determining what plants and animals occur naturally in your area.
While most land is in continents, the same geological processes produce the different types of islands. These illustrate many of the same processes, but on a smaller scale where the effects may be easier to see.
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.
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, even all the way around the world.
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. If the volcano is in tropical waters, 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 same thing can happen to coral rock on the tropical coasts of continents.
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 and coastal areas of continents has been marked by changes in sea level and the rising and falling of the ocean bottom. These may cause coasts and islands to be partly covered up or even drowned entirely, then uncovered again. When the sea level fell during the ice ages, many coastlines and 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 your land 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 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.
What kind of a land area do you live on?
Is it a mixture of several kinds of geological structures?
What kind of rock did your soils come from?
Can you tell the story of the origin of your land?
What do you think will happen to your land in the distant future?
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Last updated 14 November 2006