World’s Physical Geography

An archipelago is a group of islands closely scattered in a body of water. Usually, this body of water is the ocean, but it can also be a lake or river. For example, the Malay archipelago, the largest group of islands in the world, consists of more than 17,000 islands in Indonesia and approximately 7,000 islands in the Philippines. Types of archipelagos: Depending on the geological origin, the islands of the archipelagos can be oceanic or continental islands.

• Oceanic archipelagos: These are the group of islands that do not belong to a particular continental tectonic plate, but are of volcanic origin. These forms at a greater speed. For example, the Hawaiian archipelago.
• Continental archipelago: These islands are formed by continental fragments that have been separated from the continent by the movement of tectonic plates and other geological processes. For example, the British Isles and Balearic Islands.

Processes involved in the formation of archipelagos include:

• Volcanic activity: Volcanic activity that can form archipelagos includes a hotspot, where an area within the oceanic crust is heated and has a constant supply of magma spewing out from within and cooling off to form land. The relatively stationary hotspot acting on the shifting crust is what creates a chain of islands or archipelagos. For example, the Hawaiian archipelago is formed by a series of volcanoes.
• Erosion and sediment deposit formation: The movement of erosional agents like water can break down rock into sediments. These small sediments can be carried away to a new location and settle to form island-like structures. For example, the coral clay archipelago of Florida Keys formed out of sediments transported by ocean currents and gradually built up on the reef structure.
• Tectonic movements: Continental fragments are archipelagos that have separated from a continental landmass due to the Earth’s tectonic movements. For example, Farallon Islands off the coast of California.
  • Apart from this, archipelagos also form at a convergent plate boundary where oceanic plate gets subducted into the mantle and melt to raise convection currents. Such a group of islands is called an Island arc volcano. For example, Fiji, and Alaska’s Aleutian Islands.
• Glacial retreat: This phenomenon leads to the flooding of valleys and a rise in sea level leaving coastal mountain ranges standing tall. The coastal mountain ranges eventually become archipelagos. For example, the Malay archipelago, the largest on the Earth, was formed by a glacial retreat.
• Post-glacial rebound: In this process, land that was squashed by the weight of heavy glaciers during the Ice Age slowly regains its shape, like a sponge, and islands are formed out of this land. For example, the Finnish archipelago (the archipelago in Finland).

Apart from these major processes, archipelagos also form due to changes in the course of rivers, the drying up of lakes, etc. Archipelagos, as a geological structure, are not only important in terms of human settlement but are also pertinent for ecological preservation.

The relative position of the Earth with respect to the sun changes within a year due to Earth’s revolution. Further, due to the inclination of the Earth on its axis, there are differences in the heating of the continents and oceans, and as a result, the pressure conditions in January and July vary greatly. This consequently results in the shifting of the wind belts.

Shifting of pressure and wind belts:

• Summer and Winter Solstice:
  • The sun during the summer solstice is vertical over the Tropic of Cancer and therefore, all the pressure belts shift northward, including the Equatorial Low-Pressure Belt, and Subtropical High-Pressure Belt except the Northern Polar High-Pressure Belt. The wind belts associated with the said pressure belts also shift northward.
  • Similarly, during the winter solstice, the pressure and wind belts shift southward except the Southern Polar High-Pressure Belt.
• Autumnal and Vernal Equinox: The sun becomes vertical over the Equator and hence all the pressure belts occupy their normal positions.

These seasonal changes in the relative positions of the wind belts introduce the following typical climatic conditions: (i) The Mediterranean climatic regions are found in the western parts of the continents within the latitudinal zone of 30°-45° in both hemispheres. The Sub-Tropical High-Pressure Belts extending between 30°-35° latitudes are characterized by dry trade winds during the summer season and anti-cyclonic conditions. This belt extends up to 40° latitudes in the Northern hemisphere at the time of summer solstice and in the Southern hemisphere at the time of winter solstice. Thus, the western parts of the continents within the zone of 30°-40° latitudes do not receive rainfall during the summer season. On the other hand, the Sub-Tropical Belt shifts towards the Equator at the time of winter solstice in the Northern hemisphere and at the time of summer solstice in the Southern hemisphere. Consequently, the zone is characterized by the Westerlies, which lead to precipitation during the winter season. The Mediterranean type of climate is thus characterized by dry summers and wet winters. (ii) The regions lying between 60°-70° latitudes are characterized by two types of winds in a year. With the northward migration of the sun at the time of summer solstice, the Polar Easterlies are weakened because the Westerlies extend over these areas due to the northward shifting of Sub-Polar Low-Pressure Belts. The situation is reversed at the time of winter solstice when there is southward migration of the sun. The Polar Easterlies are re-established between 60°-70°N because of the shifting of the belt of the Westerlies southward. Consequently, it creates a climate characterized by wet summers through the Westerlies and associated cyclones and dry winters due to Polar Easterlies. (iii) Monsoon climate is also the result of the shifting of pressure and wind belts. Due to the northward migration of the sun in the Northern hemisphere at the time of summer solstice, the North Inter-Tropical Convergence Zone (NITCZ) is extended up to 30°N latitude over the Indian subcontinent, Southeast Asia, and parts of Africa. Thus, the Equatorial Westerlies are also extended over the aforesaid regions, which become the southwest or summer monsoons. These southwest monsoon winds bring much rain because they come from over the ocean and are associated with tropical cyclones. The NITCZ is withdrawn from the Indian subcontinent and Southeast Asia because of the southward shifting of pressure and wind belts due to the southward migration of the sun at the time of winter solstice. Thus, north-east trades are re-established which leads to the north-east or winter monsoons. Since they come from over the lands, they are dry.