The Indian Ocean - Michael Pearson [15]
We have noted the dense network of islands characteristic of the Malay world. The more isolated islands in the ocean play a rather different role. Geologically they are various. Some are granite fragments of larger land masses, such as Madagascar, Sri Lanka, Socotra and part of the Seychelles. Other are volcanic from submarine eruptions: Mauritius, Reunion, Comoros, Kerguelen, while others are formed by coral buildup, such as the Cocos Islands. Many were unpopulated until recent times, yet in the last few centuries several of them, taking account of the deep structure matter of their location, have acted as hinges, connecting very distant parts of the ocean. There are of course variations to do with size and distance from the continent: for example, Sri Lanka has been profoundly influenced by its larger neighbour to the north. Some smaller islands contiguous to the continent are hardly to be considered as islands at all. Kilwa, Mombasa, the islands off the Burma coast, are really just partially detached parts of the mainland. Others are so large as to share mainland characteristics, where the influence of the sea is not paramount: Madagascar, Sumatra, obviously Australia.
Even these deep structural topological characteristics of our ocean can change over time. We will consider changes in climate presently, but some coastal areas have been profoundly affected by other factors, most obviously the silting up of rivers. The Gulf of Cambay has contracted quite substantially. Once it extended up to where Ahmadabad is located. Vallabhi, now 40 kms inland, was once a riverine port. The ground level at the spot where the Tigris and Euphrates meet has risen 20 feet over the last few millennia.
The next deep structure element in the Indian Ocean which constrained human movement was the monsoon winds. Felipe Fernández-Armesto claims that what really matters in maritime history is wind systems, and especially the difference between monsoonal systems, and those with year-long prevailing winds. The monsoons follow a quite regular pattern, in the Arabian Sea essentially southwest from May to September, and northeast from November to March. This relatively predictable pattern contrasts strongly with trade wind regions like the Atlantic, where there is a regular pattern of prevailing winds year-round: essentially northeast in the northern hemisphere, southeast in the southern, though both veer more easterly nearer the equator. They are separated, around the equator, by doldrums. North and south of the trades are westerlies, especially strong in the southern hemisphere. While both oceans have predictable winds, more or less, it is clearly much easier to do a round trip in the Indian Ocean than it is in the Atlantic. 'The predictability of a homeward wind made the Indian Ocean the most benign environment in the world for long-range voyaging.'19
In simple terms, the monsoons are generated by the rotation of the earth, and by climate. Heat during the summer warms the continental land mass in the north of the ocean. Hot air rises and creates a low pressure zone at the earth's surface. Moisture-laden air from the sea then moves in to this low pressure area, rises in the upward air current, cools, and so produces clouds and rain. In winter the reverse occurs; as the sea cools more slowly than the land, winds flow out from the land. This pattern is most clearly seen in the Arabian Sea, thanks to the high plateau of Tibet to the north, and warm tropical seas to the south. Another arm of the southwest monsoon avoids southern India and flows directly over the Bay of Bengal to Bengal and Bangladesh: these areas often get the monsoon before Mumbai. It is also in this area that the monsoons sometimes progress