Week 8The Meeting of Cultures in the Pacific[1]
Part 1: The Physical Geography of the Southwest Pacific.
In this lecture, I shall try to outline the basic physical geography of the island realm that we are studying, starting with the geology, moving to the climate, and finally touching briefly on the mineral resources.
First, however, I want to deal with the three-fold division of the area into ethnic regions. Polynesia, Melanesia and Micronesia are names conferred by Europeans, as the Greek roots of the names indicate. The Shorter Oxford Dictionary dates the word, Polynesia, to 1766; a word invented by a French writer, meaning 'many islands'. The word, Melanesia, meaning 'islands inhabited by black people', dates from 1849, long after a distinction between the natives of the two regions, based on differences of skin colour and language, had been recognised by European explorers. Finally, the name, Micronesia, meaning 'region of small islands' was coined in 1896, a rather belated recognition that the islands to the north of New Guinea and Fiji, and west of Polynesia, were inhabited by people
Polynesia is roughly triangular in shape, with the apices of the triangle near Hawaii, New Zealand and Easter Island. It was Captain James Cook, who discovered the Hawaiian Islands and who explored New Zealand and Easter Island, who first commented on the affinities of language, culture and physical appearance between the peoples of this enormous area. He speculated on how they could have established themselves over such a vast and relatively empty expanse of ocean and posed the questions we now refer to as the 'Polynesian Problem': how did they get there, and when?
If we regard the base of the Polynesian triangle as the line joining Hawaii and New Zealand, passing between Melanesian Fiji on the west and the Polynesian Samoan and Tongan groups on the east, we may visualise it as an arrow-head pointing into the Pacific. The shaft of the arrow is the line separating Micronesia from Melanesia. On no account should this 'model' be seen as a set of hard-and-fast boundaries. There are a number of island communities scattered through Micronesia and Melanesia that are distinctly Polynesian, and are known collectively, as Polynesian outliers. How they came to be there is part of the 'Polynesian Problem' that I shall discuss in Part II. Note, however, that we do not find outliers of either Melanesia or Micronesia within the Polynesian Triangle.
Continental and Oceanic Islands.
The islands of Melanesia have more in common than the skin colour of their native peoples. They are typically much larger in area than the 'oceanic' islands of Polynesia and Micronesia, whether these be 'high' islands of volcanic origin or 'low' islands of coral sand, perched on coral reefs. We refer to the Melanesian islands, and New Zealand, as 'continental' islands, because the rocks have much more variety, including older sedimentary rocks, such as sandstones, and volcanic rocks other than basalt, which characterises the high islands. Geologists distinguish between acid, intermediate and basic volcanic rocks. An example of an acid rock is rhyolite; andesite is an intermediate type, while basalt is the commonest form of basic volcanic rock. These three examples are all lavas, extruded by volcanoes during eruptions, basalt being noted for the ease with which it flows while in a molten state. Rhyolite, on the other hand, is liable to clog the pipe of the volcano and cause an explosive eruption, with layers of pumice and ash being deposited around the volcano.
These extrusive rocks correspond, in their chemical composition, to three types of plutonic rocks, a term given to rocks that crystallise from the molten state without reaching the surface, though they are often exposed later by erosion. The best known acid plutonic rock is granite; diorite corresponds to andesite, and gabbro to basalt. The acid rocks are rich in quartz, the main component of sandstones (and the sand of the beaches of eastern Australia).
The distinction between continental and oceanic islands in the western Pacific was first noted by a New Zealand geologist, Patrick Marshall, who identified what he called the 'Andesite Line' separating the western margin of the Pacific Ocean, from the central Pacific proper. West of the Andesite Line - sometimes called the Marshall Line - acid and intermediate volcanic rocks are found, as well as basic rocks. To the east of it, only basalt, occurs. The line is characterised by a series of deep trenches in the ocean floor, e.g. the Aleutian and Kurile Trenches in the North Pacific; the Mariana Trench, south of Japan, which includes the Challenger Deep (10 915 metres) the greatest depth known; and the Kermadec and Tonga Trenches in the South Pacific, which also exceed 10 000 metres.
Modern geologists identify Marshall's Andesite Line as a plate boundary, marking the western edge of the very large Pacific Plate, a section of the earth's crust that is moving slowly but inexorably to the north-west from the area known as the East Pacific Rise, where the extrusion of basaltic lavas on the sea floor and a steady movement away from the Rise, in both directions, creates new areas of sea floor. The same process, along the Southeast Indian Ridge, between Australia and Antarctica, impels the Australian Plate northwards. The trenches mentioned earlier lie along the edges of the advancing Pacific plate, and are regarded as subduction zones, where the advancing plate disappears beneath the edge of the adjoining plate, a process that gives rise to earthquakes and volcanic activity. The 'Ring of Fire' - active volcanoes - that encircles the Pacific, is matched by zones where earthquakes are much more common, and violent, than we are used to in Australia.
Out on the surface of the Pacific Plate's sea floor, volcanic activity is comparatively rare, despite the evidence of unmistakeable volcanic remnants in islands such as Tahiti, Pitcairn, Rapa, and Rapanui (Easter I.). The exception is found in Hawaii, where craters full of molten lava may be seen, and where basalt flows down the mountain sides and sometimes reaches the sea. The older islands of the Hawaiian chain extend to the north-west of Hawaii itself, which is consistent with the theory that the Pacific plate is passing over a crustal 'hot spot' where eruptions have created a succession of volcanoes, Hawaii being the youngest, Midway I. the oldest. Beyond Midway, however, there is a chain of sea mounts that may represent an earlier phase of the same hot spot's activity, where the volcanoes failed to reach the (present) level of the sea. Similar sea mounts occur in the South Pacific. The movement of the Pacific Plate over other hot spots offers an explanation for the linear arrangement of other island groups.
The Coral Island
Between latitudes 30° N. and 30° S., provided that the sea temperature is above 20° C., the water is clean, well aerated and not fresh, the coral polyp can thrive down to about 60 metres below the surface. This tiny animal secretes calcium carbonate from the seawater and builds it into the variety of coral formations familiar to anyone who has visited the Great Barrier Reef. As coral colonies die, new ones are formed, building on the limestone base created by their predecessors. Because the food supply is greatest where there is a lot of water movement, corals thrive on the seaward edges of the reefs they build, despite the damage done by breaking surf. The fragments are thrown on to the inner side of the reef, and tend to become cemented into limestone. Coral colonies can stand some exposure to the air, and reefs tend to develop a flat upper surface at about the upper one-third mark of the tidal range. The reef is thus covered at high tide and exposed at low tide.
Coral reefs occur as fringing reefs attached to the shores of high, volcanic islands, (or other rocks); as barrier reefs separated from the mainland by a lagoon, from the floor of which a variety of coral formations may grow; and as atolls, where there is a more or less circular set of barrier reefs surrounding a lagoon, but with no evidence of a high island base from which the growth process might have begun. Observing atolls in the Pacific and Indian Oceans in the course of his voyage round the world as naturalist on HMS Beagle (1831-36), Charles Darwin tested a theory put forward by the pioneer geologist, Sir Charles Lyell, that atolls represented the coral-encrusted rims of extinct volcanic craters. He concluded that this was not so; that the fringing reef on the outer slopes of a volcanic island would grow upwards if the island subsided slowly into the ocean, forming a barrier reef around a lagoon, in the centre of which the tip of the volcano would remain as a much smaller island. Further subsidence of the sea floor might drown the volcano's tip but the upward growth of the reef would create the atoll. The accumulation of coral sand and other debris would create the 'cays' where plants might establish themselves.
Darwin was not overly concerned as to why the sea floor should subside. Later workers in the same field found a mechanism of which he was not aware, namely that sea level fell during the ice ages when water was locked up in the enormous ice sheets that covered so much of North America, Europe and Siberia, and then rose again, between 16000 and 6000 years ago, at a rate slow enough for the coral reefs to keep pace. (A rise of 120 m in 10 000 years averages at 12 mm per year.) It follows that during the period of lowered sea level, reefs left high and dry would have been eroded by wind and rain, while new reefs might have begun to grow at the new shorelines, provided that the sea temperature remained warm enough for the corals to survive. If these new reefs grew more or less vertically upward - and the outer face of a barrier reef is notoriously steep - as sea level rose again, barrier reefs and atolls could form , and the islands within the lagoons could be submerged.
There is evidence in some areas that the sea floor and adjoining land masses have risen even faster than the sea itself. On the Huon Peninsula, in New Guinea, there is a famous set of raised coral reefs, so well preserved that they have been dated with great accuracy. Similar rises of the sea floor in parts of the Pacific have created island forms of which we need to be aware. Nauru is a raised atoll, with the limestone of its lagoon floor now well above sea level. The phosphate rock that is the island's principal resource, is quarried from between the pillars of coral that once rose from the lagoon floor. Niue is another example of a raised atoll, as are some of the southern group of the Cook Islands. Swains Island, north of Samoa, is a partly raised atoll, with a partial lagoon (now fresh) surrounded by 'makatea', the cavernous limestone of the former lagoon floor. (The word comes from the name of a raised atoll in the Tuamotu archipelago.) And, just as an island with a surrounding lagoon and cays on its barrier reef is called an 'almost atoll', e.g. Aitutaki, so a similar combination raised from the sea is termed a 'raised almost atoll'. Atiu in the Cook Islands is one example.
Climate
As the island realm we are studying lies within thirty degrees of the Equator, its climate is dominated by the trade wind circulation, i.e. winds from the south-east or east, in the southern hemisphere, from the north-east or east in the northern hemisphere. The flow is strongest and most persistent in the winter season of the hemisphere, and the intertropical front, where these two air streams converge, tends to move north and south with the overhead sun. On the poleward edges of the trade wind belt, westerly winds can occur in winter. Near the Equator, in summer and early autumn, especially in the western Pacific, hurricanes (or tropical cyclones or typhoons) occur from time to time, typically moving westward and then swinging poleward. For example, a storm developing over the Coral Sea might approach the Queensland coast, then swing to the south, running parallel to the coast before veering away towards New Caledonia or even towards New Zealand. If they cross the Australian coast they tend to lose their strength - their energy comes from the warm sea - and they 'deteriorate into rain depressions' as the weather reports like to say. The damage that a hurricane can do to the coconut palms and houses on an atoll is quite frightening. Another hazard of atoll life is the threat of inundation by 'tsunami', the waves set up by submarine earthquakes.
The trade winds bring rain to the windward side of the high islands, while the lee side of the islands may have a much lower rainfall. On Viti Levu, in Fiji, the 'wet zone' and the 'dry zone' are notionally divided by the 2500 mm(100 inch) isohyet! Many atolls, however, lack the height to induce the trade winds to yield any rain, and often suffer long dry spells, even droughts. Some atolls lack the drinking water to sustain permanent settlement. It is the lack of rain that has preserved the soluble phosphate rock on islands such as Nauru.
The meteorological variations that make the weather a topic of conversation in temperate regions have little meaning in the trade wind belts, except that the summer or 'hurricane' season is usually the wet season, and the winter, when the trade wind flow is most persistent, is the dry season. Temperatures rarely exceed 30° C. and as rarely fall below 20° C.
Climates become more complex in Melanesia, as the islands are higher and larger, and are closer to Australia, which draws in 'monsoon' winds from the northern hemisphere in the southern summer. This flow affects New Guinea in particular. Its size, and the existence of extensive areas of highland, not to mention mountains high enough to have carried glaciers in the past, creates a diversity of climatic types.
There is a growing awareness of cyclic phenomena in matters of weather and climate in the Pacific. You may come across references in the literature to the 'El Nino' effect and/or the 'Southern Oscillation'. The two are related: the Southern Oscillation is an 'observed regularity', viz, that when mean air pressure over a period is above average in Darwin, it will be below average in Tahiti, and vice versa. Low pressures are associated with storms and rain; high pressures with droughts. When pressure is lower than usual over the eastern Pacific, there is a higher likelihood of the 'El Nino' phenomenon occurring along the coast of Peru. This is the appearance of a narrow stream of warm water moving south along the coast, inside the north-moving Peru Current. The latter is a cold current, driven by the off-shore S.E.Trade winds, its coldness reinforced by the up-welling of cold water from well below the surface, to replace the water moving off-shore. The up-welling brings nutrients up into the euphotic zone (i.e. the layers penetrated by sunlight) where they can be used by phytoplankton, which in turn are grazed by the zooplankton, on which fish feed. The Peru Current was formerly one of the great fishing grounds of the world, and its fish were also food for a large population of seabirds, who nested on rocky islands along this desert coast, depositing guano that accumulated over centuries to become a 'resource', exploited in the nineteenth century. An incursion of the Equatorial Countercurrent along the coast - an 'El Nino' event, because it typically happened about Christmas time - brought rain to the desert and the guano islands, while the warm water killed the fish, which starved the seabirds. In the same southern summer, Australia was likely to be having a drought. In the majority of years, with no El Nino, the wet season in northern Australia would be as it usually was. This complex interrelationship is being closely studied, as an aid to long range forecasting, and the meteorology of the Pacific is coming to be better understood. Sea surface temperatures - which, when high, are associated with the development of hurricanes - are now monitored from satellite information.
The 'greenhouse effect', in which increases in the carbon dioxide content of the atmosphere impedes the escape of long-wave radiation from the earth, thus raising air temperatures, holds serious threats for the inhabitants of the low islands of the Pacific (and other oceans). A rise of only a few degrees is thought to be enough to melt the Antarctic and Greenland icecaps, releasing enough water to raise sea levels all over the world by many metres. Increased storminess is another probable consequence of the rise in temperatures. The prospects for island dwellers, as for the inhabitants of coastal lowlands everywhere, are undeniably gloomy. Experts are divided on whether or not the greenhouse effect is already beginning to be felt. Unfortunately, one can only distinguish between a fluctuation and a new trend with hindsight!