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2 Geography of Bougainville and Buka Islands
2.1 General Orientation
The Solomon Islands lie on the southwestern margin of the Pacific Ocean and come within the circum- Pacific volcanic belt. They are made up mostly of Tertiary and Quaternary volcanic rocks and derived
sedimentary rocks, but also include some organic limestones. Bougainville and Buka Islands form a single land mass separated from one another by
only a shallow, half-mile strait. Together, they are about 240 kilometers long and about 64 kilometers across at their widest point. They are located along a north-west-south-east
axis.… The total land area is approximately 9000 square kilometers minus some 13 square kilometers of lakes and some other expanses of permanent freshwater swamps Oliver
1973:7.
Bougainville Island, on which the Rotokas people live, has a mountainous interior consisting of the Emperor, Crown Prince, and Deuro Ranges, the active Bagana volcano
1747 meters, the dormant Balbi 2593 meters and Loloru volcanoes, and a number of extinct volcanoes, including Billy Mitchell, Takuan 2245 meters, and Taroka 2201
meters; crater lakes occur in the centers of Loloru and Billy Mitchell volcanoes Blake and Miezitis 1967:1.
The climate is of the wet-tropical or tropical-rainfall type, and it is remarkably equable the year round. The mean annual temperature at sea level is about 26.7 degrees C; monthly
sea-level means vary only a degree or so above or below that mark, and the average diurnal range at sea level is only about 10.6 degrees C. Temperatures are lower at higher elevations
according to records from comparable places, mean temperature undergoes a drop of 1.35 degrees C with every 300 meters, but here also they change within quite narrow monthly
and diurnal ranges, and nowhere reach conditions of frost.
The alternating wind systems that affect these islands consist of a variable set from the northwest, which occurs between December and April, and a stronger more continual set
from the southeast, which prevails from May to December. These changes in wind have little discernible effect on sea level temperatures but they do exert some influence on patterns of
rainfall, particularly in the north…
[An observation made by a Rotokas man was that the west wind is a “cleaning wind” which clears off rain, while a wind from the east blowing from the Pipipaia area towards
Togarao brings rain. This only confirms the general pattern of rain and wind during the very wet season when winds coming up the Wakunai Valley from the east bring heavy rainfall.]
Average rainfall at sea level is higher in the south about 3353 millimeters per annum than in the north about 2667 millimeters per annum, and regional topographic factors serve
to extend these differences somewhat. Rainfall also tends to increase with elevation but there are too few records available to indicate how much. The north-westerly winds
December–April distribute about the same amount of rainfall over all parts of both islands. During the south-east season May–December, however, the moisture laden winds deposit
more of their water on the southern slopes of Bougainville’s mountains, thereby accounting for the higher average annual rainfall in the south. As a result of this circumstance, Buka and
north Bougainville undergo a ‘dry season’ during this part of the year. But it is only relatively dry—or rather, relatively less wet; the longest recorded period without rain
anywhere in these islands is only sixteen days, and the mean duration of rainless days for both islands is three days and under a month for all months of the year.
Finally, the equability of the climate is further indicated by figures for relative humidity. On the basis of observations at sea level stations, the mean monthly recordings range
28 between only 75 and 86 per cent, and diurnal variations for any one station are even less
Oliver 1973:9–10].
Map 2 – Rotokas area of Bougainville Island
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2.2 Physical Characteristics of the Rotokas Area
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