ISOLATION AND FINITENESS

The mystique of islands is in part due to two physical features: their isolation and their sharply defined boundaries or finiteness (Fig. 2.1). In this chapter, we discuss the physical setting of the nine island groups to provide the background needed to interpret the natural disturbances to which they are subjected (see Chapter 3) and distributions of their plants and animals (see Chapter 4). We discuss the geological origins of each island group and cover the geographic relationships among islands in each group, their topographical relief, and the nature of the coastline; each of these parameters is of key relevance to colonizing organisms, from small barnacles to humans looking for harbors. Next, we discuss the climate of the island groups, including the reasons for variation of precipitation and temperature within islands. We finish by describing how soils are shaped by geological, topographical, climatic, and biological influences on the islands.

GEOLOGY

Oceanic islands have never been in contact with a continent and are largely created when volcanoes form from magma extruded through hotspots or weak zones in crustal plates (Plate 1). Uplifted coral and limestone formed on a volcanic base can also form oceanic islands. Oceanic islands of volcanic origin form when mountains emerge above the sea after extensive deposits of lava. Subsequent movement of the plates away from the hotspots allows the formation of new islands and over time this movement results in island groups.