I think that you really have to look at the impact of human beings on the geography here. One of the major things that happens in the book, geography-wise, is when "Wild Whip" and Kamejiro Sakagawa drive the tunnel through the mountains to bring water to the leeward side of O'ahu. By doing this, they completely transform the geography of the island, particularly in the area of Waipahu and all the other places that used to be pineapple fields back in the old days.
This book also sheds some light on the impact of geographic isolation. The Hawaiian islands were essentially settled twice - once by the Polynesians, and then a second time by the white traders, explorers, and missionaries - and Michener takes you through both settlement processes.
Physical isolation has long been recognized as a factor that can drive biological evolution. In this book, Michener shows us how it can drive the evolution of a society as well.
You also learn about how the lifestyle of the inhabitants was affected by the geography of the islands. Certain groups farmed for certain crops. Other groups gathered various types of products from the ocean. Building materials, resources to make everyday implements and items, and other aspects of daily life were shaped through the impact exerted by the geography of the area.