In geological terms the two arcs of islands that form the Mariana Islands archipelago (Fig. 5.1) are situated at the junction of two tectonic plates composing the earth's crust. The islands are located on the edge of a subduction zone where the Pacific Plate, moving westwards, dips below the Philippine Plate. Guam is the largest island in the region, having a total land area of 544 square kilometres; the rest of the Marianas group has a combined land area of 478 square kilometres. The islands generally diminish in size in a south to north direction, and on a conventionally coloured map they appear to fade away to blue.

All of the islands north of Saipan, sometimes called the Gani group, are volcanic in origin, consisting of dark igneous rocks (Russell 1998a). The subduction zone, although responsible for the creation of the islands in the first place, also leads to an unstable archipelago, with many of the northern islands volcanically active and all of the islands susceptible to earthquakes. The Marianas are also known for the frequent occurrence of typhoons and droughts. Guam on average experiences a typhoon every three and a half years, and a super-typhoon once a decade. These storms can cause extensive and severe damage to both crops and structures.

The majority of the archaeological evidence derives from the larger southern islands, namely Guam, Rota, Tinian and Saipan. Guam is composed of a limestone plateau in the north and volcanic mountains in the south.