Geology of Guam

The geology of Guam formed as a result of mafic, felsic and intermediate composition volcanic rocks erupting below the ocean, building up the base of the island in the Eocene, between 33.9 and 56 million years ago.

The second crater collapsed and Guam went through a period in which it was almost entirely submerged, resembling a swampy atoll, until structural deformation slowly uplifted different parts of the island to their present topography.

The process of uplift led to widespread erosion and clay formation, as well as the deposition of different types of limestone, reflecting different water depths.

The formation contains basalt and andesite, as well as volcanic breccia with component dacite pebbles and cobbles and tuff shale cemented together with calcite.

Stabile shallow water reef-forming environments by the end of the Eocene gave way to violent volcanic eruptions in the Oligocene.

Eruptions resumed after the deposition of the limestone, generating hundreds of feet of tuff breccia and volcanic conglomerate, interspersed with periodic lava flows.

In fact, the calcareous lenses of the Maemong limestone, which hold the oldest Miocene fossils are underlain by sequences of pillow lava hundreds of feet thick.

Higher parts of the Tenjo block were probably eroded, given the presence of early Miocene reefs in central Guam.

Streams are only found in southern Guam, where volcanic rock with low-permeability slows the infiltration of rainwater into the ground.

Minor areas of perched water occur in limestone, atop nearly impermeable volcanic rocks at different points.