The geology of the Northern Mariana Islands began to form with undersea volcanic eruptions in the Eocene.
The Sankakuyama Formation is the oldest individual rock unit, which includes Eocene rhyolite found mainly on north-central part of the island.
The andesite lava flows, conglomerate and sandstone of the unit date to between 26 and 28 million years ago, during the Oligocene.
Calcareous marine tuff and interbedded volcanic flow rocks near the village of As Lito and the Fina Sisu hills was named the Fina-sisu Formation in 1956.
Most faults trend north-south and even young rocks from the Holocene show signs of offset.