The base of northern Guam (and land surface of much of souther Guam) comprises of volcanic units of the Alutom Formation. The northern Guam limestone plateau sits atop of the Alutom volcanics. The volcanic base penetrates through the limestone and forms three volcanic inliers within the plateau: Mt. Santa Rosa (shown on the photo above), Mataguac Hill, and Palii Hill. These are the only places where volcanic units can be seen at the land surface in northern Guam. They consist of tuffaceous shale, volcanic boulder conglomerate, and coarse breccia.

According to Reagan and Meijer (1984), the Alutom Formation is over 1000 feet (300 m) thick and is composed of a lower (Late Eocene) section of altered volcanic breccias inter-bedded with tuffaceaous sandstones, shales, limestones, volcanic breccias, and minor pillow lavas, with an upper (Early Oligocene) section of coarse volcanic breccias interbedded with a few fine-grained sedimentary layers, some conglomerates, and a few pillow lavas. The sandstones and shales contain angular volcanic and biogenic fragments supported by a clayey or calcareous matrix. The breccias and conglomerates include intraformational sandstones and shales, reefal and non-reefal limestones and volcanic rocks. The volcanic clasts are composed predominantly of aphyric basalt, together with highly porphyritic dacite and andesite. Reagan and Meijer reported finding a greenschist metavolcanic rock in the Alutom Formation. Shallow-water limestone clasts near the base of the Alutom Formation indicate that the edifice from which the lavas and breccias erupted was near sea level early in its eruptive history. The final Oligocene event recorded on Guam is the intrusion of sills into the Alutom Formation around 32 million years ago.

Factsheet:

Click below to open full-sized version.

Maps:

Use the menu to the right to view relevant maps.