Northern Guam geospatial information server
Volcanic units in northern Guam
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.