Mount Pleasant Caldera rock. The Intercaldera Sequence comprises formations that crop out from overlying flows in triangularly shaped area, and includes thick volcanic ash (tuff), thick breccia layers, and intermediate to felsic igneous rocks that tend to intrude the above layers and are typically located along caldera margin faults. The Exocaldera Sequence contains ash flow tuffs, mafic lavas, alluvial redbeds, and porphyritic felsic lavas that are distributed across five different layers. The late Caldera-Fill Sequence contains rocks that are similar to those of the outflows of the other, older layers, and comprises two formations and two relatively minor intrusive lava flows. The volcanic
Geology of Guam a foot, which dropped to 0.05 feet further inland. The Guam Waterworks Authority, Guam Environmental Protection Agency, the Air Force and the Navy manage the 180 wells throughout the island. A combination of high-permeability rock, over pumping and overcrowding of wells resulted in 32% of wells (measured in data from the early 1960s through 1997) exceeding a guideline of 250 mg/L chloride concentration. 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