Karst caves in northern Guam
Caves are holes in the ground. More specifically, they are natural underground voids, which people can enter. There are several types of caves, the most common of which are karst (solution) caves. Karst caves are formed by the water dissolution of limestone or similar rocks. As water moves through caves, it dissolves away the rock walls and enlarges the caves. Additional factors shaping karst caves are erosion and collapse. Most caves on Guam are karst caves. The exceptions are artificial caves (WWII era tunnels) and some sea caves, which are created by wave erosion, not dissolution.
Karst caves of northern Guam are divided into two fundamental categories:
1. vadose caves, which are created by water dissolving bedrock in the zone above the water table; and
2. phreatic caves, which forme below the water table.
In northern Guam, extensive vadose caves are found only on the flanks of Mt. Santa Rosa and Mataguac Hill, where the volcanic slopes capture meteoric water, focus it into ephemeral streams, and deliver it to depressions on the contact with the surrounding limestone. From these, it descends in stream caves that follow the contact to the water table. Although stream caves in general may fundamentally be vadose features, stream caves on Guam show ample evidence of phreatic dissolution as well. Observations in uplifted caves that show evidence of previous phreatic dissolution suggest enlarged chambers develop at the juncture with the water table. Stream caves may also undulate beneath the water table, and may undergo phreatic flow over their entire length during major storms, which are frequent in the Mariana Islands. Moreover, given the complex history of tectonic and glacio-eustatic changes in relative sea level, caves on Guam can be alternately overprinted by prolonged episodes of emergence and submergence.
Other vadose caves in northern Guam are simpler. These include dissolutionally widened fractures and vertical shafts called pit caves. These are not associated with volcanic inliers and can occur anywhere that structural and lithologic conditions are conducive to focused vadose flow. A striking example is seen at the Two Lovers Park at Amantes Point, where a walkway has been built over the deep vertical shaft.
The vast majority of caves in northern Guam are phreatic caves, produced by dissolution within the water-saturated zone of the aquifer. They are fundamentally different from vadose caves in that there is no turbulent flow of free-surface streams involved. In most places within the groundwater lens, water may be saturated with respect to calcium carbonate (mineral that comprises limestone) and thus may not be able to dissolve any voids. However, a phenomenon called mixing corrosion can create dissolutionally aggressive waters locally and produce significant caves. This happens when waters of different chemistry, each of which may even be saturated in calcium carbonate, mix to form an undersaturated solution. In northern Guam and similar places, such mixing occurs at (1) the top of the fresh water lens, where downward percolating vadose fresh water mixes with phreatic fresh water; (2) the bottom of the lens, where phreatic fresh water mixes with phreatic seawater; and (3) the outer margin of the lens, where the top and the bottom mixing zones converge. The caves that form in these three zones, respectively, are called water table caves (Mylroie & Carew 1988) or, where ceiling collapse has opened them to the surface, banana holes (Harris et al. 1995); halocline caves (Palmer & Williams 1984); and flank margin caves (Mylroie & Carew 1990). Because they form without connections to the land surface, these caves cannot be entered by people unless they are breached by erosion.
Banana holes occur in northern Guam, but are rare. Rapid uplift of the island may have brought most unbreached water table caves out of the phreatic zone before they were sufficiently extensive and thin-roofed to be vulnerable to collapse. Lenticular voids, which are common in the walls of deep quarries, where some of them appear to lie along uniform horizons, sometimes associated with changes in color and texture of the bedrock, could be either water table or halocline caves. By far the most common type are flank margin caves, because they form in the zone of continual and rapid erosion at the coast and are regularly exposed by cliff collapse along coastal cliffs and terraces. They are observed not only at modern sea level, but also below the sea level and high above in cliff faces, at elevations that indicate previous sea levels stillstands. Finally, some coastal caves may be ordinary sea caves formed by wave erosion. Given the rate at which flank margin caves are exposed by coastal processes, however, even what appear to be ordinary sea caves can actually be overprinted flank margin caves, whose origin is obscured by recent wave erosion. Flank margin caves on Guam range in size from a few meters to tens of meters. Guam’s best known caves, including Pagat Cave, Marbo Cave, and Fafai Cave, are flank margin caves.
Taboroši et al. (2003) provide a systematic
classification of caves of Guam
with illustrations and maps.
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