Northern Guam geospatial information server

KARST TOPOGRAPHY

The word “karst” refers to features and terrain where some of the rock has been dissolved by water. All features formed by dissolution, from the tiny raindrop marks on rock surfaces to the largest caves, are collectively known as karst. They can form only in soluble rocks, the most common of which is limestone

What is karst?

Karst is any topography or feature created by dissolution of rocks. What is most significant about karst is that because the land is composed of soluble rocks, it cannot support much surface water flow in the long term. In karst terrain, rainwater and streams tend to dissolve their way downward and disappear into holes in the rock. This is why karst areas usually lack surface streams. Instead of flowing at the surface, the water moves underground, via pores in the rock, small voids and conduits, and caves. The replacement of surface drainage by underground drainage is one of the defining characteristics of karst topography.

How does water create karst?

Karst forms in soluble rock, such as limestone (CaCO3). Rainwater is unsaturated with respect to CaCO3 and is, therefore, capable of dissolving limestone. This is especially so because rain absorbs atmospheric carbon dioxide (CO2), which, when added to water, makes it slightly acidic and boosts its solvent power. As it falls on limestone surfaces and percolates down, it dissolves the solid rock and removes it in solution. The result are voids where rock has been removed.

What kinds of karst features are there in Guam?

Karst features in Guam include the epikarst (heavily weathered rocks near the surface), sinkholes (points where water enters underground), caves (voids through which water moves underground), springs (points where water flows out from the aquifer), and other landforms. These features control the movement and storage of groundwater.

Where do karst features form in the NGLA?

Karst features are formed only by unsaturated water, capable of dissolving limestone. After dissolving some limestone, the water becomes saturated and can no longer dissolve more rock. Therefore, most dissolution in the NGLA occurs in places where sufficient unsaturated water exists: 1) at the land surface, where rain first comes in contact with limestone; 2) in areas where the flow of water is focused; and 3) in areas where different waters mix and become unsaturated, due to complex chemical phenomena (this occurs at the top and the bottom of the freshwater lens). Voids are produced by dissolution, and further dissolution and collapse enlarges and connects them, ultimately forming a complex underground drainage system.