Groundwater Erosion

Erosion and deposition occur below ground as well as at the surface. The processes of chemical weathering causes much groundwater erosion, including the formation of caves and sinkholes. Recall that carbon dioxide in the air combines with rainwater to form carbonic acid. As the acidic rainwater moves down through the ground, it reacts with some rocks and may become more acidic.

Limestone easily erodes away through this process, forming caves, or caverns. Most caves are small, narrow passages, but some are hundreds of feet high and wide. Caves usually form when they are in the saturated zone, below the water table. When the water table drops, the water flows downward, leaving dry caves like the one in Figure 17.

Sometimes water drips into the cavern from the rock layers above, carrying dissolved minerals. When this mineral-laden water reaches the air of the cave, some of the dissolved carbon dioxide escapes and the minerals are left behind. If the water drips from the cavern ceiling, an icicle-like formation called a stalactite (stuh LAK tyt) grows. If the water drips down to the floor, a pillar of minerals called a stalagmite (stuh LAG myt) forms.

If erosion weakens a layer of limestone, entire portions of the ground can suddenly collapse. The resulting hole is called a sinkhole. Areas of the southern and central United States have many sinkholes. Sinkholes can appear suddenly, swallowing buildings and roads.

Figure 17 Carlsbad Caverns in New Mexico contain spectacular formations, such as the stalactites and stalagmites shown here.

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