Groundwater Erosion and Deposition - Not all water that falls on the land flows through rivers and streams. When it rains, much of the water sinks into the ground and moves through pore spaces in soil and cracks and fractures in rock. This water necessarily moves slowly, mostly under the influence of gravity. Yet groundwater is still a strong erosional force, as this water works to dissolve away solid rock. If you have ever explored a cave or seen a sinkhole, you have some experience with the work of groundwater.
Formation of Caves
As groundwater moves through spaces between mineral grains, it works to dissolve and carry away different elements. Some types of minerals are easily dissolved by groundwater. Rainwater absorbs carbon dioxide (CO2) as it falls through the air. The carbon dioxide combines with water to form carbonic acid. This naturally occurring weak acid readily dissolves many types of rock, including limestone.
If you have ever watched an antacid tablet dissolve in water, you have seen an example of just how quickly this type of rock is eroded away. Caves are one of nature’s most spectacular demonstrations of erosion. Working slowly over many years, groundwater dissolves and carries away elements of once solid rock in solution. First it travels along small cracks and fractures, gradually enlarging them. In time, caverns many football fields long and as high as many meters tall can form.
A sinkhole could form if the roof of an underground cave collapses. Some sinkholes are large enough to swallow up a home or several homes in a neighborhood. As groundwater dissolves away solid rock, it carries those minerals in solution as it travels. As groundwater drips through openings, several interesting types of formations occur. Stalactites are icicle like deposits of calcium carbonate which form as layer on layer of calcite drips from the ceiling, coating the ‘icicle’.
As mineral rich material drips to the floor of a cave, stalagmites form rounded deposits of calcium carbonate on the floor of the cave. The word stalactite has a ‘C,’ so you can remember it forms from the ceiling, while the ‘G’ in stalagmite reminds you it forms on the ground. If a stalactite and stalagmite join together, they form a column. One of the wonders of visiting a cave is to witness the beauty of these amazing and strangely captivating structures.
Caves also produce a beautiful type of rock, formed from calcium carbonate called travertine. This happens when groundwater saturated with calcium carbonate suddenly precipitates out as the mineral calcite or aragonite. Mineral springs that produce travertine can be hot springs or the water may just be warm or could even be cold.
When lots of calcium carbonate is carried by groundwater, we call the water ‘hard.’ If the water in your area is hard, it might be difficult to get soap to lather or make soapsuds. Hard water might also have a taste to it, perhaps one that some people don’t like as much as pure water. If your water is ‘hard,’ you may treat your water with a filter before you drink it. Zeolites are minerals that help to absorb ions from the water as it passes through the filter. When the water passes through the filter, it comes out tasting good!
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