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Geosciences LibreTexts

9.10: Karst

  • Page ID
    11258
  • Steep rock towers left as remnants of limestone layers dissolved away by acidic rain and groundwater.
    Figure: Steep karst towers in China left as remnants as limestone is dissolved away by acidic rain and groundwater.

    Karst refers to landscapes and hydrologic features created by the dissolution of limestone. Karst can be found anywhere where there are limestone and other soluble subterranean substances like salt deposits. The dissolution of limestone creates features like sinkholes, caverns, disappearing streams, and towers.

    Sinkholes of the McCauley Sink in Northern Arizona, produced by collapse of Kaibab Limestone into caverns caused by solution of underlying salt deposits
    Figure: Sinkholes of the McCauley Sink in Northern Arizona, produced by the collapse of Kaibab Limestone into caverns caused by the solution of underlying salt deposits.

    The dissolution of underlying salt deposits has caused sinkholes to form in the Kaibab Limestone on the Colorado Plateau in Arizona.

    Sinkhole that appeared in Florida in the front yard of a home.
    Figure: Sinkhole from the collapse of the surface into an underground cavern that appeared in the front yard of this home in Florida.

    Karst forms when natural water, in combination with carbon dioxide, creates carbonic acid and dissolves calcite (calcium carbonate) in limestone. Remember that CO2 in the atmosphere dissolves readily in the water droplets that form clouds from which precipitation comes in the form of rain and snow. Thus precipitation is slightly acidic with carbonic acid.

    Water + Carbon Dioxide Gas equals Carbonic Acid in Water:

    \[\ce{H2O + CO2 -> H2CO3}\]

    Solid Calcite + Carbonic Acid in Water Dissolved equals Calcium Ion + Dissolved Bicarbonate Ion:

    \[\ce{CaCO3 + H2CO3 -> Ca^{2+} + 2HCO3^{-}}\]

    Calcium carbonate deposited at Mammoth hot springs encapsulates trees.
    Figure: Mammoth hot springs, Yellowstone National Park.

    After the slightly acidic water dissolves the calcite, changes in temperature or gas content in the water can cause the water to redeposit the calcite in a different place as tufa (travertine), often deposited by a spring or in a cave. Speleotherms are secondary deposits, typically made of travertine, deposited in a cave. Travertine speleotherms form by water dripping through cracks and dissolution openings in caves and evaporating, leaving behind the travertine deposits. Speleotherms commonly occur in the form of stalactites, when extending from the ceiling, and stalagmites, when extending from the floor.

    Cave deposits hanging and protruding from the base of a cave.
    Figure: Varieties of speleotherms.
    Stream disappears into subterranean cavern system.
    Figure: This stream disappears into a subterranean cavern system to re-emerge a few hundred yards downstream.

    Meteoric (surface) water enters the karst system through sinkholes, losing streams, and disappearing streams. Changes in base level can cause rivers running over limestone to dissolve the limestone and sink into the ground. As the water continues to dissolve its way through the limestone, it can leave behind intricate networks of caves and narrow passages. Often dissolution will follow and expand fractures in the limestone. Water exits the karst system as springs and rises. In mountainous terrane, dissolution can extend all the way through the vertical profile of the mountain, with caverns dropping thousands of feet.