Lecture 19 - Groundwater and Cave Formation

Outline

I. Groundwater and How Caves Form

II. Mammoth Caves

III. Carlsbad Caverns and Guadalupe Mtns National Parks

I. Groundwater and How Caves Form

Caves form in humid environments from dissolution of limestone by acidic groundwater, which forms when carbon dioxide from the atmosphere or from decaying organic material in the soil combines with rainwater. Limestone terranes that have dissolution features like caves or sinkholes are called karst. The chemical reactions leading to limestone dissolution are given below:

water + carbon dioxide -------> carbonic acid

H2O + CO2 --------> H2CO3

carbonic acid + limestone -----> calcium + bicarbonate

H2CO3 + CaCO3 -------> Ca + 2HCO3

When calcium and bicarbonate are carried dissolved in groundwater they can reprecipitate as another form of limestone called travertine when the water seeps into an open cavity and CO2 is released. Speleothem is the term given to travertine precipitated in caves and the terms stalactite and stalagmite are specific forms of speleothems depending on whether they form on the roof (stalactite) or the ground (stalagmite) of the cave.

Groundwater is subterranean water in pore space or voids in rock. More groundwater exists then is present in all surface water. Rain is the source of groundwater that percolates downward through a zone that still has empty pore space in the rock called the vadose zone to the region completely saturated with water called the phreatic zone. The boundary between these two regions is called the water table, which rises and falls in response to the changing amount of rainfall.

The hydrologic cycle refers to the flow of water between reservoirs on the Earth's surface (oceans, lakes, rivers), in the atmosphere and underground (groundwater). The Earth's total water supply is constant so that even though water cycles from one reservoir to another, all sources and sinks of water in each reservoir must balance out.

Water Resource Issues

For years most people in the U.S. took their water supply for granted. Recently, the threat of significant future water shortages has provoked some conservation measures but are they enough? consider this:

1. Humans can survive on about 2 liters of water per day however average use in the U.S. for all purposes is about 6000 liters per day.
2. Domestic water use in the U.S is about 2-4 times that in Western Europe.
3. Agricultural and industrial water use accounts for over 80% of all water use and has increased drastically in the last 40 years.

This high demand for water has impacted many National Parks. For instance, dams have been built or proposed within or just upstream of many National Park boundaries (Hetch Hetchy in Yosemite, Glen Canyon Dam up stream of Grand Canyon, proposed dams just outside Olympic National Park in Washington, Gunnison National Monument in Colorado, Dinosaur National Monument and Zion National Park in Utah), natural water flow has been severely altered in Everglades National Park to accommodate the increasing population and has endangered the diverse ecosystem bringing many of the formerly abundant species into near extinction (white ibis, wood storks and panthers).

Although the private sector demand for water has risen steadily in the last 40 years, agricultural demands have skyrocketed.

II. Mammoth Caves National Park

Mammoth Caves are formed in a limestone plateau that is capped by less soluble clastic rocks. The limestone was deposited in mid-Paleozoic time in a shallow continental sea. Down warping of the sediments due to the load cause the beds to dip gently to the northwest. This dip of the beds causes groundwater that seeps into the limestone to flow down slope dissolving limestone and forming the caves as nearly horizontal tunnels paralleling bedding surfaces. The groundwater emerges as springs in a few major rivers that exist on the plateau. Cave growth continues today at a rate of about 1 mm/yr.

Several kinds of speleothems are present in Mammoth such as:

1. flowstone- travertine deposit left by water flowing over a wall to form a sheet or drapery
2. stalactite- icicle-like travertine deposit from water dripping from the cave ceiling
3. stalagmite- icicle-like travertine deposit from water dripping from the cave ceiling and building mound on cave floor
4. gypsum flowers- dry cave cavities draw water out of wet limestone that evaporates very quickly and deposits gypsum (calcium sulfate)

Development of Mammoth Caves:

1. Deposition of thick limestone unit in Shallow Ocean, which is overlain by clastic sediments in mid-Paleozoic time.
2. Sub aerial exposure and erosion with beginning of karst development in latest Paleozoic time.
3. Development of cave passages as Green River cuts down plateau from late Cenozoic to present.

III. Carlsbad Caverns and Guadalupe Mtns. National Parks

Carlsbad Caverns formed in an ancient uplifted barrier reef that grew in a shallow ocean that covered much of New Mexico and west Texas during the late Paleozoic. This reef was similar to the Great Barrier Reef that exists today off the coast of Australia. The Permian (late Paleozoic) reef is a bit different from the Great Barrier Reef in that rather than consisting of primarily corals, it was built by algae, sponges and bryozoa. This reef complex was uplifted in late Mesozoic time during the Laramide orogeny and makes up the Guadeloupe Mountains.

Reef- submerged, resistant mound formed by the accumulation of plant and animal skeletons. Reefs occur in warm, clear, shallow water with wave action. They have a particular structure consisting of:

• fore reef- high wave action region facing open ocean and consisting of reef debris, piles where blocks of the reef have been worked away by wave action and have come down reef slope.
• reef crest- solid framework of reef, which is usually fairly flat due to wave action on the continuing growing reef.
• back reef- low energy environment consisting primarily of fine sediment

Sediments from all three of these reef environments are visible in the ancient reef complex that makes up Guadalupe Mtns. National Park.

Cave Formation

Unlike Mammoth Caves, Carlsbad Caverns were formed by the dissolution of limestone by sulfuric acid. Sulfuric acid is produced by the oxidation of hydrogen sulfide gas in the following reaction:

hydrogen sulfide gas + oxygen -------> sulfuric acid

H2S + 2O2 --------> 2HSO4

Hydrogen sulfide gas is associated with decaying organic material. This gas produced at depth, combined with groundwater and produced sulfuric acid that was able to travel along fractures in the reef limestone and dissolve it to create the caves that we see today. Evidence for the action of sulfuric acid is seen in the extensive gypsum (calcium sulfate) deposits, which precipitate from sulfate rich waters traveling through the caves.

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This page was last reviewed on 3/1/04.