Karst Topography The formation of caves and other associated features in limestone bedrock is called karst topography. Limestone, a sedimentary rock made mostly of the mineral calcite and small amounts of clay, is relatively easy to dissolve with dilute acids, including the weak dilute acid formed when carbon dioxide dissolves in water.
Karst Topography In order to understand karst topography we must first have a basic understanding of the water cycle, the formation of limestone (carbonate) rocks and the geological processes that cause limestone rocks to be near the surface. Having limestone rocks near the surface in Minnesota means we are talking about the driftless area so a basic review of glacial geology will be necessary.
Karst Topography Once we know how things work we will take some time to appreciate what has happened cause some of the wonderful and beautiful structures associated with karst topography. Karst topography also presents unique environmental problems. We will consider some of these and discuss ways to deal with the problems or potential problems.
The Water Cycle
The Water Cycle Water travels through the cycle once in about 10 days. It doesn t matter where we start: 1. Evaporation - Water molecules gain energy from the sun (mostly over the oceans) and change from liquid to gas. 2. Warm, moist air will rise in the atmosphere because it is less dense - it floats. As it rises the air (including the water molecules) cool. Cooling water vapor causes it to condense or change into a liquid.
The Water Cycle 3. Condensing water vapor causes clouds and eventually precipitation in the form of rain or snow. 4. The precipitation collects on the ground in lakes and rivers where it eventually runs off into the oceans.
Detours in the Water Cycle Not all of the water is directly involved in the main water cycle. Water can be taken out of the cycle when it precipitates as snow and is accumulated in snow pack or glaciers. Water can by pass runoff when plants and animals release water vapor in transpiration. Water can be added to the water cycle when snow or ice melts or when ground water is pumped to the surface.
Detours in the Water Cycle Water is also taken out of the Water Cycle when it infiltrates - soaks into the ground. Of course, evaporation doesn t only happen over oceans, it happens everywhere but the vast majority of water vapor is from water evaporating over the oceans. A minor contribution to water vapor is when snow and ice sublime - change directly from a solid form to the gas form.
Groundwater Ground water is defined as water that is under the surface of the earth, trapped in porous rock and is able to move through connecting openings in the rock. Because ground water slowly moves through the connected openings it gets filtered, removing many impurities and contaminants.
Soil Profile
Soil Profile Typical soil has three main parts called horizons A. The top part, composed mostly of decayed organic matter (plant and animal) B. The middle part, composed of highly weathered material - often clay. C. The bottom part, composed of slightly weathered material.
Groundwater
Groundwater The zone of aeration is the ground where it is not saturated with water. There is air in the spaces between rock or soil. This does not mean there is no water, only that more water could be trapped here. The zone of saturation is where the ground has no spaces filled with air. All of the available space is filled with water. The line between these zones is called the water table.
Groundwater At the bottom of the zone of aeration is the capillary fringe. This is a place where water from the zone of saturation rises up into the spaces by capillary action.
Water Table
Ground Water Porosity is the measure of the amount of spaces in the rock or soil that the water could occupy. Permeability is the measure of how fast the water can move between the interconnected spaces. Large volumes of saturated and permeable rock or soil are called aquifers. Aquifers are the source of well water. A spring forms where the water table connects with the ground surface.
Spring
Sink Hole Springs are common in areas with Karst Topography. Rivers erode into the limestone bedrock causing deep valleys that interconnect with the water table. The opposite of a spring is a sink hole. Sink holes are direct openings, almost a funnel, where surface water can run directly to the water table. Sink Holes & other Karst Features
Winona State Geology MN Sink Holes Other resources: Karst Topography 1. Find the hand out from the Minnesota Geological Survey titled Caves in Minnesota. Pay particular attention to Figure 3 Karst drainage and Figure 4 evolution of sinkholes.
Environmental Concerns What unusual concerns about the environment (pollution) are related to Karst Topography? http://www.pca.state.mn.us/water/groundwater/ karst.html