Duration: 30 to 40 minutes
Vocabulary: limestone, sink hole, continental land masses, crustal plates, geologic time, sediment
Summary: Students learn the origins of sink holes and the environmental issues surrounding them.
Objectives: Students will gain a deeper understanding of Pennsylvania geology and the effect it has on the environment, as well as the ways that human interactions often make these features worse or into garbage dumps.
Materials: USGS PA limestone maps (see Handouts), trash bags, gloves, rock hammers (or regular hammer), observation dishes, vinegar
Most of Pennsylvania is underlain by limestone. Limestone is a type of sedimentary rock made of calcium carbonate. This is calcium that combines with carbon dioxide and while it is hard it can dissolve in water. Geologically speaking, most calcium carbonate or limestone, originates in the ocean. Microscopic life uses it and causes it to precipitate out of the water while other sea creatures use it to make their shells. Eventually these small shelled animals die and fall to the ocean floor, forming layers of calcium carbonate. But how does this rock get “on land?”
Movements of crustal plates and continental land masses over geologic time pushes some land masses up while others come down, which can change the locations of oceans. Sometimes oceans become trapped inside land masses, eventually evaporating and leaving their mineral content behind. The fact that Pennsylvania has a lot of limestone tells us that for a long long time Pennsylvania used to be under an ocean!
In present time, water filters through the ground and starts to dissolve the limestone. Holes are created and start to get bigger and bigger. This is how sink holes are formed. When they become big enough under the surface, the ground can give way and create depressions in the ground. This process occurs much more quickly when there is acid in the water like in the case of acid rain. When the acid dissolves the limestone, the acid becomes buffered and the waters are protected. This is one of the reasons trout fishing is so good in this area–the pH of the water is very balanced.
Unfortunately, many people throw their trash into sinkholes. They don’t realize that these holes come in direct contact with the ground water and are the source of water for many streams in PA. In turn, the trash they throw in directly interacts with the water and contaminates it.
Ask students what they think of when they hear the word “invasive.” What are some examples of invasive species that have been damaging to natural habitats? Are there any species that have gone extinct because of invasives? Ask them what measures they would take to get rid of an invasive species that was taking over an area.
This activity focuses on two common invasive plant species, garlic mustard and multi-flora rose. Garlic mustard was originally brought here from Europe for herbal purposes, and multi-flora rose was brought over from Asia to prevent soil erosion along cattle fields. Unfortunately, it grows too quickly and takes up other desirable land as well. Multi-flora rose spreads by “layering,” which is when the plant grows straight up until it eventually falls over to the ground, and then the tip of the plant sends out roots and it grows up again. This means that their root systems can be enormous and difficult to remove.
One of the primary ways to control both of these invasive plants is to pull them up and plant something else on top of the space to compete with it.
If possible, locate a local sinkhole to take students to. If there is no sink hole to visit in person, students can compare the way water moves through a funnel filled with dirt compared to a funnel filled with sugar cubes and then dirt. In the second example, the sugar cubes act as the limestone and dissolve when in contact with water. As they dissolve, more and more dirt falls into the water, the same way soil and garbage can come in contact with ground water through a sink hole.
After explaining the way acid rain dissolves limestone, have students place drops of vinegar on pieces of limestone. They should see air bubbles and it should make a fizzing sound. This is one of the ways scientists determine if a rock is limestone or not.
If possible, have students pick up litter that has been discarded into local sink holes, and have them make signs to put up on the location explaining to visitors why they should not throw trash there–it will contaminate their water source.
Ask students to describe how the limestone rocks we have now were formed. If limestone dissolves easily, do you think it makes up the ridges or valleys in this area? (Note: Valleys! Harder sandstone makes up the ridges). Ask students to explain how sinkholes form and why they can be dangerous.
Have students do more research on the geology of the area–how long ago did the limestone form? Did there used to be more that has eroded away?