Caves and Sinkholes 13/14 Nov

Warm up: What is chemical erosion?

Warm up answer: Chemical erosion is on page 710. Also known as chemical weathering.

Vocab/essentials: water table permeable aquifer erosion chemical weathering Describe agents of erosion and differentiate between chemical and mechanical weathering

What is acidic water? Water in the form of rain can pick up CO2 in the atmosphere on its way down to the ground and become acidic. The pH of the water can chemically erode some types of rock and carve out landscapes.

http://water.usgs.gov/edu/sinkholes.html Acidic water runs through the pores, slowly eroding the material. It forms a space underground, which can fill with water. When the ground above it can no longer hold the structure together, it will collapse.

Pothole in the road This road is in need of repairs! Fractures around the edge of this pothole form concentric circles, a lot like rings around a bullseye on a dart board. Near the center of the pothole, there are a lot more fractures and the pattern is harder to see. The pothole formed when a portion of the road compacted or subsided, causes the surface of the road to collapse and fall into the hole. To get a sense of the size of this feature, you can see the painted stripe of a crosswalk in the background, and a small white ruler about 10 cm long near the center-left.

Sinkhole in Florida Here, circular shaped fractures form the margins of a massive sinkhole that catastrophically caused this house to collapse. Sinkholes are natural features that are relatively common in areas with abundant limestone near the surface. Note how the sinkhole is filled with water, hinting at what caused this catastrophe.

What is karst? http://www.esi.utexas.edu/outreach/caves/karst.php Karst is a landscape formed from the dissolution of soluble rocks including limestone, dolomite and gypsum. It is characterized by sinkholes, caves, and underground drainage systems. Rainwater becomes acidic as it comes in contact with carbon dioxide in the atmosphere and the soil. As it drains into fractures in the rock, the water begins to dissolve away the rock creating a network of passages. Over time, water flowing through the network continues to erode and enlarge the passages.

Why is Karst Important? Dissolution associated with karst development in central Texas limestone has created a complex underground water flow network that includes caves large enough for humans to access. The karstified limestone acts as an aquifer where water can be stored and later extracted by humans.

Two million people in central Texas get their drinking water from the karst aquifer known as the Edwards Aquifer (Sharp and Banner, 1997). This resource is especially important for central Texas as the region becomes more urbanized. With a higher density of people, central Texas will face higher demand and increased pollution. Just like rainwater, pollutants can easily pass through the karstified limestone. Another difficulty is that streams and surface runoff entering the aquifer via sinkholes and caves bypass the natural filtration produced by seeping through soil and bedrock. This direct recharge quickly replenishes the water supply; however, it also leaves the aquifer particularly vulnerable to contamination (Drew and Hötzl, 1999).

How Do Caves Form? natural opening, usually in rocks, that is large enough for human entry types of caves: volcanic, glacier, crevice, erosion, and solution. Solution caves occur in limestone and gypsum

Stream goes into crevasse or fracture on the right Stream goes into crevasse or fracture on the right. Cave is created by chemical erosion of rock layer. The left has a sinkhole example after the cave has been made.

Minerals vary widely in their chemical response to groundwater Minerals vary widely in their chemical response to groundwater. Calcite, for example, is soluble in a weak acid such as H2CO3 - carbonic acid. The acidic water passes through fractures, crevices, and cavities and dissolves the limestone very slowly, enlarging the network of passageways. Most solutional caves require more than 100,000 years to grow large enough for a human to be able to pass through.

Most caverns are created at or just below the water table in the zone of saturation in limestone. If the water table is stable, large openings can be created because water would contact all surfaces of the cave, dissolving the limestone at a large scale. If the water table drops, the area of active cave formation will move lower into the bedrock and the upper openings are left in the zone of aeration, only subject to dissolution from running water. Water dripping through these dry passages may deposit CaCO3 in various forms collectively referred to as speleothems.

The water table continues to lower due to use, so it continues to erode away the rock forming layers to the cave system.

Cave formations Flowstone forms as water flows over walls, floors, or other formations and builds up sheets of calcite that look like a rock waterfall.

Stalagmites rise upward from the floor of the cave passage and are usually formed by water dripping from ceiling.

Stalactites hang from the cave ceiling and form as water flows down the formation deposits layers of calcite. You can remember the difference between stalagmites and stalactites by remembering that stalactites cling “tight” to the ceiling. When a stalactite and a stalagmite join they form a column.

Reading Read page 717 Complete “Writing in Science” assignment on page 717. Your journey to the ocean will include an underground path through a cave or sinkhole. Mention the things you see, the pH of yourself and explain how you can work your way through rock. Use your vocabulary!

Vocab/essentials: water table permeable aquifer erosion chemical weathering Describe agents of erosion and differentiate between chemical and mechanical weathering