Crunch, Crunch, Crunch the Rise of the Tides
This Earthcache is located along the Bolivar Peninsula. Since this is an Earthcache, there is no physical container to find. To earn your smiley for this cache, e-mail or email your answers to the owner BEFORE logging a “found it.”
Welcome to the Bolivar Peninsula
Watch your step. What is under your feet as you walk toward the Gulf of Mexico?
Sally Sells Seashells by the Seashore
The crunch-crunch-crunch-under-your-foots crunch is the breaking of calcium carbonate, the main mineral found in shells bound to protein, as the mollusk, clam, and snails grow. The shells under your feet could have traveled thousands of miles and be thousands of years old.
Thanks to the location of the Bolivar Peninsula, which runs southwest-to-northeast, and the currents of the Gulf of Mexico, shells are stirred up and scooped onto the beach here. It is estimated that over 275 different shells can be found along the Bolivar Peninsula.
Pull of the Moon
The tide in the Gulf of Mexico refers to seawater's cyclic rise and fall. Tides are caused by slight variations in the gravitational attraction between the Earth, the moon, and the Sun in geometric relationships with the locations of the Earth’s surface. They are cyclical because of the Earth’s rotation.
The moon’s gravitational pull generates tidal force, which causes the Earth – and its water – to bulge out on the side closest to the moon and the side farthest from the moon. These bulges of water are high tides. The Bolivar Peninsula passes through high and low tides daily as the Earth rotates.
The timing of the high and low tides on the Bolivar Peninsula is related to the Earth’s rotation and the moon's revolution around the Earth. The moon revolves around the Earth every 27 days, which adds approximately 50 minutes to the tidal cycle, which is 24 hours and 50 minutes.
The most common tide is semidiurnal – two high and two low tides each day, or diurnal – one high and low tide each day. There are also mixed types of tides in places around the Earth.
Tides are important to geology because of their ability to transport sediment and seashells that carve coastlines. Over a long period, shorelines like here at GZ will shift, estuaries and rivers will be filled with sediments, and beaches will change because of the rise and fall of tides.
Washing Away
Just as the hundreds of shells have been deposited at your feet by the pull of the Moon and the tides, so do these tides wash away Bolivar Peninsula. The United States Army Corps of Engineers, Galveston District, estimates that 64% of the Texas Coast erodes an average of 5.9 feet yearly. Some parts – such as parts of Follett Island, located northeast of GZ, are eroding 30 feet a year.
Why the beach erosion? Two answers: First, it is a natural process of the ebb and flow of the Gulf of Mexico. If sand and shells are removed from Bolivar Peninsula, then the sand and shells would be deposited further southwest, say on Mustang Island. Second, recent storms such as Hurricane Ike in 2008 and Hurricane Harvey in 2017. As Hurricane Ike came ashore, storm surges were estimated to be as high as 20 feet along Galveston Island and Bolivar Peninsula.
Highway 87 and Shore Protection
Highway 87 is the hurricane evacuation route for those who live on the Bolivar Peninsula. The highway has been repeatedly washed out because of high tides and storm surges since 1990. The Gulf of Mexico has been less than 100 feet away from Highway 87 at times since 1990. If you were to take an aerial look at GZ and Bolivar Peninsula, the road is in front of the area where natural sand berms would form to protect the Peninsula. The Texas Department of Transportation is spending millions to protect Highway 87 and Bolivar Peninsula.
Riprap
Riprap describes a range of rocky materials placed along shorelines, bridges, foundations, steep slopes, and other structures to protect against waves and erosion. Rocks used range from 4 inches to over 2 feet. The rock size needed on a project depends on the steepness of the slope and how fast the water is moving. Riprap is a very durable and long-term solution.
Stone for riprap should consist of field stone or quarry stone that is angular, variably sized, and resistant to cracking during freeze and thaw cycles. Most igneous stones, such as granite, have suitable durability. Riprap ranges in shape because “riprap” can include various stone types: granite, fieldstone, limestone, and slate.
After reading the information above and making observations at GZ, please answer these required logging tasks:
- Near GZ, observe a 6-inch by 6-inch square. Describe what you see. Do you think these shells were deposited at high or low tide? Explain why.
- Is this beach a Semidiurnal Tide Zone, A Diurnal Tide Zone, or a mixed tide, and why do you think that?
- Estimate the distance between GZ and the edge of the Gulf of Mexico. Do you think the beach is eroding? What do you base your observation on?
- What type of Riprap has the Texas Department of Transportation placed between the beach and the highway? What is the color, texture, and purpose of the Riprap? Why do you think TxDOT placed the Riprap here?
- Post a picture of GZ with the shells and the Gulf of Mexico.
Confirm your findings by answering these questions and sending your answers via the message center.
**Please do not post your answers in your log!**
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Sources:
NOAA SciJinks. 2024. Tides and Oceans: What Causes Tides? https://scijinks.gov/tides/
National Geographic. 2024. Cause and Effect: Tides. https://education.nationalgeographic.org/resource/cause-effect-tides/
Randall, Keith. 2020. Texas A&M Expert: Storms Worsening State’s Beach Erosion Problem. https://today.tamu.edu/2020/10/27/texas-am-expert-storms-worsening-states-beach-erosion-problem/
US Army Corps of Engineers, Galveston District. 2024. Coastal Texas Study: Beach and Dunes Systems. https://storymaps.arcgis.com/stories/f11118bd7d9c49a690c48b98bfe9b5db
Woods Hole Oceanographic Institution. 2024. How are seashells made? https://www.whoi.edu/know-your-ocean/did-you-know/how-are-seashells-made/