The Hanging Beach - A Story of Coastal Erosion
Welcome to the hanging beach of Molokai. As you approach this beach from the designated parking area you'll find beautiful sand dunes, a scenic view and a steep drop off! As you continue along to the posted coordinates you'll find something even more amazing; the beach that you just walked across is actually hanging above the water! Throughout the course of this Earthcache you will learn about the coastal erosion processes occurring here that have lead to the formation of these hanging beaches. As with all Earthcaches there is no logbook here; rather you will make observations at the posted sets of coordinates and answer the questions located at the bottom of this cache page in order to log your find. Please use the messaging system to send me the answers before you log your cache find. Thank you in advance!
A safety note: at no point should you approach the edge of the cliff face. Erosion is actively taking place here and the edges of the cliff are unstable. Follow all posted signs and stay on the trails. You assume all risks in hiking in this area. Have fun, and be safe!
The Circle of Erosion - A Wave-cut in Action
Proceed to the posted coordinates (stage 1)This hanging beach is the intermediate stage of a continuously evolving story of erosion. As you can see in the image, a wave-cut is formed as the sea erodes away the base of a cliff. Wave action is a type of mechanical weathering process that slowly breaks down the rock. There are a number of factors that affect the rate of erosion:
- Rock Type - A more resistant rock, such as granite or igneous rock will be eroded more slowly. A less resistant rock, such as clay or mud can be eroded very quickly.
- Joining/Faulting - The more faults and joints a rock has the more susceptible it is to erosion. Faults and joints are naturally occurring during the rock formation process.
- Coastal Rock Arrangement - A coastline with rocks that run parallel to the coast is called a concordant or "Pacific" coastline. One that has rocks running at right angles to the coast is called discordant or "Atlantic" coastline.
- Rock Angle - The angle that the coastal rocks dip can also cause erosion to occur at differen rates.
Rocks that dip towards the sea produce gently sloping cliffs.- Rocks that dip away from the sea produce an uneven, steep cliff face with many overhangs.
- Rocks that are horizontally bedded produce a steep cliff with small overhangs.
- Rocks with vertical beds will produce vertical cliffs if the layer closest to the sea is resistant.
At the first set of coordinates, if you turn and face back toward the parking lot and beach access, you will see the hanging beaches. The wave action has eroded the base of the cliffs leaving the top half of the cliff (including that beach you have just walked on) hanging over the ocean! At this point you may want to look at the questions below and make the needed observations to answer questions 1-2.
A Collapsing Cliff - The Circle Continues
Proceed to the second set of coordinates (stage 2). Here you will be able to observe another stage of the erosion process. At the same time that the base of the cliff is being eroded, the cliff face and its structure are being weakened by sub aerial processes. Oxidation and carbonation are some of the chemical processes that can weaken the structure of the rock, and depending upon the climate additional physical processes such as water layer weathering can take effect.
Over time this weakens the structure of the cliff face, and coupled with the erosion of the wave cut notch at a critical point this cliff face will succumb to the influence of gravity and collapse in a process of mass movement. This material will then be carried away by the sea in the process of long shore drift by the transportation process of solution, suspension, saltation and traction (depending upon the particle sizes).
The removal of the material from the collapsed cliff means that the base of the cliff is once again exposed. Therefore, further marine and sub aerial erosion means that the whole process of cliff collapses repeats itself and the cliff retreat inland. A wave cut platform will begin to occupy the area coastline at the base of the cliff. The diagram above illustrates how a cliff can retreat inland and the location of the wave cut notch and the picture below shows an example of typical wave cut platform.
Here at the second set of coordinates you'll need to make several observations in order to answer questions 3-4.
How to Log your Earthcache!
In order to log your earthcache you must send your answers to the following questions to me via the messaging system. You do not have to wait for a response from me in order to log your cache. The answers to the first two questions can be found at stage 1 and the third and fourth questions can be found at stage 2. I hope you enjoyed your visit to these hanging beaches!
- Based on the manner in which the coastline is eroding, do you believe this is a concordant or discordant coastline? What evidence leads you to this answer.
- What is the angle of the rocks in relationship to the wave action? How has this orientation aided the erosion process?
- At stage 2 you can see some recently collapsed cliffs. What is the angle of the collapsed rock in relationship to the sea? (In other words, what is the angle of the rocks in the water).
- Based on your answer to question 3, will this hinder or facilitate the erosion process. Explain your answer.
- (Optional) Post a photo of you or your GPS in front of the hanging cliffs (stage 1). Please do not include any photos of stage 2 in your log.
Sources
The following sources were used to help develop this earthcache and are an excellent source for additional information on the coastal erosion processes observed at this earthcache!
- http://www.geocoops.com/coasts-of-erosion.html
- https://www.s-cool.co.uk/gcse/geography/coasts/revise-it/coastal-erosion-features
- https://web.archive.org/web/20030218074552/http://cse.cosm.sc.edu/erth_sci/Coasts/Erode.htm