Hawaiian Beach Diversity
Quartz is the second most abundant mineral in the earth's crust. So, it's not surprising that most of the world's beaches are composed of grains of eroded quartz. However, the Hawaiian Islands are volcanic, and therefore formed almost entirely of basalt. Basaltic rock is extremely low in quartz. This means that our beaches are made from different (shall we say "more exotic?") materials. On Big Island, you will find black sand beaches (composed of eroded volcanic rock), green sand beaches (made from small grains of the mineral olivine), and white sand beaches like Hapuna. Many other beaches exist here as mixtures of these types of sand.
White Sand
The white beach at Hapuna is a stunning example of sand that is composed almost entirely of the carbonate shells of marine organisms. This reef-derived material is termed "calcareous" sediment. It originates primarily from corals and the calcium carbonate forming algae that live on them. Many coral reefs surround the Hawaiian islands. They get broken down into calcareous sediment that becomes white sand by two processes. 1) Mechanical erosion - when waves create friction that physically breaks down the reef. 2) Bioerosion - when marine organisms break down the reef as part of their life functions.
The Parrotfish
Around 90 different species of parrotfish inhabit shallow tropical and subtropical oceans. These herbavores have teeth that are adapted to enable them to graze the algae living on coral reefs. This provides a tremendous benefit to the coral, as too much algae will cause it to smother. However, some of the coral gets broken off and ingested by the fish as it forages. To cope, the parrotfish actually has a special toothy plate in its throat called a pharyngeal mil which grinds up the coral and allows it to pass through the fish's digestive tract. When this material is excreted by the fish it becomes the calcareous sediment that will eventually wash up on the beach and become sand. Parrotfish can reach lengths of 4 feet and one individual fish can produce as much as two tons of sand in one year! An estimated 70% of the sand on Hapuna Beach was secreted by parrotfish.
Categorizing Sand
Scientists can learn a great deal about the physical, chemical and biological properties of a beach by observing it's sand. Sand is categorized by it's size, shape and probable source.
Size
One factor that determines sand grain size at a beach is wave activity. High energy waves produce sediments with larger grains. This is because high energy waves recede with a higher velocity, taking fine sediments back out to sea and leaving larger grains behind. Lower energy waves, with lower receding velocity, deposit finer sediments along with larger grains. Another factor that affects grain size is the steepness of the beach. On steeper beaches, water rushes back to the ocean with higher velocity that removes finer sediments leaving larger grains. Water recedes at lower velocity on flatter beaches allowing finer sediments to remain with larger sediments. These larger grains then get rolled back and forth more frequently on flatter beaches, further eroding them, producing finer sediments. Wave energy also affects homogeneity of grain size. High energy waves produce more homogeneous sand while low energy waves form sand with a mixed grain size. The Wentworth Scale is used to classify sediment by it's grain size:
Shape
The shape of sand grains is determined by the material from which they are composed and their physical environment. Sand that comes from beaches with high wave action is more rounded than sand that comes from beaches with low wave action which will be more angular. Also, steeper beaches have more angular shaped sand versus flatter beaches which have more rounded sand.
Probable Source
We know that Hapuna is composed mostly of calcareous sediment originating from the break down of coral. However, parts of other marine animals such as sea urchins, gastropods, foraminifera (single-cell protozoans), mollusks, and sponges may also contribute to the sediment.
Activity
Please travel to the listed coordinates and examine a handful of sand. You may log this cache immediately after messaging me with the answers to the following questions:
1) According to the Wentworth Scale, what is the category, type and grain diameter (mm) of the material found on Hapuna Beach?
2) Are the waves you observe high energy or low energy? Does your observation of wave energy match your observation of grain size? Explain
3) Is the grain size homogeneous or mixed? Does this observation match the wave energy of the beach? Explain
4) Is this beach steep or flat? Does this observation match the grain size you observed? Explain
5) Referencing the chart above, what is the shape type of the material found on Hapuna Beach? Does this shape type agree with your observations of the wave activity and steepness of the beach? Please explain.
6) Aside from light colored grains, what else did you see in your handful? Did you observe the remains of any tiny marine animals that you could identify using the above chart? If yes, please state the identity of the organism and include a picture of your find along with your answers (not your log!)
REFERENCES:
https://oceanservice.noaa.gov/facts/sand.html
https://www.southkohala.com/blog/big-island-info/all-about-hawaiis-many-sand-colors-black-green-sand-beaches
https://dlnr.hawaii.gov/occl/files/2013/08/Sand-in-Hawai.pdf
https://www.to-hawaii.com/hawaiibeachsand.php
https://manoa.hawaii.edu/exploringourfluidearth/physical/coastal-interactions/beaches-and-sand/weird-science-parrotfish-and-sand
https://manoa.hawaii.edu/exploringourfluidearth/physical/coastal-interactions/beaches-and-sand