On the south-eastern coast of the island of Guam at the mouth of the Talofofo River lies an inlet known as Talofofo Bay. Limestone cliffs border the long, narrow bay. The beach consists of chocolate colored sand deposited by the river. After Supertyphoon Paka in December of 1997, the beach began to quickly erode. A wall of giant boulders was placed to protect the road, however, the rest of the beach is rapidly disappearing. If you enjoyed your visit to this cache, please consider awarding it a Favorite Point.
The bay is one of only four green sand beaches in the world and is easily accessible. Other green sand beaches are located on Punta Cormorant on Floreana Island in the Galapagos Islands, Hornindalsvatnet in Norway, and Papakōlea Beach on the island of Hawaii. The sand at Talofofo Bay gets its distinctive coloring from the mineral olivine that erodes out of basalt, a product of the island's lava flow.
Usually green in color, the rock-forming mineral olivine is responsible for the color of the sand. It can also be yellow-green, greenish yellow, or brown. Olivine is familiar to many people because it is the mineral of the popular green gemstone known as peridot. Because of its high crystallization temperature, olivine is one of the first minerals to crystallize from basaltic magma. As the slow cooling process occurs, the crystals of olivine may form and because of their high density, settle to the bottom of the magma chamber. The accumulation of this mineral can result in the formation of olivine-rich rocks. It is easiest found in the beach sand as tiny, fine, sand-sized, rounded crystals that under a microscope look like green bottle glass.
In addition to the olivine found in the sand, you will also find magnetite. Magnetite appears as tiny, fine, sand-sized black octahedral crystals. It is easily attracted to a magnet. Close to the water you see the usual brown color of the mixed sand, however, in places, wind action may separate the brown sand of the beach into a green, olivine-rich layer, and a black, magnetite-rich layer.
Logging requirements:
From the parking lot, follow the short trail to the beach. Pick up some of the dry sand and complete the activities listed, sending me your answers through my geocaching profile:
1. "Talofofo Bay Earthcache" on the first line.
2. Describe the consistency of the sand.
3. Look carefully at the sample in your hand. What is the predominant color you note?
4. Based on the color, which mineral do you feel is predominant?
5. Pass a magnet over a small amount of sand in the palm of your hand. Do you feel you are holding more magnetite or olivine?
Please don’t be intimidated about answering the questions. They are meant to improve your understanding of the geological aspects of the earth and provide you with new knowledge and/or insights about the location. You don't have to write a term paper. Just provide enough information to show me that you were there and tried to observe the details. But, most importantly have fun. I hope you take away some new knowledge with a smile on your face!
The above information was compiled from the following resources:
USGS -- [http://geomaps.wr.usgs.gov/parks/coast/sand/greensand.html]
Geology.com -- [http://geology.com/minerals/olivine.shtml]
ROCKS and MINERALS of GUAM -- [http://www.heptune.com/guamrock.html]