This cache is intentionally similar to GC3GQ9B Black Sand Beach (Big Island), and if you've done that one (or intend to), this will seem very familiar. However, you don't need to do that cache to work on this one. If you do intend to visit that location, keep what you learn here in mind as the two beaches are quite different.
Sand of varying colors appears on beaches the world over. Most sands are formed by centuries of waves crashing on ordinary rocks, producing white, gray, or golden-colored beaches, sometimes with other colors mixed in. For example, the green sand beach near South Point contains olivine crystals (see GC1M15H Olivine (Mg,Fe)2SiO4 - DP/EC38), while reddish-tinted beaches possess a high iron content (see GC2BT93 Red Sand Beach over on Maui). Beaches aren't always made entirely out of eroded rock, either - they can be formed from pulverized coral and shells, either in whole or in part. Sometimes a beach will have many different colors, depending on the makeup of the rocks nearby and the amount of biological material present. And, occasionally, a beach will be made entirely of black sand.
What is black sand, anyway? Sure, it's black in color, but why is that? While black elements exist in many kinds of sand, the black sand in Hawaii is of volcanic origin. There are two common ways volcanic activity leads to the formation of black sand:
Lava-Ocean Collision: When an active lava flow hits the ocean, the extreme temperature difference causes the lava to basically explode into tiny fragments that look like glass or crystal. These chunks and shards are then beaten down by wave action into rounded grains, forming a beach. Under the right conditions, a lava flow can create a black sand beach virtually overnight (although you wouldn't want to walk on it barefoot right away, for obvious reasons).
Flow Erosion: Lava flowing into the ocean in sufficient volume can also form high walls or cliffs, which are then eroded over time. Hardened lava (volcanic igneous rock) is generally less dense and more porous than other kinds of rock, primarily because it has not been subjected to extreme pressure underground. As a result it erodes much more quickly (in a geological sense), breaking down rapidly into grains and clasts by wave action to form beaches.
Logging Requirements
In order to log this earthcache, go to the posted coordinates (or close enough that you're standing on the black sand beach) and answer the questions below. You can send your answers to me either by email or using the messenger system. You don't need to wait for a response before logging your find. However, if you don't send answers within 48 hours of logging your find, I reserve the right to delete your log. No armchair caching, please!
1. Examine the black sand carefully. Is it coarse or fine? If you had to describe its texture in one word, what would that word be? What other colors are present in the sand?
2. Do you think this beach was formed by lava-ocean collision or flow erosion? Explain how you arrived at your conclusion.
3. Take off your shoes and go for a walk in the wet sand. Describe the experience in your own words. Does anything curious happen when you step in the sand? Why do you suppose this is?
4. (Optional) If you've been to any other black sand beaches, compare their appearance and the sand texture with what you found here. If you haven't been to one, or don't remember the details, that's okay - but remember this beach in case you do visit others. The earthcaches there might ask you to make a comparison with this one!
5. (Optional) Please post any photos of you or your team along with your log (just avoid close-ups of the sand as that will give away the answers to some of the questions above).
Thanks for visiting and enjoy your trip to Waipio Valley!
References: In addition to the earthcaches mentioned in the text, see also the Wikipedia entry for Black Sand Beaches.