Beaches come in all shapes and sizes, however the vast majority of beach sands are made up of quartz. These are the beaches where the sand is the color of, well, sand. It is that light orangish hue that sometimes grades towards white. It is a hue so common in the world that the name "desert sand" was even adopted as an official palette color.
|Desert Sand Crayon. Image courtesy of Ranker.|
This "typical" sand is made up of quartz grains that had been eroded down into "sand-sized" grains, coated frequently in a microscopically thin layer of iron-oxide, giving it its unique hue. The reason why sand is frequently comprised of quartz is that quartz is one of the most abundant minerals on the surface of the earth, making up a large majority of the rocks on the surface such as granite and sandstones. It is also extremely hard compared to other minerals (a 7 out of 10 on Moh's hardness scale), it has a fairly simple crystal structure comprised of only two elements (SiO2), and it doesn't have cleavage, the ability to break along plains of weakness in the mineral. This means that as it erodes, it erodes down into a small ball, instead of breaking down along flat edged blocks.
With granite being one of the most abundant rock in mountains, and beaches composed of sand from the breakdown of mountain rocks, then that being the case, it is rather rare to have a beach NOT comprised of quartz sand. Beaches get their sand from rivers that carry eroded sediment from local or distant mountain ranges. Over the course of the river, the rocks from the mountains are broken down into smaller and smaller pieces, until all that is left is the most resistant mineral grains.
However, sometimes you can get beaches comprised of different minerals or chunks of rock than quartz. There are a few ways this can happen. One way is that these minerals and rock chunks can come from sources closer and more local to the beach, so that they haven't had the distance and time to erode down all the way. Or you can have an instance where quartz is not present in the source rock at all, or much less abundant than other minerals. Both of these occurrences are what we have in Hawaii.
The Geological Context of Hawaii
|Diagram of the creation of the Hawaiian Islands. Image courtesy of Clark Science.|
Hawaii is a volcanic island made up from the volcanic eruptions of a hotspot deep within the mantle. A hotspot is a non-moving plume within the mantle that melts the crust on top of it. However, the Earth's crust moves around on top of these hotspots. As the Pacific plate moves over the Hawaiian hotspot, it melts the crust and creates a series of volcanoes as it passes over. That is the string of Hawaiian Island, where the only active volcanoes are located on the Big Island, where the hotspot is currently located.
Hawaii's volcanic rocks are composed of Basalt. This is a silica-poor rock, meaning that it lacks any quartz in it whatsoever. Basalt is also very black in color. And being located nearly in the center of the Pacific Ocean, away from any other sources of rocks, all of the sand on Hawaii needs to come from the islands themselves. So since Hawaii doesn't have quartz, what is the sand comprised of? Well, that depends heavily on which island you are on and where you are located on that island.
Hawaiian Beach Sands
Black Sands Beach
|Location of Punalu'u Black Sands Beach|
|Map of the Big Island volcanoes. Image courtesy of the USGS.|