Note: This is an Earthcache. There is no physical cache to find. Logging this Earthcache requires that you undertake an educational task relating to the specific Earth Science at the site.
Prior to logging this cache, click on Message this owner, or send an email with answers to the following questions:
- Before you examine the sand, look around and make a guess as to how this volcanic sand got here. Lava flow into the ocean, or erosion of the volcanic hillsides?
- In general, is the sand well sorted or poorly sorted?
- Now grab a handful of sand, the wetter the better. Look closely at the grains of sand. I found that taking a photo and zooming in helped me to see it better. Kind of like a low-powered microscope. You should notice that it’s not all black volcanic rock. What do you think that other stuff is?
- REQUIRED: A photo! Groundspeak now allows this requirement. A selfie, a thumbs up, a ship in the background, your traveling companion (TB?), whatever your imagination can come up with.
Extra credit: What do you call people who collect sand as a hobby?
St. Kitts Black Sand Beach
This is not the most picturesque black sand beach. In fact, where the sand is dry, the beach appears more grey than black. However, it is easily accessible, making it a good earthcache location for tourists and cruisers. I promise you don’t have to be a geologist to answer the required questions.
This earthcache lesson is not so much about defining sand. We all know sand when we see it, and we can easily see the difference between gravel, sand and silt. Rather this is to get you to take a closer look at the composition of the sand that makes up this black sand beach.
The composition of sand varies, depending on the local rock sources and conditions. Nature forms sand constantly. Tiny chips of rock are always breaking off from boulders, cliffs, and mountainsides. As the weather and water wear down the chips, they create sand. This is the most common kind of sand.
A second type of sand comes from living things, such as corals and clams. After the animal dies, the ocean slowly wears down its skeleton or shell into tiny particles.
A third type of sand comes from minerals dissolved in water. When the water dries up, it leaves behind the minerals in the form of sand-like crystals. This is the least common type of sand.
Being a volcanic island, the darker grains are no doubt volcanic in origin. These could have gotten to this beach in two different ways:
Liquid lava flowing into the ocean is one of the ways black sand beaches can be formed. The lava is so hot that when it runs into the cold water, it causes explosions that shatter tiny fragments of volcanic sand/glass around. This process can happen so fast that a black sand beach can be formed overnight! The size of these fragments can be from small boulders to fine sand, dependent on the temperature of the lava and the water.
The other way black sand beaches are formed, is through erosion of black volcanic rocks by rivers, which then carry the black grains of sand down to the ocean. Depending on which other types of rock the rivers flow over, other minerals will be added to the mix, resulting in sand colors that can range from pitch black to dark brown or even beige or green.
Looking closely at the individual grains can reveal historical information as to the origin and kind of transport of the grain. Sand that is recently weathered, will be angular. Sand that is transported long distances by water or wind will be rounded.
The most common forms of transport are wind and water; the velocity of either force will determine the greatest particle size able to be transported. In other words, the faster the water, the bigger the particle that can be transported.
As the velocity decreases particles will begin to drop out based on size. This natural separation of sediment based on size is known as sorting. Sediment that is well sorted will be composed of material that is all similar in size. Poorly sorted sediment, will be composed of sediment of all different sizes. In general well sorted sediments are typically deposited by moving water or wind.
Well Sorted Sediment Poorly Sorted Sediment
As an added bonus, there’s litter in the form of sea glass on this beach. Help yourself!
