This is an EarthCache and you MUST complete the educational portion in order to log this cache. EarthCache sites adhere to the principles of Leave No Trace outdoor ethics. You will need your GPSr, a camera (optional), a clear container (optional), a little deductive reasoning and a desire to learn something new. And please do not hint at the answers to any of the questions in your "Found it" log.
About Sand In General
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles. It is defined by size, being finer than gravel and coarser than silt. Sand can also refer to a textural class of soil or soil type; i.e. a soil containing more than 85% sand-sized particles (by mass).
The composition of sand varies, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal settings is silica (silicon dioxide, or SiO2), usually in the form of quartz. The second most common type of sand is calcium carbonate, for example aragonite, which has mostly been created, over the past half billion years, by various forms of life, like coral and shellfish. It is, for example, the primary form of sand apparent in areas where reefs have dominated the ecosystem for millions of years like the Caribbean.
In terms of particle size as used by geologists, sand particles range in diameter from 0.0625 mm (or 1⁄16 mm) to 2 mm. An individual particle in this range size is termed a sand grain. Sand grains are between gravel (with particles ranging from 2 mm up to 64 mm) and silt (particles smaller than 0.0625 mm down to 0.004 mm). The size specification between sand and gravel has remained constant for more than a century, but particle diameters as small as 0.02 mm were considered sand under the Albert Atterberg standard in use during the early 20th century.
Puerto Rico's Beach Geology
The simplest description of the Puerto Rico coast involves a division into three basic categories: Rocky cliffs and headlands; mangrove coasts; and sand or gravel beaches. La Guancha is a combination of the last two, as it is located in a transitional zone. The shoreline development is closely related to whether the adjacent land area of the coast is Createceous-Tertiary limestones, igneous rock (partly responsible for some of the black sands in the southern coast) or low Tertiary fans and alluvian (made up of or found in the materials that are left by the water of rivers, floods, etc.) plains.
The coastal zone of Puerto Rico is remarkably diverse. Unlike the shorelines of many major continents, there are no long interrupted stretches of basically similar beach. The beaches of Puerto Rico are relatively short and are divided into separate and distinct beach systems that have restricted communication with one another. Each is a closed or semi-closed unit receiving its supply of sediment from limited local sources and transmitting little of its longshore moving sand to another beach system. In analyzing the sand and beach systems to determine the extent of isolation, mineralogy, metal content and biogenic (produced by living organisms or biological processes) constituents have been studied; also the actual physical parameters of separation have been analyzed.
The beaches of Puerto Rico contain sand grains derived from several major sources. Erosion of land areas and transport of rock material by rivers to the beach supplies terrigenous (those derived from the erosion of rocks on land; that is, they are derived from terrestrial environments) sand grains. The composition variances are a function of the source area where the material is eroded and added to the river sand system. Areas of river drainage underlain by basaltic (a dark gray to black dense to fine-grained igneous rock that consists of basic plagioclase, augite, and usually magnetite) type rocks supply dark minerals and dark igneous (formed when hot, liquid rock cools and becomes hard) rock fragments. Granite rock outcrops supply quartz and feldspar to the beach.
Along parts of the coast containing major rivers, there is an increase in terrigenous content in the beach sands. There is also a shift toward terrigenous beaches where an offshore carbonate source is lacking. By analyzing the availability of supply we can draw conclusions about the transport system and beach dynamics. Larger concentrations of calcium carbonate can indicate shoreward transport. In an area such as Guanajibo Beach (Mayaguez) the absence of carbonate grains in the beach sands coupled with the presence of reefs just offshore shows that shoreward transport of beach material is negligible. This is part of the reason for the erosion on this relatively protected beach environment.
To claim the find:
Send me your answers BEFORE logging the find.
Email the CO with your answers to the following questions:
1. Grab a handful of sand at the posted coordinates and observe it or place it in a clear container. In your opinion, what type of rocks are partly responsible for the darker colored sands in the area you are standing? Look around you. Do you see a difference in sand coloration? What, in your own words, could be the cause of this difference in color?
2. Given the information above, is the size of the grains of sand closer to gravel or silt?
3. La Guancha is located in a ____________ zone.
4. What two natural mechanisms supply terrigenous sand grains to the beaches of Puerto Rico?
5. (Optional) Take a picture of yourself at the spot and post it on your log.
Resources:
Glossary of terms in soil science. Ottawa: Agriculture Canada. 1976. p. 35. ISBN 0662015339.
Urquhart, Leonard Church, "Civil Engineering Handbook" McGraw-Hill Book Company (1959) p. 8-2
Morelock, J., Ph.D. 1978. “Shorelines of Puerto Rico"
Acknowledgements:
Thanks to my hometown, the city of Ponce, for their gracious responses to mine and my wife's inquiries regarding the placement of our Earthcaches in public domain lands.
Special thanks to reviewer GeoawareUSA4 for guiding us in the changes that needed to be made in order to create this Earthcache. Your help is greatly appreciated!
