The Sands of Chile

Or more specifically, the beach sands of Chile are what we have collected as part of a larger suite of beach sands. The header image on this post is of our modest sand collection. The top row contains samples from around the world, the second row down are odd interior sands not from ocean beaches, the third row down are sands from the Hawaiian islands, the fourth row down has samples from California beaches, the fifth row down are a few samples from Peru, and the bottom row is a fairly complete set of Chilean beach sands.

 

 

Search online for photographs of beach sand collections and you will find many examples with a great amount of care taken in collecting and archiving the sand. The pursuit becomes more interesting in comparing sand types between beaches and seeing the variety of colors. It is easy for the non-specialist to quickly develop fascination with this topic particularly because as a souvenir it links the sample to the experience of a fine day spent on the beach. For the geologist a bottle of sand contains a wealth of information encompassing far more than the material dumped along the coastline, a sand sample provides a record of the world.

As part of my geological studies at university, courses included sedimentary geology, sedimentary petrology, and basin analysis- sandstones, the geological record of past sand deposits, give information about the environment in which they form (deposition environment) and data about the type of geology that the sand was originally eroded from (sandstone provenance). Both of these conditions combine to make the overall hue, color, and texture of beach sand and it explains the wide variety that people dutifully store into glass bottles.

One of the more fundamental guides on the description of sand can be found in Robert Folk’s 1974 laboratory manual called “Petrology of Sedimentary Rocks.” The two most immediate observably features of a sand sample include its color and grain size.

Color classification is somewhat subjective, even when using schema like the Munsell color charts. White balanced photographs can be measured for their average RGB colors. For some extreme examples, sand made entire of quartz grains will appear fairly white. The most intense white sand is derived from clean carbonate or limestone, such as the sandy beaches at the Bahamas. The black sand beaches of Hawaii have extremely short erosion site to deposition place distances for material transport, and have grains derived almost exclusively from black somewhat glassy basalt lava flows. The rare examples of green sand beaches (there are four of them in the world) generally result from the mechanical separation and concentration of denser green minerals in basalt flows (olivine). The sands of Chile range in different shades of gray, with a few exceptions, and they reflect the bulk composition of Andean volcanism being an intermediate composition rock (based on silica content) that is called an andesite.

Grain size in sand is determined by the water energy level during deposition. Beach sands in general, if collected from near the swash zone of moving waves, will have similar sizes between samples. Sand grain sizes are measured in millimeters for the grain diameter. Exhausting characterization of sand sizes can be done through sieving the sand through different mesh sizes and weighing each population of grain sizes, the size fractions. These weight fractions by mesh size can be plotted on a graph. Most sand samples will have a normal distribution around a single rounded peak in the graph, though complexly sourced sands can have bimodal populations. Again given the same deposition environment is being sampled from beach sands the variation in grain size from sample to sample should not be great. Although, some higher energy beaches with very immature sand can result in very coarse-grained sand, such can be found basement rock comprised coarse-grained granitic rock providing locally derived sand. Other higher energy beaches will be comprised of pebbles to cobbles and contain little sand. Geologists use a general size description scheme that can be used from visual inspection, sometimes with use of a hand lens, which has the divisions of fine-grained sand (0.125 to 0.25 mm), medium-grained sand (0.25 to 0.5 mm), coarse-grained sand (0.5 to 1 mm), and very coarse-grained sand (1 to 2 mm). While fine sand may at first appear very small indeed, in geology finer material is termed silt, and then as the grain size goes even smaller we are then discussing the material of clays. For quick identification reference cards are used that either have glued on examples of the grain sizes or black and white graphic showing the size ranges. These are very useful until one learns or calibrates the eye to be able to estimate the size by the sample appearance. The variation of the average grain size in sand is termed sorting. Sand that has nearly all the same grain sizes is termed “well sorted” whereas those with a mixture of grain sizes are called “poorly sorted.” Additionally, geologists will look at the shape of the sand grains, classifying them as rounded, subrounded, subangular, and angular- which has direct significance in terms of the distance the material was transported while considering the compositional hardness of the grains.

On the source of sand- its provenance. Several hallmark papers gave in-depth analysis of sand grain composition populations linking them to provenance charts showing the type of broad geological settings, tectonic settings. These are determined by the relative abundance of quartz (Q), feldspar (F), and rock fragments (RF) or lithics typically as observed under a microscope and systematically measuring the composition in a procedure that is called point counting. Dickinson and Suczek (1979) paper “Plate tectonics and sandstone compositions” marks on ternary plots the general categories of recycled orogeny (subduction, collision orogeny, and foreland uplift), magmatic arc (in various stages of erosion), and continental block (craton interior, uplifted basement). More exotic included minerals (such as garnet, tourmaline, magnetite, etc.) in the sand grains (generally a small minority population termed accessories) can provide very dialogistic information on provenance. The sands of Chile all have their source within ten to 150 km from the beach, and they all come an active plate margin that includes arc magmatism and deformation that developed the Andean mountains. The sands should contain an abundance of young material derived from volcanic deposits, and given the very short transport distances, the sands should also incorporate what are called lithic fragments. A sandstone comprised mainly of sand-sized lithic fragments is termed an “arkose.” Conversely, sand that is comprised of greater than 90% quartz grains is termed a “quartz arenite.” Quartz-rich samples represent “mature” sources, and in many cases where eroded from previous sandstone formations from the center regions of continents.

 

 

Chilean sand samples (beaches from north to south)

Arica: gray color

Iquique: light medium gray, mixed shell fragments

Playa Grande, Mejillones: light medium gray, mixed shell fragments

Cifuncho: medium gray, salt and pepper coloration

Chañaral: light gray- may contain abundant tailings material from the El Salvador mine

Bahia Inglesa: light gray, contains abundant shell fragments

Huasco: gray

La Serena: gray

Reñaca: beige-gray

Isla Negra: beige, very coarse-grained, sand derived from granitic local source

Pichilemu: black

Cucao, Chiloe: dark gray

 

Nearly all of these Chilean beaches are of the river mouth variety, having narrow extent boxed in by the canyon walls that form eroded cliffs to the north and south of the beach. The few exceptions that have longer strands are Arica, Playa Grande, La Serena, and Cucao. Much of the Chilean coast is actively being eroded because of ongoing uplift of the Andes plus tectonic erosion of the leading edge of the South American plate.

Closing remarks. Overall the general color or hue of the Chilean sand collection is similar to the material found on beaches in Peru, which likewise shares the Andean mountains as the source for the beach sands. From arm’s length, the Chilean sands are similar appearing to many of the beaches of California, though California has a more complex provenance. In geology the concept from James Hutton “the present is the key to the past”, or uniformitarianism, provides additional incentive to gather a suite of modern sand samples to have as reference when inspecting older sandstone during a geological study. The suite of Chilean sand we have collected has not gone through exhaustive characterization under microscope, though perhaps someday the effort will be made.

 

James M. Wise- February, 2019

 

Do connect with us:

ResearchGate: James M. Wise 

Author´s page: James M. Wise

Photography page: JamesM.Wise.com 

Author´s page: Yanira K. Wise

 

 

South America seems to refuse to show its inexhaustible creative force.