Posted by: Rob Viens | March 17, 2012

The Singing Sands of Bahia

March 17th – Darwin’s first encounter with the New World (in Bahia) was coming to an end as the Beagle prepared to set sail for Rio (and many more New World adventures). Today he was a bit reflective, writing about the contrast of what was both good and bad about Bahia:

“Took a farewell stroll with King: the evening was bright & exceedingly clear; not a breath of air moved the leaves; every thing was quiet; nothing could be better adapted for fixing in the mind the last & glorious remembrances of Bahia. — If to what Nature has granted the Brazils, man added his just & proper efforts, of what a country might the inhabitants boast. But where the greater parts are in a state of slavery, & where this system is maintained by an entire stop to education, the mainspring of human actions, what can be expected; but that the whole would be polluted by its part.” (Mar 17)

Up until this point in the journey, it is very clear that these two issues (natural beauty and slavery) generate the most intense emotions in Darwin.  The natural world is beautiful and he revels in describing its wonders (see Oh verdure New World, that has such forests in’t!). On the other hand, the cruelty of human beings , particularly in the form of slavery, turns him red with anger (see Darwin the Abolitionist). It all seems to come together in this one short diary entry.

Bahia beach (from BACC Travel web site):

Bahia beach

Meanwhile in his zoological notebooks Darwin delves into the realm of physics for a brief time, describing the thermal properties of the light sand beaches of Bahia (compared to the black sands of many tropical islands):

“The sand on the beach is of a brilliant white colour & composed of minute grains of quartz: when walked over the friction of the particles caused a curious high note or chirp: The temperature of this sand a few inches beneath the surface was 108 in the open rays of the sun.— A person in a hot country might with closed eyes tell what colour the ground was on which he was walking.— The effects of reflection from a white surface preponderating those of radiation from a dark.” (Zoological Notebook)

The most interesting thing that pops out of this entry is the “curious high note or chirp” sound that the sand makes.  People today often refer to this as “singing sand”, and it has been described as occurring at both sand dunes and on beaches (as in this case).  From what I can tell, there is still some question about the exact cause of the sound, but the beach variety is most common on quartz sand beaches where the grains are well rounded, spherical and dry. All the hypothesizes that explain the phenomenon have to do with how the grains of sand rub against one another when stepped on.

Here is an audio file of a person walking on the “singing sands” of a beach of Kotogahama, Japan (a “featured sound” from Wikipedia commons). I have to say – it does sound like a “chirp”. Is there nothing this man can’t describe accurately?

Incidentally, the white-sand beaches of Brazil are exactly what you would expect in a region where (1) the source of the rock is largely granite and gneiss (see Geologizing on the South American Craton) and (2) the rivers that carry the sand to the beach are typically traveling a long distance. This is because (1) felsic source rocks are primarily made of lighter colored minerals, including quartz, and (2) as a general rule, darker-colored minerals weather and break down faster than lighter-colored minerals. In other words, if you threw a handful of minerals into the headwaters of a river and let them travel a 1000 miles or so downstream, you’ll find that all that is left at the end of the trip is the most common felsic mineral quartz. Since both of these conditions exist in Brazil, the beaches are typically made up of light-colored quartz sand – just as Darwin describes. (On the other hand, many of the tropical islands Darwin visited were made of dark black volcanic rock, that weathers into black sand beaches.)

Sand is a wonderful substance – I’m sure I’ll have more to say about it in the future. (RJV)

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Responses

  1. Ah yes, the man can write:

    “…nothing could be better adapted for fixing in the mind…”

    “…but that the whole would be polluted by its part.”

    I enjoy knowing that Darwin was a tremendous scientist and that he had some level of political consciousness as well. However, I doubt that be acted on his beliefs like another modern day hero, Richard Dawkins.

    Chuck’s description of the sands of Bahia reminded me of Rob’s exploration of different types of sand in the summer science camp a few years back. What a world of wonderment opened up under the dissecting scopes! Sands from around the world varied in color, texture, shape as well as assorted entrees like shark’s teeth. Clearly it was no longer appropriate to take sand for granite…

  2. I don’t think Darwin was very active in a vocal sense, but I think he knew that his book would reach a large audience and purposely included comments and examples of the horrors of slavery. He knew that he could help make an impact that way. (And he sure says a lot more against it later in the book, too.) His dad and grandfather were also pretty active in the movement, too.

    Thanks for the sand comments and the pun 🙂 One of these days I’ll have to include some close up pictures of different types of sand. I’m sure the opportunity will come up again.

  3. […] the margin of the diary he mentions the “chirping sands” (see more at The Singing Sands of Bahia). In Voyage, Darwin describes the sand a little further writing: “Leaving Socêgo, during the […]

  4. […] and in deserts (especially the white or tan type that is common) is made of the mineral quartz (see The Singing Sands of Bahia). Quartz has the chemical formula SiO2 (one silica atom for every two oxygen atoms). Natural glass […]


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