Sunday sand: neighbourhood differences and an important ingredient

 
Continuing the arenaceously fecund theme of my recent sojourn on the island of Bangka, once again an illustration of how coastal sands can change dramatically from one beach to another just a short distance away. I explored this earlier with an example from southern California where a submarine canyon stole all the sand travelling down the coast with the result that the beaches to its north and south were made of  different ingredients of different sizes. But Bangka faces the shallow waters of the South China Sea that separate it from Kalimantan – no canyons here, but nevertheless contrasting sands.

Bangka’s bedrock geology is dominated by granites, a couple of hundred million years old and quite distinctive – we shall return to them in a later thematic post. These granites make for some wonderful stretches of coast, the rounded boulders and rocky headlands shaping an extremely photogenic landscape.

These granites are, of course, the parent material for much of the island’s sands, weathering and rotting until their tough little quartz grains are liberated to join their companions in the rivers and along the coasts, a process that has been going on for millions of years. But once at the coast, the young quartz sands are joined by other ingredients – shell and coral fragments and a variety of other grains of different sizes that have been participants in the great game of coastal erosion for a long time. Variations in local ingredients and in the local forces of this great game – waves, tides, currents, and storms – conspire with the shape of the coast to sort and winnow sands on a grand scale: the result is that one beach will be built from distinctly different sands from the next. The sand on the left in the image at the head of this post is from one of the small beaches nestled in between the granite headlands of Parai (see the Google Earth image below), whereas the one on the right is from the broad sweep of Matras beach just a couple af kilometres along the coast. Parai’s sands are quite coarse and dominated by chunky quartz grains; those of Matras are white and extremely fine, almost powdery in feel when you pick up a handful. The beaches are different on the topographic scale, the sands different on the microscopic:

 

Left, Parai, right, Matras:

Furthermore, on the beaches of Parai, there are opportunities to observe natural scales in a different way – miniature dynamic landscapes evolving as streams of water flow down to the sea. I find these fractal landscapes compelling and endlessly watchable – as I described a couple of years ago from the Atlantic coast of France; I realise that this compulsion is somewhat weird, but I make no excuses. One of the great – and, as we shall see, important - things about the sands at Parai is that they contain scattered black grains of a heavier mineral than the ubiquitous quartz; a few such grains can be seen in the microscope photos above. As the water flows and ripples in the sand form, these heavier grains tend to get left behind on the crests of the ripples as their lighter quartz companions are hustled away. This not only makes for wonderful designs, but it makes it easy to watch the way in which the ripples are constantly shape-shifting and migrating downstream.

But so what are these heavy black minerals? The complete answer will be revealed and discussed in some detail in a later post, for they are representatives of what has, for centuries, been the driver of the island’s economy and provided its primary claim to fame. I collected a bag of sand plucked from the dark patterns on the crests of the ripples, labelled it “Bangka heavies,” brought it home, and set it out to dry. But, once dried, it seemed to me that I must have made a mistake – it looked to the naked eye no different from the usual sand, no sign of a concentration of dark grains. So I decided to repair to the laboratory (i.e., as usual, the kitchen) and attempt an amateur panning exercise. I put some of the sand in a mixing bowl, flooded it with an ample volume of water, and began gently swirling. Very satisfyingly, lo and behold, there were dark smears of the heavy grains slowly being separated from their lighter companions. I removed some of the lighter sand and repeated the exercise – and here are the results:

Just as the forty-niners of the California gold rush did, I had succeeded in a achieving a preliminary concentration of grains of interest – a process begun by nature. And here they are, some retaining much of their original crystal form, nestled – but now in much greater numbers - in among the quartz grains.

So, what is this mineral? Well, that will have to wait for another time, but meanwhile I will leave you with a couple more images of this spectacular coast.

Comments

[Karen]

What a delightful place! I can feel my feet itching to walk along those shores, and I, too delight in watching ripples. To top it off, you're in an area where the water is warm and your feet don't freeze instantly if you get them wet -- what a deal!

[Malcom Karibad]

Hi! I really like this blog, just read some pages and post and I already got soooo much joy from it! Keep up this great work, it's was useful for me and I'm sure it has been for many others! take care :)