Last week I had a discussion with a friend. We were talking about the origin of sand. I believed that sand comes from the ocean as the result of erosion, and the world once was covered with water, so the origin of sand is the ocean. My friend didn’t agree with the theory. He says it would take a lot more time for that much sand exists in the world now. He argued that sand comes from all kind of thing, mountains, rock,…
I know I didn’t present the case very well, please forgive me.
Any doper sand experts could give a correct answer ?
A boulder is a fairly large rock. One can easily imagine a location – some have seen one – where a large number of boulders are present.
The next size of rock down from a boulder is a cobble. Typically cobbles are about 8" in diameter, though they come larger and smaller.
Next item down is the pebble. And a typical rocky shoreline is composed of pebbles. There is a sedimentary rock composed largely of pebbles and cement called conglomerate.
Then you get to gravel, particles smaller than pebbles but clearly individual rocks which can be seen at a distance.
Silty soil consists of fairly fine particles, generally with humus composed of decaying and decayed vegetation mixed in.
Getting even smaller than silt, we have clay, which consists of extremely fine particles, nearly always with humus.
Now, fill in the gap. The missing category of particle is sand. When rock is ground, it turns into coarser or finer particles, ranging from boulders down to clay.
Typically, sand is composed of silica and feldspar, though other minerals can stop at the sand level of coarseness. White Sands N.M. is composed largely of sand-size gypsum particles.
The ultimate origins of sand are in granite-type igneous rocks. (Someone will have to identify the various rocks that can act as sources.) But most sand over the millennia has been compressed and cemented into sandstone, and most present-day sand has been eroded from sandstone.
Sand generally comes from erosion. Sand is produced by the action of water - fresh water in rivers and streams, and also by the action of waves on the shore, - as well as by wind scouring rock with other bits of blowing sand. It can also be produced by the action of glaciers grinding down rocks. Sand-type deposits can also be produced directly by volcanic eruptions. So your friend is right.
As far as we know, the world was never entirely covered by water.
Despite the popular image, most of the Sahara is a rocky desert, not sandy. And northern Africa has been around for a very long time - plenty of time for freshwater erosion to have produced lots and lots of sand.
This said, wind erosion has probably been responsible for some of the Sahara’s sands. In any case, wind can blow sand around (whether produced by water or wind) and cause it to accumulate in dunes in particular areas away from where it first formed.
I think most deserts are rock, gravel and what I would call coarse sand on the surface. The fines have long since been blown away. At least that’s the case around here and in what I see of the Sahara and Gobi in pictures.
I was just remembering Geobabe, who told me once she doesn’t post much in General Questions and she was hoping for more geology questions. I remembered her when I saw the thread title. This is a job for Geobabe!