Harrow and Hillingdon Geological Society
There’s more to sand than you might think.
This presentation was first presented to the Hillingdon U3A Geology group prior to sending them off for the summer break with a request to bring back samples of sand for examination. This resulted in about 50 samples.
Sand is extremely variable and its examination can tell you a lot about its history. Different sands include dune sand, beach sand, river sand, green sand on Hawaii, black volcanic sand, pink sand, white sand etc.
What is sand and where does it come from?
A lot of sand comes from granite, which is essentially composed of 3 minerals – feldspar, biotite and quartz. It is formed in plutons when magma rises up, stops at depth and cools slowly. The overlying rocks are eroded away and the granite is subject to alteration and weathering, both chemical and physical. Feldspar is unstable at low temperatures and pressures and breaks down to clay minerals. Biotite is more stable but still breaks down but quartz is much more stable. Frost-shattered rocks are washed downstream by rivers and abraded in transit before being deposited on the river/sea bed. The most resistant mineral to this abrasion is quartz.
The energy of the river and the size of particles determines what gets transported and what gets deposited. In fast-flowing streams finer particles are carried downstream and gravel is deposited but in slower-flowing streams the sand is deposited. Very quiet water is required for clay to be deposited and this often is on meeting the sea, where salt causes the clay particles to clump together and settle to the sea bed.
The bedload of rivers moves by saltation, rolling along the bed and bouncing up when hitting other particles. Sand grains may bounce up to 2-3mm above the bed. As it travels the sand is abraded and becomes more rounded, the classes of rounding being – very angular, angular, sub-angular, subrounded, rounded and well rounded. In some rocks, such as metamorphic rocks, the original minerals are often elongate and hence long grains may be an indication of derivation from a metamorphic source area. As sand moves downstream it becomes sorted by grain size, the range being – very well sorted, well sorted, moderately sorted and poorly sorted. The farther sand travels, the better is the sorting. Sands also differ in grain size, ranging from fine sand to medium and coarse sand. The major constituent of sand is quartz, but other minerals can remain in chemically immature sands. Sand is described using the percentage of different minerals. Sediment that falls on the surface of a glacier does not change in transport but the bed load is ground down, often to fine rock flour and deposited as moraine, which may then be moved and sorted by meltwater.
Some examples of different sands
In Hawaii, the source rock is basalt, often with larger crystals of olivine, the resulting sand then being dominated by the green colour of olivine. Black volcanic sand comprises small fragments of basaltic lava.
Pink sand occurs in the Bahamas and other coral islands. The actual colour depends on the species of coral. Its origin lies in the activities of the parrot fish, which bites off chunks of living coral, digests the animal matter and excretes the hard exoskeleton.
Broken-up sea shells are often mixed in with beach sands and in some areas may dominate to form a shell sand.
Dune sand is very well sorted because of the limited range capable of being moved by the wind and grains often have a frosted appearance due to abrasion. They are rounded and often coated in iron oxide to give a reddish colour.
At the end of the day sand usually ends up as sandstone, which may or may not be cemented, (or limestone in the case of shell sands) and like the original source rocks is subject to further erosion and grains may be re-worked. Examination of sand grains may give indications of this past history.