Sorting Sand Grains by Grain Size.
by
Bill Andrake, Swampscott Middle School
From the size and shape of its grains to its composition, sand has a story
to tell. Analyzing a sample of sand can reveal how it was transported, the source of its
journey, the type of environment and climate from which it came. Uncovering the story
in a sample of sand is a science lesson that incorporates all the disciplines in
science.
Background
from.... What is Sand? (Information from Dr. Dave Douglas: Pasadena
Community College, CA)
http://faculty.pasadena.edu/dndouglass/sand/SANDHP.htm
Sorting - The sorting of a sand sample is simply how well the grains are separated
out by size.
For example, if all the grains in a sediment sample are very nearly the same size,
then we say the sample is "well sorted."
If a sediment sample were to contain pieces of gravel, as well as sand and silt, it
would be a "poorly sorted" sample.
Sorting is somewhat dependent on the distance of transport, but it is mostly affected
by how the sand was transported to its location.
Water’s ability to sort particles by size depends on the strength of the waves or
currents pushing the sand. Stronger waves push bigger particles along with smaller
grains creating poorly sorted sand. Weaker waves can only deposit finer grains
resulting in a well sorted sample. Wind is probably the best at sorting grains; all but
only on the finer grain sized (not much gravel is moved by wind transport).
The stronger the transport medium... the more poorly the sediment is sorted.
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Very Poorly Sorted Sediment... a mix of large boulders, cobbles , pebbles, gravel,
sand and silt. Deposited by glaciers or very strong
waves and currents of water.
Poorly Sorted Sediment.......... a mix of gravel, coarse and fine sand: Deposited by
strong waves and currents.... silt is often washed
away in these conditions.
Well Sorted Sediment............ maybe some coarse sand but mostly medium sized
and finer grains of sand.... moderate waves and currents.
Very Well Sorted Sediment... Mostly fine grains of sand and silt...weak transport
medium such as quiet waters or wind blown sand such
as that found in dunes or high up on the beach.
Activity: Sand Sort
We can determine the sort and size of sand grains by pouring and sifting sand
through a series of sieves.... (screens)... with different mesh sizes.
Sieves
The mesh size of the screens becomes
smaller and smaller as the sand passes
through each sieve.
Larger or “coarse” sediments are
filtered by the sreens with bigger
mesh sizes allowing finer grains to pass
though to the next screen and so on....
Coarse Grains
Fine grains
In this way a sample of sand can be sorted by grain size. This informaton is useful
when analyzing a sample of sand as to its history and the way in which this
sand was brought and deposited.
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Materials:
• gram balance, small paint brush, plain paper, graph paper, samples of dry
beach sand, and a sand sieve set.*
The sand sieve set that I use has
seven trays range from gravel, with
grains greater than 4mm in diameter
to fine sediment less than 0.1 mm .
Procedure:
1) Place a piece of paper on a gram scale,, rezero.. and
measure out 100 grams of your sand sample.
Make sure that the sieves are stacked in
the proper order from largest to smallest
mesh size, top to bottom.
Coarse Grains
2) Take the 100 g sand sample on the paper
and pour the sand through the series of
screens shaking the stack side to side
for about 5 minutes to make sure the
finer grains pass through the sorter.
Fine grains
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3) Carefully take the stack apart and empty
the sand in each sieve on to its own
sheet of paper...
BE SURE TO LABEL EACH SHEET OF PAPER WITH THE PROPER
GRAIN SIZE.
4) Next get the mass of the sand from each sieve using a gram scale.
RECORD THIS IN THE DATA TABLE BELOW
Location of sand sample: ___________________________________________
Date of the collection: __________________
Grain Size
Grams in
100g Sample
% of Sample
> 4mm (gravel)
2 to 4mm
(fine gravel)
0.5 to 2mm
(coarse sand)
0.25 to 0.5mm
(medium)
0.15 to 0.25 mm
(fine sand)
0.1 to 0.15 mm
(very fine)
< 0.1 mm
(silt-clay)
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Shape of the grains
Analyze your sand samples.
• Prepare bar graphs showing the sort of the sand grains in
our samples. Set up your scale on graph paper with X- Axis labeled “Grain Size”
Y- Axis “labeled Mass (g) ”. Also, include an informative title.
mass
(g)
(coarse grains)
(fine sands)
grain size
Discussion
After graphing the sample of sand one can see a pattern. Fine, well sorted sands
such as windblown sands near dunes, will have a bar graph showing a higher
percentage of the sample with finer grains.
Poorly sorted sands such as those collected lower on a beach will have a mix
of gravel, fine gravel, coarse, and finer sand grains. This is because sands lower
on the beach are transported by waves, which can transport coarse as well as finer
grains.
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The Story in the Shape of Sand Grains
Background: from.... What is Sand? (Information from Dr. Dave Douglas: Pasadena
Community College, CA)
When first formed... all rock fragments have sharp corners and edges.
Rounding - As material is transported, it is subject to abrasion (wearing away) and
impact (or collision) with other particles which tends to "round off" the sharp edges or
corners.
Be careful not to confuse rounding with sphericity (shaped like a sphere).
A well-rounded grain may or may not resemble a sphere.
Source of the sand and age of the grains.....
A well rounded sand grain has probably traveled a great distance from its original
source area, while an angular grain (sharp edges) has probably only been
transported locally. Well rounded sand grains are probably older than those with
sharp edges.
Rounding is also related to the size of the grains...
Larger rock fragments tend to round much more quickly than smaller sand grains
because they strike each other with much greater force. It is common to find
well rounded gravel mixed with smaller finer grains that are angular with sharp edges
and corners.... all grains being the same age and source.
The hardness of the minerals making up sand grains is also a major
factor in rounding.....
Quartz is a very hard mineral and more difficult to round off. If you come across
small rounded sand grains of quartz... they have traveled far, are older grains, and
may have been exposed to strong currents, waves, etc.
Sand grains made from seashells, coral fragments etc. are made of the mineral
calcite... Ca CO3 (calcium carbonate)... this is a much softer mineral than quartz and
wears down easily.... (thus relatively young, local, sand grains formed from sea shells
or coral fragments may have rounded edges).
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Photo of fine quartz sand grains from Siesta Key, Florida magnified 100 times.
The shape of the grains are rounded and subrounded. In order to round off
the tiny grains of this hard mineral (quartz) these grains must have traveled
a long distance; most likely from the Appalachin Mountains, as there is no
natural bedrock with quartz in this area of Florida. (Note the absence of
any grains from softer minerals, which would have been worn away during
this long journey.
By contrast the angular grains in this sand from Maine are relatively young and
were not transported very far as they still have sharp defined edges. They were
from local granite rock formations consisting feldspar and quartz.
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Procedure:
After sorting your sand, take a small sample from each grain size and observe it
through a stereo or compound microscope. Match the shape of the
grains in your sample to those in the diagram below. Record this in the table on p. 4.
Diagram from “World Atlas of Sand”
http://www.sand-atlas.com/en/shape-of-sand-grains/
Discussion Questions from the Shape of your Sand Grains.
Sand Sample Location __________________________________________
1. In your sand sample...was there a relationship between the size of your sand
grains and their shape or degree of rounding ?
____________ Explain ____________________________________
_______________________________________________________
_______________________________________________________
_______________________________________________________
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2. From the shape of your grains would you consider your sand grains to be from a
local source or did they travel a great distance to this location ?
(remember focus on the shape of the smaller grains for this answer... as larger
grains can be rounded off much easier).
___________________________________
Explain your answer. ________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
3. From the shape of your grains would you consider your sand grains to be an
older sand or relatively young sand ?
(remember focus on the smaller grains for this answer... as larger grains can be
rounded off in less time).
___________________________________
Explain your answer. ________________________________________
______________________________________________________
______________________________________________________
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