Enquiring into earthy things

Activities

Angle of rest in sand
Dune science
"Faking a fossil"
Salt in sand
Stalactites model
Other pages in this series
Other pages on this site

How to do it

Note: if you are looking for science fair or science project areas, this set of Web pages may help you with ideas for techniques you might use: read with a prepared mind! Alternatively, look at the projects collection which is part of this series.
Salty sand or sandy salt?
You will need some samples of sand, taken from different parts of a surf beach: near the water's edge, right up to the very back of the beach. Collect the sand when there has been no rain for a week or two, so that salt spray has had a chance to "salt" the upper levels. Use standard coffee jars or something else, so that you get the same quantity of sand.

You will also need access to a fairly sensitive balance -- how you find your way to one of these is up to you. If you cannot get a balance, you will need to modify this, and make eyeball estimates of the salt from each sample.

Dry and weigh your samples of sand, then wash and filter the sand carefully to dissolve out any salt, dry the sand again, and weigh it to find out how much salt you have washed out. As a check, crystallise the salt and weigh it.

This will help you understand


The angle of rest
Particles of matter will develop a maximum slope which depends on local gravity, the attractive forces between the particles, their shapes, friction, and maybe a few other bits and pieces. This angle is called the angle of rest , or the angle of repose .

Sand dunes and sand banks are controlled by this angle. So what is it for sand? Put some clean dry sand in a cylindrical glass jar with a lid. You will get the best results if the jar is about half full. After you have put a lid on the jar, tip it on its side and roll it along a table. You will notice the way in which the sand builds up to the angle of initial yield, then avalanches down the slope, and settles out at the angle of repose.

This will help you extend your ideas


Dune science
How are the sand dunes near where you live? Are they healthy? Are they stable? What angle does the sand lie at? How much water is available in the dunes at different places?

What lives there, what is changing about the life forms in the area, and what are their prospects? What tracks can you find in the sand?

You will need to make a number of visits at different times of day, and in different weather conditions. Try taking photographs from the same spot at regular intervals. If you are going to do this, choose a stable point, like a walkway, just outside a fenced area, and centre all of your shots on distant landmarks, so that as far as possible you are taking equivalent shots each time.

This will help you understand


Stalactites in a hurry
This is a method for making a model of a stalagmite-stalactite system. You need some magnesium sulfate (Epsom salts), two glasses, a piece of string, and a large plate to catch all the drips.

Mix up a saturated solution of Epsom salts (or at least a strong solution) and put half in each of the glasses. Sit the two glasses on opposite sides of the plate, and wash the string in water (some string is bound together with glue, and you want to get rid of this). The best string to use is a natural fibre string.

Let the string sag a little between the glasses, and leave nature to take its course. Over several days, you should see stalactites, and maybe even stalagmites form: the chances of stalagmites are much better if you have a piece of cardboard under the string.

This will help you understand


Let's fake a fossil
This is about faking a fossil, where the fakery leaves a bit be desired, but the recovery method is very similar to what real palaeontologists do.

You will need some seashells, plaster of Paris, cement (just cement, not sand and cement), a margarine tub (or similar -- it has to be tapered ), dilute hydrochloric acid, safety glasses and gloves, very small chisels and sharpened needles.

Some fossils form when buried shells are dissolved out by acidic ground water, and then the hole is slowly filled by other minerals. We will do this much faster, and much more blatantly -- this is where the fake is poor.

Warning: plaster is messy stuff, and usually spills. Choose your work place carefully!!!

Wash the margarine tub to make sure it is clean, add about 1 - 1.5 cm of water, then mix some plaster of Paris in the tub by adding plaster to the water in the tub, and mixing gently until the plaster is creamy. Do not stir too much, or you will get air bubbles in the plaster. Next, wet some shells under a running tap, and push them firmly into the plaster, making sure that a bit of each shell is sticking out of the plaster.

There will be air bubbles on the shells, so this would be a good time to bounce the tub onto a firm surface a few times, to shake the bubbles loose. After that, leave the tub for 24 hours, so the plaster can set properly.

Warning: handling acid requires adult assistance, safety glasses and gloves!! This is especially important when you are diluting the concentrated acid.
The next day, add some dilute (about 1 in 10) hydrochloric acid to the tub, until the acid is about 2 cm deep, and put the tub somewhere safe from small people and pets. The shells will begin to bubble as the acid dissolves the calcium carbonate.

You will probably need to change the acid once or twice: I find that it is a good idea to wash the plaster and shells under running water before I add new acid, but eventually you will have a set of clean moulds for your shells. Now you are ready to make the fake fossils. Give the tub one last clean and rinse.

Add about 1 cm of water to the tub, and then start to add powdered cement in small amounts, bouncing the tub to shake cement down into the holes. Use a toothpick to poke more cement down into the holes. Keep adding cement until all the water has been sopped up, bounce the tub a few more times to settle everything, then set it aside for a few days.

I find it best to take the plaster-cement disc out after about three days, but I usually leave it seven days before I start to work on it (this is when you will realise why the tub had to be tapered!). At this stage, you are in the same position as somebody working on a fossil-bearing rock: you have to put the disc down, plaster-side up on some newspaper, and begin clearing away the plaster that surrounds the "fossils".

A small chisel, made by flattening the point of a 3" (8 cm) nail, ground down on a wheel, or filed flat, works well. With a small hammer, you can use the chisel to take away small pieces from the outside, always remembering that you do not know where the fossils are, so you have to go slowly .

After a while, you will start to spot the first hints of the "fossils", and you may be able to clear away slightly larger chunks of plaster. Then you will have to begin with a needle. You can use a sharpened knitting needle, or a small nail mounted in a wooden handle. Real palaeontologists use "wheat bag" needles, the large curved needles that were once used to sew up wheat bags. You can also use a dissecting needle, although this is a waste of a good needle, since it will wear out.

Finish off by giving the cement fossil collection a good scrub with water and an old toothbrush.

This will give you some of the details



And now for some help

Salt in the sand
In theory, sand near the sea should be more salty, but is this the case? Because salt dissolves in water, you should be able to flush most of the sand out by two or three washings. When this water evaporates, the salt will be left behind.
The sources of error: some sand samples may be more tightly packed than others, which is why it is a good idea to weigh the samples if you can.
Where does the salt come from? Think about the spray that comes off the ocean waves. Do you see why you were told to pick a surf beach?

Probes

There may be other things in the sand.
If you heat the sand at 105 degrees C, any organic matter in the soil is not affected. You could also try dissolving out any shell grit with hydrochloric acid, and then heating in a furnace to get rid of any organic matter in the sand. Think carefully about the order in which you do these tests, and get some advice from an adult.

You could extend this study in sand dunes.
Sand dune plants only grow so far down towards the sea. Is this because of salt in the sand? Do you have enough information to go and find out?

Making eyeball estimates of salt
With a bit of practice, you can estimate the amount of salt in a sample by drying out the filtered wash from a known volume of dried sand on a "clock" glass, and comparing the results. Weighing is best, but make sure all the samples are equally dry.

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More about the angle of rest
There is probably an interesting science project for somebody here, collecting sand from various places, and carefully measuring the angles of initial yield and repose for each sand, and the difference between these, which is called the angle of dilatation. Typical published values for this measure are around 8 to 13 degrees. You would probably need to link this to the shape of the sand, and maybe the amount of salt, organic matter or shell grit in the sand.

You begin with the sand surface horizontal, with the jar on a sheet of blank paper on a table. Roll the jar several times to mix the sand well, and return it to the horizontal position. Mark where the jar touches the sheet of blank paper, and then roll the jar very slowly, until you see a single grain tumble down the slope. Mark the point where the jar touches the paper, and keep turning the jar, and the rest of the sand will tumble down to the angle of repose. Continue rolling until there is a second avalanche, a third, and so on, marking the paper each time. The distances between the marks along the paper will then tell their own story. The angle of rest effect is also important to animals. Find out more about ant lionsto see why.

Probe

On a slope The textbook definition of angle of repose refers to sand etc pouring onto a flat surface. Obviously, if the surface has a slope greater than the angle of repose, this would make quite a difference. But does it make any difference to the angle of repose when the surface is at a small angle?

What is the angle of rest of rice grains? Wheat? Macaroni of assorted shapes?

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Life on the sand dunes
Sand dunes are not just piles of sand: they are living ecosystems, where there are many tough plants which shelter many more animals than you may imagine. Sometimes the angle of the sand slope on a mature dune may be greater than the angle of rest for pure sand, because the dune is held up and held together by plant roots.

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Stalactites and stalagmites
The real thing forms extremely slowly, as soluble bicarbonate ions are converted into insoluble calcium carbonate. This quick model depends on capillary flow of solution along the string, combined with evaporation and crystallisation.

Do you have trouble telling stalactites from stalagmites? Think of ants in the pants: the mites go up, and the tites go down :-)

Probe

Greenhouse? More carbon dioxide in the atmosphere should mean more bicarbonate ions dissolved in the water that flows over limestone. What effect will higher carbon dioxide levels have on real cave systems with calcium carbonate formations?

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About faking a fossil
The extraction part is close to the real thing, although bedded rocks like shales are easier to work on than plaster, and limestones can often be cleaned away with dilute acid, leaving the fossils behind.

Many plants and leaves can be dissolved out with a weak alkali, though this usually does not affect the "veins" of leaves. I have yet to experiment with acids on plant material: perhaps you can beat me to it. Or maybe you can do some localised boiling of vegetable material with a microwave: there is plenty of room to develop this scheme.

Remember that real fossils form very slowly, by replacing one lot of material (the shell) with another material (the rock), atom by atom.

The fossil plates that you end up with make rather different paperweights: if you are artistic, you may wish to paint them up.

Curiously, a few of the organic bits from the outsides of shells may be left behind, giving real colour to your "fossils". Andrew Parker, formerly at the Australian Museum has been investigating a similar effect in real fossils, which sometimes lets him deduce what colour the fossils are!

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This file is http://www.ozemail.com.au/~macinnis/scifun/earth.htm, first created on September 19, 1998. Last recorded revision (well I get lazy and forget sometimes!) was on December 8, 2001.
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