Iron Sand
The reason that beaches along the west coast of the North Island of New Zealand consist of dark, and often entirely black, sand is the presence of iron oxide compounds in various concentrations. Of the iron compounds present, the most prolific is titanomagnetite - a highly magnetic compound.
The iron oxide compounds have originated from rocks deposited from volcanic activity, both coastal and inland, followed by subsequent erosion by wind, rain and waves. The magnetite eroded from the volcanic rock has been distributed along the coastline by the prevailing wind and sea currents.
The high concentrations of magnetite found in various locations along the west coast have made it viable to mine the sand and extract the iron compounds for the production of iron and steel.
The iron oxide compounds, including titano magnetite, found in the sand at Muriwai Beach do not originate in the local area, but have been carried here by the sea currents, which along with the prevailing wind, have deposited the minerals along this area of coast line.
This Earthcache
The concentration of magnetite in the sand at Muriwai is not as high as other areas, but still significant enough for us to perform an experiment to determine both the concentration, and the direction of the prevailing sea currents that carry and deposit the magnetite here.
By undertaking this earthcache, you will be participating in a project to research the iron compound concentration in the black sand of Muriwai Beach, and the observations and results that you record will be used in an ongoing scientific study.
Periodically, the experimental results, received from finders of this earthcache, will be compiled into a graph published on this cache page.
In order to claim a find on this earthcache you will need to repeatedly perform an experiment, using samples of sand obtained at various points along the beach, designed to determine the concentration of magnetic iron compounds in the sand. You will use the results from these experiments to make conclusions based on how the concentrations differ at the various points along the beach.
You will perform the experiment outlined further down this page using sand samples collected near the published coordinates, near the coordinates at Waypoint 1, and at a minimum of 3 other places along the beach between those points. You can choose where those other places are, but they should be reasonably spread out along the beach.
All sand samples used in the experiments must be taken from the intertidal area on the beach (below the high tide mark) and should be collected as a scraping perpendicular to the shore line, particularly where there is visual banding of colour in the sand.
Driving on the Beach
In order to complete this earthcache, you will need to drive the length of Muriwai Beach. For this, you'll need a vehicle capable of driving onto and along the beach.
The speed limit on the beach is 60kph, or 30kph near the southern entrance onto the beach beside the golf course. Please watch out for, and respect, other beach users and give them a wide berth when driving on the beach.
Be very careful driving on the beach - watch out for buried debris and driftwood, and do not make any sudden turns when driving at speed as your front wheels can dig into the sand and flip your vehicle. You will want to time your trip so that you are off the beach no later than 1-2 hours after low tide (depending on how low the low tide is). You be the judge, but please bear in mind that for most of the upper half of the beach, high tide reaches the base of the dunes, so there is no escape from the tide if your vehicle is stranded there.
Equipment Required
You can complete this earthcache while on the beach, or at home. The equipment you will require will depend on how you intend to undertake the experiment.
You will definitely need:
- A vehicle capable of safely driving up the beach
- A number of clean, non-metallic, containers to collect sand samples (preferably all the same size and capable of being labelled if you intend to perform the experiments at home)
- A strong magnet
- Other materials to make an apparatus with your magnet in order to separate the magnetite from the sand (eg. sheets of paper or plastic bags)
- An accurate graduated cylinder with a fine scale suitable for use in chemistry experiments (a measuring cup or jug will NOT be suitable as it will not be accurate enough)
- A gas burner (or your stove top/oven at home) and a heatproof, non-metallic, container that you can use to dry out your sand samples
- Plenty of clean fresh water
Experimental Procedure
To complete this earthcache, you will need to perform this experiment using sand samples from various points along Muriwai Beach.
1. Collect a sample of sand from the intertidal area of the beach (below the high tide mark). Where visual banding of the sand occurs, make sure you scrape a sample perpendicular to the shoreline across multiple bands.
2. Gently heat the sand sample in order to completely dry the sand sample. This step is important in order to make the sand easy to separate (so that it does not clump, and the magnetic and non-magnetic particles do not stick together).
3. Carefully separate the dried sand sample using your magnet. There are a number of ways to do this, but perhaps the simplest method is to put your magnet inside a plastic bag and dip the plastic bag in the sand. The magnet will collect magnetite particles, and pulling the magnet away from the plastic will release these particles. You will need to be quite thorough to get all of the magnetite out of the sand, and then you will need to repeat this process to refine, as much as possible, the magnetite you have extracted from the sand. It is important that you handle the sand and magnetite carefully and do not spill or lose any of the particles.
4. Clean and carefully fill the graduated cylinder up to a specific known volume with clean, fresh water. Record the water level as value A.
5. Add the extracted magnetite (with all sand removed) to the graduated cylinder. Gently agitate the cylinder so that all air is removed from the magnetite in the water, and then record the new water level as value B.
6. Add the remaining sand (that you extracted the magnetite from) to the graduated cylinder that already contains the magnetite in water and gently agitate to remove air. Record the new water level as value C.
7. The percentage of iron-laden compounds, principally magnetite, in the sand at this point on the beach can now be calculated as the volume of extracted magnetite divided by the total volume of the sand sample collected, or: Percentage of iron-laden compounds = (B - A) / (C - A). We'll call this percentage D.
Logging Requirements
To log a find on this earthcache you will need to visit at least 5 spots along Muriwai Beach including the published coordinates, the coordinates of Waypoint 1, and at least 3 other spots randomly chosen by you but spread out along the beach. At each location, you must perform the experiment outlined above.
Each visitor must collect their own sand samples and perform the experiment themselves. Group answers are not allowed on this earthcache. We need the most experimental results we can possibly get in order to have the best spread of results and logging as a group is counter to that goal. Also, this earthcache has a high difficulty rating for a reason, and it is not reasonable to log a find if you do not complete the tasks yourself.
You can log this cache straight away after you have emailed your answers to the cache owner, no need to wait for confirmation. Please include the name of this earthcache in the email - you'd be surprised how many people forget. Also, when contacting us with answers, if you want a reply, please include your email address. Any problems with your answers we'll be in touch.
Email to the cache owner:
1. Having performed the above experiment at least 5 times, using sand collected from at least 5 locations along the beach, you must email the cache owner with the following information:
- The coordinates where each sand sample was collected
- The values A, B, C and percentage D, at each location
2. In addition to the above, and using your results from the experiments, formulate a conclusion about the prevailing sea currents and therefore the direction of origin of the magnetite deposited along Muriwai Beach. This conclusion should also be emailed to the cache owner.
Please remember to state the name of this earthcache in your email to us, and also please include your return email address if you want a response. You'd be surprised how many people forget to include these!
Online log:
After you have emailed the cache owner with the information requested above, you can go ahead and log your find on this cache page. When you log your find online you must include a description of your method of extraction of the magnetite from the sand. This is important as it will be helpful for future finders.
Please also give a description of your adventure on the beach and tell us if you enjoyed the experience.
Photos:
Please take photos as we would very much appreciate photos uploaded with your online log showing your experimental equipment and procedure. The photos will help guide us as to the expected accuracy of your results.
Experimental Results
Periodically, after a number of sets of results have been received, we will upload a scatter graph of results to be displayed here on the cache page.
Happy Earthcaching!
YuGeo4, Sp8sm8 & Greenblat