* Visiting GZ right on high tide might be difficult, two hours either side of high tide will be fine. Tides can be checked here.
Our aim with this particular Earth Cache is to take you to an interesting spot and teach you some basics, we have presented this earth science lesson on a basic level to accommodate those who avoid EC’s ‘cos they are too hard’ and also to accommodate those travelling with Children (TAWKers) who incorporate Earth caches into their curriculum.
Geo-daughter was amazed by the black sand that decorated the beach here at 1770, so she decided to find out more about it. She soon found black sand was a very general term used for lots of different things and there could be 1770 possibilities.
Generally speaking, black sands can be broadly divided into two parts, Volcanic and Mineral. The most widespread type of black sand is composed of volcanic minerals and lava fragments. Such sands are especially common on the coasts of volcanic islands for example Hawaii and the Canary Islands.
Black sand beaches are black because many volcanic minerals and rocks are dark-coloured. Common rock types of volcanic islands are basalt (black when fresh), andesite (usually dark grey) and volcanic glass (often black in colour). The minerals that give black colour to these rocks are predominantly pyroxenes (mostly augite), amphiboles (mostly hornblende) and iron oxides (mostly magnetite). Such sands are heavier than ‘normal’ light-coloured sands and become very hot on a sunny day. Dark colour and heaviness are both caused by high iron content. Iron gives black colour to most minerals because it absorbs light very well and it is also heavy.
Black volcanic sands may contain many non-black grains like green olivine crystals, volcanic rocks of other colours, light-coloured quartz and carbonate biogenic grains (coral sand). Most volcanic minerals are not very stable. They decompose rapidly. These sands are said to be compositionally immature (mature sands are composed of quartz and other minerals very resistant to weathering.
Heavy Mineral sand is the other type of black sand, usually appearing as streaks or pockets rather than whole beaches, it occurs mostly but not only in continental (inland) settings.
Heavy minerals are minerals which have a specific gravity above 2.9. There are almost all colours present among the heavy minerals, but they seem to be dark compared to usually light-coloured quartz sand. Heavy mineral sands are usually composed of minerals that are relatively resistant to weathering. Some of these minerals are tourmaline, magnetite, garnet, rutile, ilmenite, zircon, epidote, staurolite. Heavy minerals are in most cases spread among the light-coloured (and usually much larger) quartz grains but in certain conditions they tend to accumulate.
These streaks of black sand are composed of tiny gems that were carried high on the beach either by big waves or streams, but they successfully managed to avoid flowing back with the receding waves because of their above average density.
Now she knows what black sand generally is, she still couldn’t work out what the 1770 grains were, finding a local geology student, she was able to find out that the sands here are in fact mineral sands, further reading taught her they were derived from the Agnes Water Volcanics which are the predominant rocks in the area. These rocks are Late Triassic in age or about 220 million years old. At that time the area was dominated by explosive volcanoes. Agnes itself lies within a 20 km diameter caldera - a big crater caused by a collapsed volcano; she felt this information might help you with your answers.
To log this EarthCache we require you to visit GZ, consider the information given and make some observations, then message us with the following answers to the best of your ability.
As identifying minerals in the field is difficult, geo daughter has tried to make the answers easy for you:
1. Finish the sentence: The mineral sands of 1770 are mainly made up of I________ and a small amount of R________.
2. Find some black sand on the beach: Scoop some up with your hand, describe what you see and feel. After rinsing your hand off, is it clean or does it take some scrubbing off? Explain why this might be.
3. Looking at the beach where you have scooped from, what does the sand underneath look like? Is it darker or lighter, explain why this might be.
4. Photo requirement, we had fun making handprints, we would love to see your handprints on the beach too OR another creative way of leaving a non-permanent mark on the beach that represents your team.
You are welcome to log your find straight away to keep your TB's and Stats in order but please message us with your answers within 24 hours. Cachers who do not fulfil the Earth Cache requirement will have their logs deleted.
