Did you order some rocks? EarthCache
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Did you order some rocks?
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There are four large rocks originating in Western Australia located in this corner of the Museum grounds, so please check the website provided for up-to-date admission details.
When I visited on the date this Earthcache was placed, there was no information about the rocks, so it is your job to identify the rocks and e-mail me the answers requested below.
Please note that these rocks are in a temporary location, so could be moved at any time!
Permission for this Earthcache has been kindly granted by the Western Australian Museum – Perth.
Lesson Goals: You will learn how to identify and compare different rocks types and become familiar with several iconic rocks that can be found in various places throughout Western Australia.
It is unusual to find these four rock types together, so it is a good opportunity to see them side-by-side to immediately recognise how different they are.
There are four rocks present near the Listed Coordinates. For the purpose of this Earthcache, they will be numbered from 1-4, with boulder number '1' being the most southerly one (closest to Museum 'Gate 4'). You will need to assign the descriptions in the table below to the appropriate rock.
| Name: |
Banded Iron Formation (BIF). |
| From: |
Pilbara, WA. |
| Info: |
The Pilbara Banded Iron Formations [or BIFs] formed between about 2.4 and 2.7 billion years ago. Today, enriched BIFs are the main source of iron ore in Western Australia, and a major contributor to the State’s economy. The red bands represent iron-oxide rich layers, the lighter bands are silica-rich [cherts].The banding resulted from rhythmic chemical precipitation in ancient oceans. The formation of vast deposits of iron oxides not only in Western Australia, but also in Brazil, South Africa and Russia, may have resulted, in part, from the action of the earliest life forms on Earth. Many of the earliest life forms were bacteria, including cyanobacteria, that are photosynthetic. The oxygen that was generated by these organisms in what was then an oxygen-poor environment resulted in iron present in the oceans being oxidised. In a sense, these early oceans ‘rusted’.
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| Name: |
Orbicular Granodiorite. |
| From: |
Boogardie Station, WA. |
| Info: |
A one tonne slab of orbicular granodiorite [a type of granite] quarried from the Murchison District of Western Australia. The rounded, concentrically layered structures are called 'orbs', from which the rock gets its name. These curious structures sometimes form at the edge of a large volume of molten magma as it crystallises deep within the Earth's crust. One of the ways orbicular rocks can form is when early 'sand' crystals act as centres around which successive layers of different minerals crystallise from the solidifying magma. The small vein of paler granitic rock that cuts across the slab was injected after the orbs formed.
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| Name: |
Fossil Stromatolite (Limestone). |
| From: |
Pilbara, WA. |
| Info: |
The layered structure of stromatolites is caused by the diurnal secretion of a sticky mucus and precipitation of calcium carbonate by photosynthetic cyanobacteria. The mucus traps sediment particles and the lime acts as cement, hardening the structure. Stromatolites often form dome-shaped structures, and can grow in a wide variety of shallow water environments, including hyper-saline bays and freshwater lakes. Western Australia is renowned for modem, active stromatolites, and Shark Bay is the best known locality in Australia where these intriguing structures built by organisms can be seen. Western Australia has an extraordinary fossil record of stromatolites. The oldest known stromatolites were found in the Pilbara and formed 3.45 billion years ago. This example is about 2 billion years old.
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| Name: |
Gold in Quartz boulder. |
| From: |
Orient Mine, Murchison Goldfield, WA. |
| Info: |
This boulder of quartz containing a thin vein of gold was removed from the Orient Mine in the Murchison goldfield of Western Australia in 1892. The boulder was presented to the Museum by Messrs, Moony and Lawrence in the same year.
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Sources: Text in table taken from signs previously displayed with the rocks at the Western Australian Museum – Perth.
Once you have identified the correct rocks, please e-mail me the answers to the following:
- The correct order of the rocks from left to right (e.g. '1=BIF')
- Examine the Orbicular Granite. a) What is the range of diameters of the 'orbs'? b) In your opinion, are they similar or various sizes (and suggest a reason why this could be)?
- What do the deeper red-coloured bands in the BIF suggest in relation to the other bands in the rock?
- Cyanobacteria are understood to be responsible for producing something that is vital for human existence - what do you think this is?
- Find the vein in the Quartz boulder. How wide is it (on the side facing you)?
Please use the following link to submit answers to this Earthcache: GeologyBoy.Earthcache@gmail.com
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