The Moeraki Boulders (North Otago)
In South Island, New Zealand
Size:  (not chosen)
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It looks like a scene from a science fiction movie; hundreds of huge marble-like boulders are strewn along the windswept beach like giant turtle eggs. That's what's been dumped on the sands at Moeraki, just south of Oamaru.
Best seen at low tide, these beautiful spherical stones - some up to 4 metres in circumference - can be seen emerging from the cliffs and disappearing into the sand and the sea. The coastal elements are slowly eroding the boulders into fascinating shapes and uncovering new ones.
There's been a bit of debate as to how the Moeraki Boulders got there - much of it rather colourful. According to Maori tradition, the boulders are gourds and calabashes washed ashore from the voyaging canoe Araiteuru when it was wrecked upon landfall some 1,000 years ago.
The reality is more about geology. These boulders were formed over millions of years on the sea floor by a layering process similar to the formation of oyster pearls. The seabed was uplifted to form coastal cliffs that have eroded over time so the boulders tumbled down onto the beach.
The most striking aspect of the boulders is their unusually large size and highly spherical shape, with a distinct bimodal size distribution. About one-third of the boulders range in size from about 0.5 to 1.0 metres in diameter, the other two-thirds from 1.5 to 2.2 metres, the majority being nearly to almost perfectly spherical. A minority of them are not spherical, being slightly elongated parallel to the bedding of the mudstone that once enclosed them.
These concretions were created by the cementation of the Paleocene mudstone and the precipitation of calcite from pore waters of the Moeraki Formation, from which they have been exhumed by coastal erosion. The spherical shape of these concretions indicates that the source of calcium was mass diffusion and not fluid flow. Studies of the oxygen and carbon comprising the calcite cement and spar comprising the Moeraki Boulders demonstrates that the main body of these concretions started forming in marine mud near the surface of the Paleocene seafloor. The larger, 2-metre in diameter, Moeraki Boulders are estimated to have taken 4 to 5.5 million years to grow while 10 to 50 metres of marine mud accumulated on the seafloor above them.
After the concretions formed, large cracks called septaria, formed in them. Brown calcite, yellow calcite, and, in rare cases, dolomite and quartz progressively filled these cracks when a drop in sea level allowed fresh groundwater to flow through the mudstone enclosing them. The process or processes that created septaria within Moeraki Boulders, and in other septarian concretions, remain an unresolved matter for which a number of possible explanations have been proposed. These cracks radiate and thin outward from the centre of the typical boulder and are typically filled with calcite, which often, but not always, completely fills the cracks.
To claim this Earthcache, please complete the following requirements:
1. Take a picture with your group posing beside a boulder that you find most striking. Make sure your GPS is visible in the picture. Post your picture with your log.
2. Find a Moeraki Boulder that demonstrates a good example of septaria cracks and take a picture of it. Post this with your log and e-mail what material you think has filled these septaria cracks.
3. Since the boulders are bimodal in size distribution, find a boulder of each size group as listed above. Take the circumference and diameter of the two boulders and include this information in your e-mail. You will need to bring a tool to take these measurements. Send this e-mail when you post your visit.
Much of this information was taken with thanks from: http://www.answers.com/topic/moeraki-boulders
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Last Updated: on 6/11/2017 1:19:59 PM Pacific Daylight Time (8:19 PM GMT)
Coordinates are in the WGS84 datum