Millstream Volcanics EarthCache
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This cache is at Millstream Falls on the Atherton Tablelands between Ravenshoe and Mount Garnet.
Millstream Falls, besides being a spectacular water falls - depending largely on the prevailing weather - is of interest because of volcanic features
Three periods of lava flows can be seen. The lava came from the volcano at S 17° 35.415 E 145° 31.751, now known as “Windy Hill” and home to the wind turbine farm that has become a prominent part of the Ravenshoe landscape.
Some three million years ago the first flow followed the ancient valley over granite and rhyolite rocks, both of which are formed deep in the earth’s crust, and so, to have been on the surface, must have been formed many millions of years before that.
Your first task.
Take a photo of the falls and mark a line that shows the boundary between two flows in the actual drop of the falls.
A more recent period of activity, about one and a half million years ago, produced the material that forms the valley sides and the surrounding lands. A small cliff formed by these rocks can be seen across the top of the falls. This cliff shows the feature that is called “Columnar Basalt”. Lava flows tend to fracture into mostly hexagonal columns due to lateral shrinkage during the cooling process. The distance across the column can vary from a few centimetres to over a metre according to the speed of cooling, which in turn is largely influenced by the depth of the flow. Fast cooling forms relatively small columns while slow cooling forms larger columns.
Your second task.
Without leaving the viewing platform, decide whether the cooling process in the rocks of the cliff was relatively quick or relatively slow.
To the layman in general a rock, is a rock, is a rock, and there is not much difference between them if one has just stubbed one’s toe on a member of the species, but there is one type of rock that is fairly easily recognised as being of volcanic origion. Because this one has not been formed by the mere cooling of lava flows, but has been blasted from a volcano as molton rock mixed with gaseous substances, it is pocked with small holes from which the gas escaped as the rock cooled. It is often referred to as “Honeycomb Basalt”. Since this site is a considerable distance from the main volcanic eruption, there is not a lot of Honeycomb Basalt but there is some.
Your third task
Find and photograph a rock showing this character in the rocks lining the path to the viewing platform or in the picnic area above the valley. Show yourself, or another member of your party, standing by, touching, or holding the said rock.
To log this cache, please email me the two photos and your answer to the second task.
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Treasures
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