An Earth cache is a special type of Virtual Cache that is meant to be educational. Therefore to log a find you must demonstrate that you have learnt something from the site and experience.
Send your answers to us in an email via our profile page.
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Logging Tasks:
The listed co-ordinates will take you to a view point of the boulders on either side of you.
- Describe the rounded boulders in terms of colour and texture?
- What do you think was the main cause of the rounding of these dolerite boulders and give your reasons for the answer?
- At the coordinates S 33 02.672 E 027 53.373 describe the rocks in terms of colour, texture and bedding.
- On the seaside of the road why do think the rocks on either side of the dolerite is so different?
On the land side of the road you will see a low but conspicuous outcrop of rock that extends below the road along the rocky beach. This outcrop is dolerite and stretches for some two hundred meters after which the coastal outcrop is Beaufort mudrock and sandstone.
The formation of Dolerite (Igneous intrusions)
Molten rock or magma is generated far below surface in Earth’s mantle or the deep roots of mountain belts. Magma rises because it is slightly less dense than the surrounding rock and may erupt on surface, forming volcanoes. Near the site of generation, the rocks are hot and soft, or plastic. Here the magma rises as a blob, much like an air balloon. If the magma solidifies at this position, the resulting body is referred to as a pluton, or if very large, a batholith.
At higher levels in the crust, rocks are cooler and much stiffer or even brittle, so magmas move in a different way. In these regions, magma move upwards by injecting into cracks, often forcing the sides apart. Magma may solidify in the cracks to form what is termed a dyke. Dykes vary in width from centimetres upwards. Magma injecting into layered sedimentary rocks may be forced between the layers to form a sill. Sills can reach kilometres in thickness.
Intrusive rocks tend to be coarse grained, since the magma cools slowly and has sufficient time to form large crystals. Extrusive igneous rocks cool rapidly at the earth’s surface, producing fine grained or glassy rocks. Basalt is fine grained because it cools very rapidly, whereas gabbro and granite are coarse grained because they have cooled slowly. Dolerite is medium grained not cooling too rapidly or slowly.
A type of Weathering
Spheroidal weathering is a form of chemical weathering that affects jointed bedrock and results in the formation of concentric or spherical layers of highly decayed rock within weathered bedrock that is known as saprolite. When saprolite is exposed by physical erosion, these concentric layers peel (spall) off as concentric shells much like the layers of a peeled onion. Within saprolite, spheroidal weathering often creates rounded boulders, known as corestones or woolsack, of relatively unweathered rock. Spheroid weathering has also been called either onion skin weathering, concentric weathering, spherical weathering and woolsack weathering and is the result of chemical weathering of systematically jointed, massive rocks, including granite, dolerite, basalt and sedimentary rocks such as silicified sandstone.
The boulders
Compared to the patches of pebbles, cobbles and boulders (up to the size of a rugby ball) found in places along our beaches these are gigantic. Few weigh less than a tonne, some a few tonnes. Not only are they impressive in size, but show an almost bizarre degree of rounding here and they are almost perfect spheres. These are dolerite boulders that were formed from magma that was forced up through weak places in the earth’s crust where it cooled to form these rocks. Subsequently erosion has taken place and exposed this outcrop as the dolerite is harder and more resistant to weathering than the surrounding formations.
During the geological past these boulders have come to lie on the beach. Their enormous size has prevented them from moving very far up or down the beach. They have been rolled about by violent storm-wave action which may have been the main agent of their rounding or it may merely have perfected a ‘pre-rounding’ by the spheroid weathering so typical of dolerite. Either way, this beach is a wonder of geology.
Acknowledgements
Geology off the beaten track by Nick Norman
Wikipedia – Spheroidal weathering
Story of Earth and Life By Terence McCarthy, Bruce Rubisge
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