Dykes are discordant tabular or sheet-like bodies of magma that cut vertically or almost vertically through and across strata, though some dykes are steeply inclined.
They are formed when molten rock of one type or other is injected at high pressure in between earlier flows of magma which have cooled or are in the process of cooling. They form a layer of mineralised rock sandwiched between the existing layers of rock forming a band of stripe like formation, often of a greatly contrasting colour, which has a striking appearance.
Hundreds of dykes can invade the cone and inner core of a volcano. Dykes may occur in swarms of parallel dykes, particularly where there has been crustal extension. In regions of crustal extension, fracturing may open the route for filling by magma from a deep source, or intrusive magma may promote the fracturing and extension of the crust. Outcrops of dykes can range from a few metres to many kilometres in length, and can spread lateral distances from a few centimetres wide to over 100 m. Very thin dykes or dykelets are sometimes called veins. The Great Dyke of Zimbabwe is a gabbroic mass nearly 500 km long and about 8 km wide.
The dyke we have here is nothing like this size of course.
Because dykes intrude relatively cool country rocks, they frequently display a chilled margin, with grain size becoming coarser towards the centre where the rate of cooling has been slower. If the dike cooled very slowly at great depth, the large crystals of pegmatite dykes have had time to form.
Pegmatite dykes represent crystallization from a residual melt fraction, but pegmatites are formed from a water-rich fluid, so are very coarse grained. Most pegmatites contain quartz, alkali feldspar, micas, and tourmaline. However, some pegmatites contain minerals such as tourmaline, garnets, apatite, beryl, topaz, spodumene, magnetite, sphene (titanite), and zircon, and various other rare minerals. This occurrence of rare minerals results from progressive concentration of trace elements into the last fraction of melt because these elements have not been removed by earlier crystallization during the solidification of the bulk of the magma.
Aplite dykes are commonly found in granitic bodies. Aplites are light coloured, fine to medium grained, and equigranular. Aplites formed from the ultimate residual melt after most of the crystallization of the granitoid was completed, so aplites are rich in quartz and alkali feldspar and sometimes muscovite.
Source: STRUCTURAL GEOLOGY – TECTONICS, VOLCANOLOGY
AND GEOTHERMAL ACTIVITY
Kristján Saemundsson
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To qualify to log a find on this earthcache please email the answers to the following questions to me.
1. Based on the information provided above, and your observation of the colour and other properties of the dyke material and surrounding rock, would you say this is a Pegmatite Dyke or an Aplite Dyke?
2. What is your estimate of the width of the Dyke in centimetres, to the nearest 10 cms?
3. About midway between where the dyke emerges on the north side of Quart Pot Creek, and where it disappears beneath the overburden at the bank, there is a man made object inserted into the dyke. What do you think this object is? Why do think it was placed here, in the dyke?
4. Please attach a non spoiler photo of yourself or a personal object, near the dyke. This is now compulsory following recent changes in the guidelines.
If you email your answers to earthcaches@jamieson.id.au rather than to my profile address, this will keep them out of my bulk watch list email and should ensure a much faster response than if you use my profile contact details, but of course it's up to you.
Note: It's quite okay to log an earthcache find immediately, so that your order of finds is preserved, but I must receive your answers with a week to ten days, or your log may be deleted..
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If you are interested enough to wish to see another dyke like this one, there is a great example in Girraween National Park. You need to visit the Underground Creek and climb around behind and up above the rock formations to see it. It's a great sight.