Geology: More unpredictable than a hungry Jeremy Clarkson When you first start learning about geolog

Geology: More unpredictable than a hungry Jeremy ClarksonWhen you first start learning about geology you are given a few basic rules to help you along the way. As you progress you begin to discover exceptions to these rules, and as you delve even deeper you simple throw the rule book out of the window. Below are some examples of how I learnt the hard way that geology is about as simple as teaching a cat to do the Macarena.LIMESTONE VS DOLOMITELimestone and dolomite/dolostone can often look very similar, the difference between the two being the presence or absence of magnesium within the mineral structure. This means the two react differently when hydrochloric acid is applied. Limestone will react vigorously, providing an obvious fizz when acid is applied to the surface. Dolomite on the other hand, will only react when the acid is heated or if the sample has been powdered first.Seems simple? Well this test works perfectly in the UK where summer is fleeting and our idea of hot weather is seen as abnormally cold across the rest of Europe. When mapping in the Spanish Pyrenees I came across a fatal flaw in this rule; warm weather renders it useless. Even at 1000m elevation the temperatures were above 30oC (I class this as rather warm) and this meant the dark blue limestone or dolomite would be heated throughout the day. Therefore, the response of the rocks was the same as if you had used hot acid, meaning both would fizz, rendering the test completely and utterly useless during the day.IF IT’S CARBONATE IT’S NOT IGNEOUSWhen using the acid test, if the rock fizzes it is usually assumed that the rock is not igneous in origin. The rock could be metamorphic, marble will fizz when acid is applied, or it could be sedimentary, and further investigation will be required to separate the two.Now there are two exceptions to the rule here. If igneous rocks are overlain with a carbonate rich succession then weathering can result in calcareous fluids coating the surface of unit below. These will then fizz with acid if a fresh sample can’t be obtained, which can be misleading at best.The second issue with this rule is a small selection of African volcanoes which, believe it or not, erupt carbonate rich lava (such as Ol Doinyo Lengai in the picture below). These carbonatite volcanoes are still poorly understood, but they appear to buck all trends, from the temperature at which these lavas erupt to the amount of silica they contain (% compared to more than 40% for most volcanoes). While these volcanoes are rare they still are an interesting exception to the rule.VOLCANIC MINERALS = VOLCANIC ROCKSWhen observing a thin section under a microscope I noticed that the minerals suggested a volcanic origin. After noting the relative abundances, I concluded that the rock must have been an extrusive intermediate specimen. I suggested this to my lecturer who laughed. Turns out the rock in question was a volcaniclastic, that’s right a clastic rock comprised of volcanic minerals. These form when a volcanic rock is eroded at the surface, and the fragments are then transported and reassembled as a new rock. A new rock with predominantly volcanic components. Cheers geology.When studying rocks the phrase ‘the more you see the more you know’ definitely applies, and venturing out into the field is often the only way to further your understanding. If anyone has their own examples of exceptions to the rule please comment below, after all there are probably still plenty I am yet to discover!WatsonReferences:http://bit.ly/1xKYciCFurther Reading:http://on.doi.gov/1a6zPkUhttp://bit.ly/1HVuj2fImage Credit: de Moor, J. M. et al., 2013. Volatile-rich silicate melts from oldoinyo lengai volcano (tanzania): Implications for carbonatite genesis and eruptive behavior. Earth and Planetary Science Letters, Volume 361, pp. 379-390. -- source link

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