Listening to Enchanted Rock When visiting a site for its geology, we tend to look at it and maybe ev

Listening to Enchanted RockWhen visiting a site for its geology, we tend to look at it and maybe even touch it, but sometimes we should listen to it as well. Enchanted Rock in Central Texas got its name from the Tonkawa Indian Tribe because it “creaks and groans”; leading them to think it is haunted by spirits.Enchanted Rock is the largest of several exfoliation domes that give us a glimpse of something mostly hidden underground - a 160 square kilometer (62 square mile) batholith. Batholiths are very large igneous intrusions, meaning they are made from a large quantity of magma. Recent studies have shown that batholiths may be 10 to 15 km (6 to 9 miles) thick. The Enchanted Rock Batholith was created a billion years ago when molten rock cooled and crystallized under a layer of rock without breaching the surface. The magma pushed up into the rock above in some places creating the domes, which are also plutons. Plutons are intrusive igneous rock a few to tens of km across that cooled below the surface of the Earth.Exfoliation domes have layers that peel away over time. When the batholith formed it was buried within the earth and held at pressure, but as the surface rocks eroded that pressure was reduced to almost nothing. When the pressure drops on a rock like this, it begins to expand outward, allowing the dome to break into exfoliation sheets that form parallel to the surface of the pluton (see photo). When these cracks form, they produce a crackling sound called a rock burst – the creaks and groans that led to spirit legends. Over time the layer of rock that has broken off the surface erodes and falls away due to gravity, wind, and water, thereby reducing pressure on the next layer and beginning the process all over again.Enchanted Rock is now part of a 665-hectare (1,643-acres) park known as Enchanted Rock State Natural Area and has been designated a National Natural Landmark. This will ensure it is not quarried like many other domes that were once nearby.REPhoto Credit: Alan Cressler (alan_cressler on Flickr)References:http://bit.ly/1H1SU42http://bit.ly/1zrnnUuhttp://bit.ly/1agwzCShttp://bit.ly/1InSAhV -- source link

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