#oolith

Wikipedia word of the day is oolith : (geology) A spherical granule of which oolite is composed, formed by concentric accretion of thin layers of a mineral (usually calcium carbonate (limestone) but also others such as dolomite and silica) around a core; an ooid. (rare) Oolite.

oolith , n : (geology) A spherical granule of which oolite is composed, formed by concentric accretion of thin layers of a mineral (usually calcium carbonate (limestone) but also others such as dolomite and silica) around a core; an ooid. (rare) Oolite.

More Mysterious Ooliths We recently posted an article about ooliths (http://on.fb.me/1yWGLbD), and mentioned that one criteria for formation of ooids is a rather active sedimentary environment, one in which waves or currents can roll them around and around and around, and thus induce them to grow symmetrically to form their spherical shapes.  Now imagine another environment. One where sedimentary deposition is sooooo slow that it takes ~5000 years to accumulate a single 5 cm layer. We've written of this environment in the past as well, the most boring environment on Earth, the Abyssal Plains (goo.gl/S8UUDq). Now imagine – wait, this is hard. Another sort of rock in which ooids are found within formations formed from the abyssal ooze. Ummm… how can we form ooids, that apparently require an active sedimentary environment, in the Earth’s most inactive sedimentary environment? Several speculations on how these silicious ooids could form include: -- Could there be an upwelling of current within the abyssal zone that lasts thousands of years? Perhaps near a hydrothermal field near a spreading ridge? -- Could we cheat, and say that these must have been originally formed as limestone ooliths, but these limestone ooliths were then transposed (through plate action such as near a subduction zone) to such great depths that the calcite dissolved and was replaced by silica (that is, chert?) In the field area where this particular sample was found, there are great thick deposits of abbysal ooze (radiolarian cherts) that occur in association with spreading ridge formations. Alas, since this sample is from a tectonic mélange zone, it could also have come from a carbonate that was partially subducted into the depths where calcite dissolves… Oh dear. I suppose there might be some way to solve this mystery, but then it wouldn't be a mystery any more, would it?  Please feel free to add your own speculations. Annie R Photo: mine (a scanned image of the rock, actually) from sample found by D. Ghikas within Vourinos Ophiolite sole mélange. Read also to enter the mysterious world of rocks: http://www.sciencedirect.com/science/article/pii/S0037073803001957 https://wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/viewglossrecord.php?gID=00000000253 http://www.sandatlas.org/oolite/