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Geology/Unusual Rock?

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Question
mystery rock image 1
mystery rock image 1  

mystery rock image 2
mystery rock image 2  
I found this looking for Lake Superior agates in east central Minnesota close to the St. Croix river.  I've never come across anything like this before and am totally mystified by what it is.

The rock is about 18-20 inches long and about 12 inches in diameter. It weighs roughly 22lbs.


I have a video of the rock on YouTube found here.  http://www.youtube.com/watch?v=VJGzEBgVlL0

Answer
Dale:

What you have looks to be a siliceous concretion.  The irregular crystals look to be silica, and as you observed there is also some iron cement thrown in as well.  Most concretions form during digenesis when dissolved silica, iron or calcite move through sediments in percolating ground water and then precipitate out around an organic nucleus, say a shell or leaf and continue to grow.  As observed in your specimen, anything in the surrounding sediment gets incorporated into the concretion, gravel, rocks, etc.

Since the area you were searching in is know for micro crystalline quartz it stands to reason that the geochemistry of the area is dominated by quartz and ground water would have a lot of free quartz in it.  We are not talking near surface here, digenesis occurs at great depth, where temperatures and pressure enhance the solubility of quartz.  You've seen travertine deposits near springs or caverns, where calcium carbonate laden water comes to the surface and deposits dissolved calcium carbonate as travertine limestone. Dunn's River Falls in Jamaica is a great example.  This same sort thing occurs with silica but at greater depths.  As you can imagine the solubility of quartz is low under near surface conditions, so any kind of microcrystalline quartz needs high temperatures and pressures.  Chert nodules found in limestones usually long bands form the same way.  Silica deposited with the limestone, in the form of siliceous skeleton parts of micro organisms migrate and coalesce over time to form the chert nodules interbedded with the limestone.  

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Keith Patton

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I can answer questions concerning physical and historical geology, environmental geology/hydrology, environmental consulting, remote sensing/aerial photo interpretation, G&G computer applications, petroleum exploration, drilling, geochemistry, geochemical and microbiological prospecting, 3D reservoir modeling, computer mapping and drilling.I am not a geophysicist.

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I have 24 years experience split between the petroleum and environmental industries. I have served as an expert witness in remote sensing, developmental geologist, exploration geologist, enviromental project manager, and subject matter expert in geology and geophysical software development.

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