Concretions

If you read the link in the post I am lichen rocks, you now know that concretions are formed in a variety of ways. However, whatever the way it formed they provide entertainment. 🙂 Also here is a link to Macrostrat which is a platform for geological data exploration, integration, and analysis. Additionally, there is app, Rockd for your smartphones.

So I had been trying to learn more how these rocks form from Dr. Andy Madden (University of Oklahoma). Here was parts of our conversation:

At first look, they look like sandstone concretions, either of carbonate or iron oxide.  If you had acid and could drop some on it, the carbonate should fizz (vinegar might be strong enough if it was pure-ish calcium carbonate). Iron oxide might be darker and not reactive to acid. Andy

Hi Andy, I guess my question is more how were these concretions formed. Is it like a piece of plant or some other cause that causes the mud to roll around to make its shape? I know or think I knew that concretions sometimes were formed from tunnels of critters. Am I on the right track?

Next time I’m there I will collect a small rock to test with vinegar.

Thanks a bunch Andy!

Mary

Good morning Andy,

Well, as I was sitting drinking my tea this morning, I did some thinking. 😱 I was wondering if the analogy of a larva lamp might be a way to think of how the concretions formed?

Also some other types of rocks are in the area to give you idea of the “habitat”.  I like to call them composite rocks because they are like concrete with lots of little pebbles embedded in them. Technically, those are probably sedimentary rocks? I attached a couple photos of those rocks.

It boggles the mind to think of all the churning going on to form the rocks! Appreciate it so much for your insights.

Hi Mary,

It’s a bit hard to see the rocks clearly since they’re covered in the beautiful lichen, but I completely agree they look like ‘clastic’ sedimentary rocks like sandstone and conglomerate, which are made of sand, silt, and pebbles glued together.

I completely see why you would make the lava lamp analogy, the shapes are very similar. I think there’s something different at work with the concretions. For lava lamps, the blobs and surrounding liquid have very similar density. So when the light bulb at the bottom heats the blobs, they expand and rise to the top. Then they cool and eventually sink.  We use this analogy to describe how the Earth’s mantle moves around below us and helps drive the motion of continents and plate tectonics.

…I should give a caveat so I don’t propagate a common misconception about the Earth. The Earth’s mantle does convect in many ways similar to a lava lamp. However, it is solid rock that very slowly flows; very little of the Earth’s mantle is actually liquid.  The motion of solid Earth materials from the bottom to the top of the mantle likely takes ~35 million years or so – just as continents move about as fast as your fingernail grows – a pretty boring lava lamp but fun to think about in the abstract.

For the concretions, I believe it more starts with a ‘nucleus’ of typically some kind of organic something that was buried in the sediment. When the organic matter decomposes, it alters the environment around it in such away that it causes new minerals to form around the outside.

Here’s a kind of picture that goes with that idea:

And here’s another example that’s very technical but shows a solid concretion forming:

It seems like the ones you found are more like the top case than the bottom case where it’s more of a rind that a solid ball, is that true? (Yes I responded in a follow-up email with Andy)

I was looking for some photos of hollow features I found previously in the local Norman Garber sandstone for comparison but couldn’t find them.

Fun exploring!

Andy

So many shapes and forms. And always fun to learn more about our world.

Love the entertainment and exploring too!

And a big thank you to Dr. Andy Madden (University of Oklahoma) for sharing your knowledge! Andy put me on to the Macrostrat as well. And also thanks to Claire!

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