Monday, January 24, 2011

Accretionary Wedge #31 - Call for Posts

So I was having a conversation with a fellow grad student who came into my office asking if I had ever heard of an euchario..something, eudiachario..something. I said I wasn't sure, I would need the real name. Then someone in the office chimed in with Ediacharian? She said yea, and everyone else is like "Oh, yea, we know that." And that leads me in to this month's Accretionary Wedge Topic:

What geological concept or idea did you hear about that you had no notion of before (and likely surprised you in some way).

I have a few ideas for my own topic, but I would be interested in what people have to say about this. Some things we think are obvious and just know about other people have never heard of.

The deadline for this is February 18th. Please post all your submissions in the comments section. And don't forget to get your submission in for Accretionary Wedge #30 which is due January 28th.


  1. Here is my contribution: Thanks for hosting!

  2. Here 'tis. Thank you for not making us bake again! ;-)

  3. I hold a PhD in karst hydrogeology and geochemistry, so I thought I was pretty down with how caves form in carbonate rocks… basically, that water containing acidity of some flavor dissolves limestone through an inorganic chemical process of acid neutralization via reaction with an alkaline mineral (calcite). It's like what happens when you take an antacid tablet to relieve heartburn; the calcium carbonate dissolves and neutralizes your stomach acid, and you feel better. Pretty simple chemistry: acid-base neutralization.

    The neat thing about this process is that it can reverse. If a solution becomes oversaturated with the dissolved constituents of the mineral phase, the mineral will precipitate out of the solution. Think of stalagmites or stalactites (collectively called speleothems) in a cave. Those pretty formations are exhibiting the reverse process of how the cave formed in the first place. The mineral calcite precipitates out of water dripping or flowing into the cave. In the case of calcite speleothems, the chemical driver for the mineral precipitation is the loss of acidity as carbon dioxide gas escapes into the cave atmosphere. The loss of acidity means the solution can't hold all of the mineral constituents (the calcium and carbonate ions) in a dissolved form, so some drop out, and a new layer of speleothem calcite is produced.

    Ok, so those are the basics of cave mineral dissolution and precipitation, or so I thought… that is, until I saw a presentation by Annette Summers-Engel at the GSA meeting in Houston in 2008 on the work she and her students were doing on a cave in Texas. This experiment was so simple, yet so profound…

    The water in this cave happened to be chemically oversaturated with calcite, and undersaturated with the mineral gypsum (hydrated calcium sulfate), meaning that no more calcium could dissolve, but gypsum still could. So Annette and her students thought, what if we put in crystals of calcite? Geochemical theory would predict that new calcite would form on the seed crystals of calcite, but theory neglected one key component: biology.

    The cave was swimming with microbes. Homeless bacteria quickly colonized the newly introduced calcite crystals. Once established, they went to work doing what microbes (and all life forms) do: exploiting geochemical reactions to obtain energy for living. But what they did astounded me… the work of the microbes caused the calcite to dissolve, and gypsum to precipitate! The evidence was undeniable, captured in beautiful scanning electron micrograph photos.

    What was happening in this case was that the microbes were exploiting the energetics of sulfur to grow. Some of them were reducing sulfate ions in the water into hydrogen sulfide, and some others were taking in the hydrogen sulfide and oxidizing it into sulfuric acid. Because the microbes were colonizing the calcite, they produced sulfuric acid in a zone around them, which caused the calcite to dissolve, and with the added calcium ions in the water gypsum was forced to precipitate nearby. Gypsum crystals were observed growing on the introduced calcite, which had become pitted through dissolution.

    Bottom line: microbes eat rocks (sort of).

    More importantly for my field of science, microbes colonizing cave walls can do a lot of the work when it comes to forming caves. And, as it turns out, they do a lot when it comes to the reverse process of forming speleothems and lots of other carbonate mineral deposits as well!

    Hope you enjoyed the story of bacteria forming caves as much as I did the first time I learned of it.

    -Dr. Doctor

  4. Tis my first accretionary wedgie:
    Thx for the topic!

  5. Here's mine:

  6. Thanks for hosting. Here's mine:

  7. Thank you for hosting this. Fun reading. My first ever contribution to an Accretionary Wedge is humbly offered here.

  8. Here's mine:

  9. I finished this one yesterday, but fell asleep at the computer last night before I got a chance to submit it.


  10. Here we go:

  11. Thank you everyone for your entries. I have read through some of them already and they seem fantastic. Hopefully I should have the official post up sometime early next week.

  12. The official post is up!!!


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