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7 Souvenirs of August

EarthCache

Rock Bands

A cache by Sterreman
Hidden : 4/28/2010
In Western Cape, South Africa
Difficulty:
2 out of 5
Terrain:
1 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

This phenomenon, which has aroused the curiosity of chemists for more than a century, is visible in many rock-types as pretty concentric rings. It was accidentally discovered in a lab over 100 years ago and also occurs in humans.


It was the year 1896. The German chemist, Raphael E. Liesegang, was busy in his lab when he accidentally dropped a drop of silver nitrate solution on a layer of gel containing potassium dichromate. This caused concentric rings of silver dichromate to form (like in the picture below, left). When formed in a test tube by diffusing one component from the top, layers or bands (rather than rings) of precipitate form (see below, right).

Liesegang rings in gel

Liesegang banding/rings can sometimes be seen in weathered rock as very eye-catching colour banding. Examples can be fan-shaped, others closely resemble sedimentary cross-bedding (but their orientation can be seen to cut primary, sedimentary features). Although easily mistaken for bedding, a close look will usually show that the banding is in fact something quite different.

From Geological Journeys, p.208
This picture from Geological Journeys by Norman & Whitfield was taken near the cache location. However, I could not find this particular example when placing the cache. Judging by the differences in the pictures from the logs, it seems like the phenomena is constantly changing as the (relatively soft) rock erodes.

Liesegang banding forms deep underground by the rhythmic deposition of various iron and manganese compounds from mineral-rich groundwater that once flowed within the permeable rock and along bedding and joint planes. Over long periods of time, the water leaches soluble material - mainly iron - from the rock and carries it along in solution. Sooner or later the water evaporates or the chemical environment changes, the material is no longer stable in solution, and it precipitates. Water moves along a 'front', as though in a wave. If anything were equal, these fronts would be straight. However, there are subtle but important differences in permeability, so the front becomes distorted and sinuous. This results in either colour banding, sometimes in very beautiful brightly hued patterns, or hardening alongside joint-planes, giving rise to a pitted, box-work appearance.

In sedimentary rocks, Liesegang bands thus appear well after the sediment has become rock. Stratification and lamination within the sandstone, or Malmesbury shale in this case, are typically cross-cut by the Liesegang bands which entirely 'ignore' the original lamination; fractures usually have a more obvious effect on the distribution and orientation of these. Liesegang banding is even seen in granite, a non-sedimentary rock.

In humans Liesegang rings occur in inflammatory breast lesions.


Sources:


To claim "Found it" you must email me satisfactory responses to the following:
Any logs not accompanied by an email will be deleted.

  1. Find the best example of Liesegang banding that you can spot in the area around the designated coordinates and send me a close-up picture of it with your navigation device for scale.
  2. If you were to keep licking a multi-layered candy ball on its one side only, concentric rings will be revealed. Is this the same mechanism as with Liesegang rings? Explain your answer.
  3. What substance do you think is responsible for the red colour in these bands?
  4. Optional: Send me a picture of you/your party taken with this feature in the background.

Note: Do not post any spoiler pictures/hints to this page, even if encrypted.

Additional Hints (Decrypt)

Ernq gur grkg pnershyyl, nyy gur pyhrf ner va gurer. Rznvy lbhe nafjref gb jcx@fnnb.np.mn

Decryption Key

A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z

(letter above equals below, and vice versa)

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Inventory

There are no Trackables in this cache.

 

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42 Logged Visits

Found it 39     Write note 2     Publish Listing 1     

View Logbook | View the Image Gallery of 16 images

**Warning! Spoilers may be included in the descriptions or links.

Current Time:
Last Updated: on 8/30/2014 9:31:29 AM Pacific Daylight Time (4:31 PM GMT)
Rendered From:Unknown
Coordinates are in the WGS84 datum