Icelandic Honeycombing - A Study of Erosion 

The land of Fire and Ice is a geological laboratory unlike any other. This earthcache offers you the opportunity to learn a bit more about a specific type of erosion that has occurred on the rock in front of you. After reading the description and answering a series of short questions you will gain a bit of understanding about a cool erosion factor while also enjoying some amazing views of the nearby water and distant mountains. Whether you are a local or a tourist, this walking path is beautiful and highly recommended! 

Honeycomb Weathering

The rock at this area sports a geological feature unique to soft stones in the maritime environment. Honeycomb weathering, also known as fretting, is a form of salt weathering common on coastal and semi-arid granites, sandstones and limestones.  The rate of erosion can be as fast as several centimeters in 100 years. Honeycomb weathering occurs throughout the world from the polar regions to the equator. It produces pits in the weathered material, from which it derives its name. 

For honeycomb weathering to occur, research indicates that a source of salt is required as the basic mechanism for this kind of weathering is salt heaving. Salt heaving occurs as salt is deposited on the surface of the rock by saltwater spray or by wind. Moisture must be present to allow the salt to settle on the rocks so that as the salt solution evaporates the salt begins to crystallize within the naturally occurring pore-spaces of the rock. Permeable rock such as sandstone or limestone is required so that there are pore-spaces for the salt to crystallize within; however in certain cases where additional weathering types are present (i.e. frost heaving) this weathering pattern can be observed in harder igneous based rocks such as basalt. These salt crystals pry apart the mineral grains, leaving them vulnerable to other forms of weathering. It takes prolonged periods for this weathering to become visible, as the rock goes through cycles of wetting and drying. 

Intertidal honeycomb weathering is found on horizontal planes in rock within the tidal zone. This type of honeycomb weathering is limited in its growth by the rate of evaporation from the sun. Once the depressions have grown large enough that the sun can not evaporate all of the water left in the gap by the retreating wave, the holes are as large as they will get, because the salt can not dry out and wedge grains apart any longer.

Polar Variation on Honeycomb Weathering

Here at the coordinates you'll see a large boulder that exhibits the classic honeybombing pattern that has given this form of erosion it's name. As we noted above, this type of weathering is typically regulated to areas that are either in the tidal region exposed to the ebb and flow of the tides as it's only in these regions that a constant supply of salt water can be delivered to the rocks so that salt heaving through evaporation can occur. 

While honeycomb weathering still obviously occurs nearer to the poles (As is the case in front of you), one major ingredient to the traditional honeycomb weathering process is missing - sunlight. In the latitudes nearer to the equator, the sun constantly dries out the saltwater on the rock allowing for the erosion to take place, but closer to the poles where sunlight and heat are scarce resources, how does this erosion still occur? 

A 2012 study conducted just a few miles away in Holmsberg showed that in southern climates while honeycomb weathering relied on evaporation, that in the northern areas it relied more heavily on salt heaving combined with frost weathering in the winter months to create the same erosion patterns. From their paper:

"Unlike in sandstones, intact, igneous basalt has low permeability, thus limiting or eliminating the process of drawing salts through capillary action from inside the rock to the rock surface... In our study area, the direct salt water source is sea aerosol. Upon drying of rock, salt crystalizes inside the pores leading to mechanical disintegration of basalts and enlargement of caverns. During winter, frost weathering and salt weathering cooperate on the basis of mutual enforcement..."

As you examine the rock in front of you, you'll notice a significant amount of honeycomb weathering, despite this rock not being located within the tidal zone. This shows that even though the affected region is relatively displaced from the ocean by several meters, that this form of weathering can still occur. This is due in large part to the winds in this region. Looking at the below table we can see that the area experiences heavy winds (given in m/s) throughout the year which ensures that this rock and others in the region receive a steady supply of aerosolized sea spray to deliver salt to the rocks.

It is thanks to the unique combination of wind, proximity to the sea, and the cold climate that this honeycomb weathering can occur outside of the tidal zone. If you are staying in Iceland and planning to travel the Ring Road take a look along the cliff faces and see if you can spot more occurrences of honeycomb weathering - you'll be amazed at how far away from the water you'll be able to find this type of erosion - all thanks to the aerosolized salt carried on the wind!

How to Log this Earthcache

Please use the message system to send the answers to the following questions to the CO. You do not need to wait for a response from me to log your smiley. If I find errors with your answers, I will email you directly and we can discuss how you came to reach your answers. As with all my Earthcaches; if I fail to receive answers, I reserve the right to delete your log without notice!

1) At the posted coordinates look for some examples of honeycombing in the rock. Approximately how wide in diameter are the eroded portions of the largest honeycombs in the rock? The smallest?

2) In your own words (using the information you've learned in the above lesson) tell me if you think that honeycombing at this rock has reached it's final state. In other words, can the honeycombs that already exist get any deeper? Why or why not

3) Based on the information in the reading and your observations on the various sizes of the weathering pattern, do you think that the weathering occurs more in the winter or in the summer? Do those forms of erosions differ in terms of processes? 

4) Take a photo of yourself or your geocaching crew at the GZ. Do not to include any evidence of the weathering process in your picture to preserve the novelty of the earthcache for future seekers. (Hint - there is a cool object right next to the subject material with which you could pose!)

References: 

1. Mustoe, G.E., 1982, The Origin of Honeycomb Weathering, Geological Society of America Bulletin, v.93, p. 108-115.
2. Cavernous Weathering Forms in Southwest Iceland

A big thank you goes out to D_Leslie_A whose nearby cache GC7ZB60 inspired my trek out this way from downtown Reykjavík!