GeoLog

weathering

Imaggeo on Mondays: the rocks that look like Swiss cheese

Imaggeo on Mondays: the rocks that look like Swiss cheese

Over the course of centuries and millennia, the force of winds, seas, ice and rains, sculpt rock formations around the globe. From the world-famous glacier carved landscapes of Yosemite National Park, to the freeze-thawed hoodoos at Bryce National Park, through to the wind battered stone pillars of South China Karst, boundless geological formations have been transformed by the power of erosion and weathering.

When the force of winds and salty waters combine, their effect on the surface of rocks is quite unique. In some costal environments, a network of holes, of all shapes and sizes, puncture otherwise smooth and silky rocks. This form of weathering is aptly known as honeycomb weathering (though some of you might be more familiar with terms such as cavernous weathering, alveoli/alveolar weathering, stone lattice, stone lace or miniature tafoni weathering). Limestones, sandstones and granites are most affected.

Exactly how the interaction of the sea breeze and the salt in ocean waters results in the distinctive ‘Swiss cheese’ weathered pattern remains a bit of a mystery.  One of the front running theories proposes that it is the culmination of physical and chemical weathering.

Evaporation of leachate causes a deposition of the rocks minerals on its surface which leads to a decomposition of the rocks interior. Additionally salt weathering caused by oceanic brackish water as well as temperature changes support the formation of this feature,” explains Michael Grund, a researcher at Karlsruhe Institute of Technology.

In Corsican, Tafoni, means hole or perforated rock, so it is not surprising that this form of weathering sometimes takes its name after the Tafoni rock formation on the southern coast of Sardinia, where Michael snapped a superb example of the potholed intrusives which dominate the area.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

Imaggeo on Mondays: The place where water runs through rocks

Imaggeo on Mondays: The place where water runs through rocks

Antelope Canyon, located in Arizona, USA, was formed by erosion of the Navajo Sandstone, primarily due to flash flooding and secondarily due to other sub-aerial processes (think of physical weathering processes such as freeze-thaw weathering exfoliation and salt crystallisation). Rainwater runs into the extensive basin above the slot canyon sections, picking up speed and sand as it rushes into the narrow passageways. Over time the passageways are eroded away, making the corridors deeper and smoothing hard edges in such a way as to form characteristic ‘flowing’ shapes in the rock.

The Navajo Sandstone was deposited in an aeolian (wind-blown) environment composed of large sand dunes: imagine a sea of sand, or an erg, as it is known scientifically, not dissimilar to the present Sarah desert landscape. The exact age of the Navajo Sandstone is controversial, with dated ages ranging from Triassic to early Jurassic, spanning a time period between 250 million years ago to approximately 175 million years ago. The difficulty in determining the exact age of the unit lies in its lack of age diagnostic fossils. The Navajo Sandstone is not alone in this quandary, dating is a common problem in aeolian sediments.

“The picture was taken during a three week Southwest USA road trip in summer 2012. One of the highlights was the visit to Antelope slot canyon, which is located on Navajo land east of Page, Arizona. The Navajo name for Upper Antelope Canyon is Tsé bighánílíní, which means the place where water runs through rocks,” explains Frederik Tack, an atmospheric scientist from the Belgian Institute for Space Aeronomy and author of today’s Imaggeo on Monday’s photograph.

The erosive processes which form the canyon are still ongoing. There is an elevated risk of flash floods, meaning the canyon can only be visited as part of guide tours.

“The canyon was actually quite crowded which made taking this picture challenging, especially as I wanted to capture the peace and solitude of the landscape,” describes Tack.

The effort was worth it: Waved rocks of Antelope slot canyon was one of the EGU’s 2015 Photo Contest finalists!

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

Imaggeo on Mondays: Pitter-patter of little paws in Patomsky crater

This week’s Imaggeo on Mondays is brought to you by Dmitry Demezhko, who describes how Patomsky crater may have formed and why it keeps scientists puzzling…

Patomsky crater, also known as Patomskiy crater or the Patom cone, sits in the Irkutsk Region of Eastern Siberia. The site is a curious cone with a crater at the top and a small mound in the center. The cone totals some 39 metres in height and stretches more than 100 metres in diameter (at the base of the cone).

Patomsky crater – view from a helicopter. (Credit: Dmitry Semenov)

Patomsky crater – view from a helicopter. (Credit: Dmitry Semenov)

The crater was discovered in 1949 by Russian geologist Vadim Kolpakov and for a long time it was considered to be an impact structure with a meteoric origin. Later, Viktor Antipin suggested it could be a nascent volcano. But neither meteoritic nor volcanic matter was found there. The crater consists of proterozoic limestone and sandstone debris and, to date, there is no consensus among scientists regarding the crater’s origin.

View from the crater. (Credit: Dmitry Demezhko, distributed via imaggeo.egu.eu)

View from the crater. (Credit: Dmitry Demezhko, distributed via imaggeo.egu.eu)

During a short expedition in August 2010 we conducted a gravimetric survey at the crater and surrounding area, aiming to evaluate its internal structure. The gravity field shows that surface negative anomalies, where the gravity is unusually low, have deep “roots” and a joint source at depth. But the crater’s gravity field differs greatly from the fields of other well-known impact structures, suggesting that it may not have formed during a meteoric impact.

Downward continuation of the Patomsky crater (left) and Popigay impact structure gravity fields (right). (Credit: Demezhko et al., 2011)

Downward continuation of the Patomsky crater (left) and Popigay impact structure gravity fields (right), (click for larger). (Credit: Demezhko et al., 2011)

We suggest this structure formed in two stages. During the first stage tectonic processes similar to mud volcanism created a porous vertical channel. In the second stage, cryogenic processes would have played an important role in breaking apart the rocks to form the cone and crater.

There is a lot of mysticism and superstition surrounding Patomsky. Local residents call the crater “a fabulous Eagle’s Nest” and say that both people and animals bypass it. We didn’t sense anything mystical while working in the crater though – and this cute little animal lives quite comfortably there.

Downward continuation of the Patomsky crater (left) and Popigay impact structure gravity fields (right). (Credit: Demezhko et al., 2011)

“Inside Patomsky crater: a chipmunk” by Dmitry Demezhko. This image is distributed via imaggeo.egu.eu.

By Dmitry Demezhko, Institute of Geophysics UB RAS, Yekaterinburg

References:

Alekseyev, V. R.  Cryovolcanism and the mystery of the Patom Cone, Geodynamics and Tectonophysics, 3, 289-307, 2012 (in Russian)

Demezhko D.Y., Ugryumov I.A., Bychkov S.G.: Gravimetric studies of Patom Crater. In: Patom Crater. Research in the 21st Century. Publishing House of the Irkutsk State University, Irkutsk, p. 42–50, 2011 (in Russian)

If you are pre-registered for the 2014 General Assembly (Vienna, 27 April – 2 May), you can take part in our annual photo competition! Up until 1 March, every participant pre-registered for the General Assembly can submit up three original photos and one moving image on any broad theme related to the Earth, planetary, and space sciences in competition for free registration to next year’s General Assembly!  These can include fantastic field photos, a stunning shot of your favourite thin section, what you’ve captured out on holiday or under the electron microscope – if it’s geoscientific, it fits the bill. Find out more about how to take part at http://imaggeo.egu.eu/photo-contest/information/.

Imaggeo on Mondays: Long live the lichen!

Lichens are amazing organisms. They are a composite of algae and fungi, each of which supports the other through the exchange of nutrients (fungi to algae) and carbon (algae to fungi). They are also capable of making a home out of seemingly inhospitable rock surfaces – and what’s more – making the most of these surfaces to release the nutrients they need to grow.

“Pioneer Settler Organisms” by Juan Antonio Campos, distributed by the EGU via imaggeo.egu.eu.

“Pioneer Settler Organisms” by Juan Antonio Campos, distributed by the EGU via imaggeo.egu.eu.

The quartzite above is home to the yellow cracked lichen (aka Acarospora hilaris) and a grey lichen species, better known as Caloplaca carphinea, both of which are busily breaking down the rock. They do this using their physical and chemical armoury:

Physical breakdown

The fungi consist of fine, branching tubes called hyphae. These hyphae penetrate the rock and make their way between both mineral grains and cleavage planes to tease it apart. Over time, expansion and contraction of the lichen body as it goes from wet to dry and back again, slowly wedges parts of the rock apart. Swelling of salts released by the lichen has a similar effect.

A closer look at fungal hyphae. This example is from the fungus penicillium, used to produce the antibiotic penicillin (Credit: GFDL)

A closer look at fungal hyphae. This example is from the fungus penicillium, used to produce the antibiotic penicillin (Credit: GFDL)

Chemical breakdown

In order to obtain much needed minerals from the rock, lichens release a variety of organic acids capable of dissolving the rock’s structure. Their main weapon is oxalic acid, which makes minerals more susceptible to breakdown by water (a process known as hydrolysis). Not all lichens produce this though; instead they rely on simpler compounds (like citric acid) that are secreted by lichen fungi for acid attack.

This colonisation of rocks by lichens, and the weathering of rock surfaces that follows, represents one of the first steps in soil development. Find out what happens next in this Biogeosciences paper.

Reference:

Chen, J., Blume, H. P., & Beyer, L.: Weathering of rocks induced by lichen colonization—a review. Catena, 39, 121-146 (2000)

The EGU’s open access geoscience image repository has a new and improved home at imaggeo.egu.eu! We’ve redesigned the website to give the database a more modern, image-based layout and have implemented a fully responsive page design. This means the new website adapts to the visitor’s screen size and looks good whether you’re using a smartphone, tablet or laptop.

Photos uploaded to Imaggeo are licensed under Creative Commons, meaning they can be used by scientists, the public, and even the press, provided the original author is credited. Further, you can now choose how you would like to licence your work. Users can also connect to Imaggeo through their social media accounts too! Find out more about the relaunch on the EGU website.