Tuesday, 23 September 2014

Fracturing of ancient bedrock surfaces during an extremely hot summer

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New exfoliation fractures formed in glacially polished bedrock at Långören Island in Finland, July 2014 (image credit: GTK).

NOAA reports that August 2014 was the hottest since records began. Long periods of exceptionally high temperatures in California and Finland this summer have been associated with the formation of large ‘exfoliation’ or ‘sheeting’ fractures in bedrock surfaces that may have remained largely unchanged since the last ice age. Recent videos show sharp fractures forming along the edge of thin (<1 m) bedrock sheets several meters across, before the rock surface appears to jump and buckle in the hot summer sun. Long scratch marks (‘striations’) visible on the surface of rock in the Långören (Fin.) case are the result of boulders being dragged over the landscape during the last glacial period (>15,000 years ago), hinting at the rarity of the recent events on the otherwise undamaged surfaces. A more hummocky surface scattered with isolated boulders at Twain Harte (Calif.) indicates (somewhat ambiguously) that although rare, this process may have occurred sporadically over a similar time interval.

Extensional fracture development leading to buckling of a rock slab at Twain Harte, California. August 2014 (credit: youtube/dotysan)

While geologists in both the USA and Finland work to understand conditions that contributed to these events, this type of fracture is well known from other settings (e.g. quarries, mines, and tunnels), and the mechanism of joint formation is therefore relatively clear. Better termed extensional fractures, these features form as a result of very high stresses (pressures) acting parallel to the bedrock surface, while very low gravitational stresses hold the bedrock in place. Much like a metal plate buckling on a hot element (often with a surprising “pop”), thermal expansion of horizontally confined rock surfaces causes an increase in horizontal stresses, and can lead to a buckling of bedrock once conditions become critical. 

Examples of extensional fracture formation in various environments.  
σ1 refers to the maximum principal stress, σ3 is the minimum (typically gravitational) stress. A) 'Sheeting' fractures developing in relatively flat-lying topography B) 'Exfoliation' fractures forming on the side of a U-shaped glacial valley C) 'Spalling' of highly stressed bedrock in a borehole or tunnel. (Leith et al., 2014)

Unlike the metal plate, however, joints separating the bedrock surface from underlying rock are generally not continuous, and many intact ‘rock bridges’ need to be broken before the surficial plate is large enough to “pop" (see video) (the stress required for buckling decreases as plates become larger). A pre-requisite for these spectacular events is therefore the destruction of rock bridges, which in this case likely occurred as thermal expansion caused stresses at the tips of existing cracks to exceeded critical levels. The subsequent propagation of fractures sub-parallel to the ground surface will then increase the area of crack tips (thereby dissipating fracture energy) until either the fracture again becomes stable, or the sheet becomes large enough to buckle.

Kallio repeää video of fracture formation and buckling at Långören Island in Finland. Glacial striations run from the upper left to lower right of the image. From Geologian tutkimuskeskus on Vimeo

The gently undulating bedrock landscapes of the Finnish (map) and Californian (map) events are classic examples of regions that have evolved through erosion of hard, unweathered bedrock, and although heating and thermal expansion almost certainly triggered these events, bedrock in both regions is known to already maintain very high stresses resulting from millions of years of exhumation (erosion) and tectonic strain (essentially being stretched and squashed by plate tectonics). In some cases these stresses can equal to the load of a more than 1 km high mountain (~25 – 50 MPa). These long-term stresses are often limited by the strength of the local bedrock, and thermal stresses therefore need only push loading conditions ‘over the edge’, providing enough additional stress to initiate the active fracturing process. Interestingly, the current period of fracturing indicates stable fracture systems were not fully developed at these ancient bedrock sites, suggesting either peak stress levels have increased over time, or fracture toughness has decreased (i.e. through stress corrosion or weathering). These will be key questions for scientists in the coming months.

Global temperature anomalies for August 2014. Both California and Finland are noted to be 'Much Warmer than average', and local weather stations in both locations record temperatures during the July - August 2014 period. (Source: NOAA)

Climatically-induced extensional fracturing events are rare in both geological, and historical records – the Finnish Geological Survey report no similar events are known in Finnish history. However, the observation that thermal stresses can cause extensional fracturing in rock is not new, and some quarries in India still use fires set on the quarry floor to generate new extensional fractures and speed up the extraction of rock. In other regions where rock is known to carry high stresses, traditional quarries commonly observe the expansion of pit walls or floors into the working area as pre-existing stresses are relieved by the removal of adjacent blocks.

New exfoliation fractures creating a 'pop-up' structure beneath the diving board at Twain Harte lake, August 2014. The artificial lake was drained as a result of the formation of extensional fractures beneath the dam abutment. (image credit: G. Hayes).

Extensional fractures are common features in post-glacial landscapes, and in research published early this year, my colleagues and I find that this mode of fracture formation is likely to be an essential factor in the progression of glacial erosion. Unlike the events of this summer, however, sub-glacial environments can preserve much higher stresses, and extensional fracturing in association with glacial erosion is therefore likely to be much more energetic (similar to ‘spalling’ which frequently causes rock to explode and fracture steel supports in new tunnels). While this more energetic form of fracturing is almost impossible at the Finnish and Californian sites, these two new videos provide a fantastic example of how small stress changes can cause dynamic fracturing in natural bedrock, and provide insight into the explosive fracturing and rapid erosion that may sometimes take place beneath large glaciers. Perhaps more importantly, however, these videos demonstrate how critical it is for scientists to understand the physics of natural environments in order to predict changes resulting from natural or human-induced changes to the Earth surface.

The topics discussed in this post form the basis of a new session at the European Geosciences Union General Assembly early next year 'Geomechanics in natural environments: quantifying environmental stresses and physical soil or rock behavior'. Myself and the other convenors welcome anyone who has an interest in environmental stresses and the mechanics of Earth surface materials to submit an abstract, or come along to participate in discussion at the session.

Saturday, 6 September 2014

Randa rockfall update: One of those rare cases

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Image describing the upward-propagation of failure during the first minute of the 29/08/2014 event. The background image is a pre-failure photo taken two weeks prior (12/08/14, credit: J. Beutel).

One of the fortunate things about working in the European Alps is that many valleys are densely populated, and few large events go unnoticed. The Randa rock slope in the Matter Valley is a particularly well-studied location, and nearby scientific installations are currently used to monitor climate, permafrost, debris flows, and other alpine hazards.

In the case of the 29/08/14 failure, members of the ETH Zurich had captured images of the rock slope two weeks prior to the event, and as the SLF was working in the region at the time, images immediately (approx. 30 min) after the event are also available. This is a rare case for such a large alpine rockfall, and in addition to the video I posted earlier could offer opportunities to investigate the driving mechanism and failure process with more detail than is usually possible. Such data can be very useful for scientists and engineers seeking to better understand rock slope behaviour, and may help us more accurately predict future activity of similar rock slopes from pre-failure observations.

Although there is no substitute for on-site inspection, by comparing pre- and post-failure images we can make a first-pass at delineating the approximate region of failure. And by comparing blocks outlined by large fractures to stages of failure in the video we can also make a reasonable estimation of the sequence of failure (see above).

Randa rock slope immediately after the 20/08/14 failure (credit: M. Phillips). The region that failed during the 29/08/14 video is highlighted in dark red, while the region highlighted in orange corresponds to a potential instability noted in my previous post. dashed grey lines marked on the grass above the scarp delineate regions of exposed soil and possibly disturbed vegetation, suggesting relatively recent movement. Colored circles can be used to reference pre- and post-failure photographs. Click here to view the original image.

The dark silty soil generated during the event makes it difficult to correlate features on the rock face beneath the failure, however, it seems like it initiated at a site just above the soil-covered region of the rock slope (i.e. above the light green marker). The slope crest was then the last to collapse. Often such an upward-propagation is associated with removal of a key block that has been weakened by high tensile or shear stresses at the toe of a creeping rock slope. This would be consistent with excellent work by members of the Engineering Geology group at ETH Zurich, which identifies slow creep in the upper section of the slope, and correlates this to bedrock structure and seasonal thermomechanical effects.

As in the lower image, the failed region appears to lie immediately above a long undulating discontinuity (black lines in inset) that slopes downhill at an angle slightly greater than that of the upper grass-covered surface. White arrows indicate where the discontinuity seems to have opened up as a result of the upper section creeping downslope relative to the lower rock mass. This kind of movement would be consistent with the initiation of failure immediately above a step in the sliding surface. The current activity could therefore be the result of slow creep that has been ongoing since the Randa rockslides in 1991. The wet summer may have accelerated this process, and although not strictly a 'trigger' (as the failure occurred on the first sunny day in several weeks), increased pore pressure and weathering on the sliding surface may have bought the slope to failure more rapidly, and therefore increased the probability of a collapse on any given day.

If this event is the result of deeper slope movement (as opposed to an isolated collapse of surficial material), then possible soil disturbance 10's of meters back from the active cliff face (see image above) may be an indicator of much larger failures in the future (although no-where near the volume of the 1991 events). Irrespective of the driver or mechanisms, this apparent re-activation of the slope is interesting, and further observation or investigation could be warranted.

An alternative view of the Randa rock slope prior to failure (12/08/14, credit: J. Beutel). Two apparently open fractures are indicated by white arrows. The inset provides an interpretation of creep that may have contributed to the opening of steeper sections of the undulating discontinuities. As in the previous image, the region that failed during the 29/08/14 video is highlighted in dark red. Colored circles can be used to reference pre- and post-failure photographs. Click here to view the original image, additional annotated and un-annotated pre-failure images are also available.

Monday, 1 September 2014

Ongoing instability of the Randa rock slope (Switzerland)

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Ongoing rockfall activity from the Randa rock slope at 1 pm on the 29th of August 2014

Two very large rockfalls in 1991 left an almost 1,000 m high cliff on the western side of the Matter Valley in Southern Switzerland. The valley is one of the most travelled in the Alps as it serves as access to the town of Zermatt and the skifields immediately adjacent to the Matterhorn.

Research into the cause of the 1991 failures and current state of stability has been ongoing for more than 20 years, and the site is now a classic example of alpine rock slope failure. Although regular rockfall is common from the remaining scarp, the event captured on video is one of the larger failures since 1991. Rockfall was ongoing throughout the day, and the event in the video occurred just after midday.

Large failure from near the crest of the remaining rock slope

The video was captured during a scientific workshop with members of the Chair of Landslide Research from the Technische Universität München, and the Geomorphological and Environmental Research Group at the Universität Bonn.

Randa rock slope in early August (05/08/2014)

Close up of the region of current activity (this is estimated to be perhaps 60 m high). Dark red indicates the approximate region of the present failure. Orange indicates a region of rock that may have been destabilised as a result of the activity.

A pre-failure photo taken two weeks prior to the event (12/08/14). The photo was taken by members of ETH Zurich as they descended by helicopter from the Randa in situ rock laboratory (credit: J. Beutel). Inset indicates the approximate failure location, as well as apparently open cracks on adjacent failure planes.