Natural processes of mechanical weathering cause solid rock to break up. However, there are a range of forces that cause cracks, different rates of cracking and several processes by which cracks grow. A recent article in Reviews of Geophysics looked specifically at how low-level forces and climate may combine to break up rock. The journal’s editors invited the authors – an earth scientist and a mechanical engineer – to answer some questions about this stage in the rock cycle and to explain what their interdisciplinary collaboration has revealed.
What is mechanical weathering and what role does it play on Earth?
Mechanical weathering comprises the in situ physical breakup of rock at and near (within about 100 meters) the Earth’s surface. That breakup occurs when environmental, gravitational or tectonic stresses act to sever molecular bonds within the rock, causing cracks to form or grow. As cracks intersect, smaller pieces of rock are separated. The process happens over and again, releasing bedrock from the crust of the Earth and downsizing boulders. It is a crucial step in the entire rock cycle that enables everything from life-sustaining soil formation to erosion of mountains.
What is the difference between critical and subcritial cracking?
Imagine holding a pencil at each end and trying to break it. The amount of force, or “stress” you would need to fully break it all in one go in a simple way represents the “critical” strength of the pencil. If you apply a much lower stress – enough to bend the pencil – and continue over a longer time you will still eventually break the pencil. This slower breaking process represents “subcritical” cracking.
Loads of experimental data from rock physicists have demonstrated the same thing is true of rock. Given sufficient time – and in geology we have plenty of that – cracks will form and propagate under stresses that are much lower (even less than 10%) of the rock’s critical strength.
Are any particular processes of mechanical weathering more significant than others?
The stresses that lead to mechanical weathering vary from location to location. Tensional (pulling) stresses are most efficient in the critical or subcritical cracking of any material, and tensional stresses in bedrock due to gravity, for example, will be larger on slopes than flat terrane.
The environmental stresses that any rock experiences will also vary by geography, for example between the equator and the poles, or between the side of a boulder that experiences more sunshine and the other that is mostly in shadow.
One motivation for our research, however, is that these variations in stresses do not always equate to the ‘correct’ variation in mechanical weathering – like observing more cracking in locations that are wetter, but with lower thermal or freezing stresses.
In contrast, the suite of actual processes that break bonds, subcritically grow cracks and thus mechanically weather rock are likely universal for any given rock type. Thus by recognizing that most mechanical weathering is facilitated by subcritical cracking, we open the door to better understanding which combination of stresses, rock and environmental properties will lead to more significant mechanical weathering. We are not there yet.
Does climate matter to cracking?
In terms of the relationship between climate and cracking rock, you might think of ‘freezing temperatures’ or ‘wet and dry cycles’ because these are conditions that produce environmental stresses on rock that can cause it to mechanically weather. This is true, but this limited view does not take into account the fact that mechanical bond-breaking at crack tips actually involves chemical reactions when stresses are low.
When stresses are present but are not critical – which is almost always the case for rocks at and near Earth’s surface – molecular bonds right around crack tips get so stretched that their constitute atoms become reactive with water or water vapor found in the adjacent pore. The result is that those bonds become weaker just at that location and can therefore then be broken by the low stresses. The process repeats, and slow subcritical cracking occurs.
Because it is a chemo-physical process, any condition like temperature or humidity that might influence chemical reactions strongly influences the rate and efficacy of subcritical cracking. Thus climate may have an enormous influence on mechanical weathering styles and rates above and beyond the influence that it has on which stresses may or may not be present.
How does an interdisciplinary approach help us better understand these processes?
Bringing together scientists who have spent their careers exploring disparate but related topics such as geological weathering and theoretical mechanical engineering allows us to build on the knowledge and confidence that arises from each other’s education and experience. For example, to build a physical model of rock cracking based on fracture mechanics theory, you must make assumptions in order to simplify the model sufficiently to gain new insight on the problem. In such a case it is invaluable to have someone around to say, “Hey, that assumption is pretty realistic; I know because I have looked at 3000 rock cracks!” This combination of disciplines thus provides the momentum – as well as the checks and balances – necessary for novel combinations of ideas to manifest as significant headway in our understanding of the Earth.
What are some of the unresolved questions in this field and what is needed to answer them?
We know that subcritical cracking occurs in rock. We know that its rates and occurrence are stress-, rock- and environment dependent. The fact that such climate-dependent subcritical cracking is likely how most mechanical weathering occurs day in and day out, however, is a new idea laid out in our article [Eppes and Keanini, 2017].
Thus, there are plentiful questions left to answer about subcritical cracking processes in nature including:
- Given that gravitational, tectonic and environmental stresses can be added together in order to overcome stress thresholds necessary to initiate subcritical cracking, what are their relative roles?
- When and/or how does stress trump moisture or vice-versa?
- To what extent does confining (pushing) pressure serve to slow down subcritical cracking in the very shallow crust?
- What is the role of subcritical cracking in the subsurface conversion of rock to regolith or soil?
- How do subcritical cracking rates and processes change as rocks weather, and thus contain longer and denser cracks?
To answer these questions, we ultimately will need both field and modeling observations of cracking rates and how those rates translate into rock erosion and regolith production. To make these connections, we will also need more and better characterization of rock and mineral mechanical parameters as well as better long-term environmental data obtained on and within rocks themselves. It will truly be an effort that will include interests and disciplines throughout the geosciences. Fun stuff!
—Martha Cary Eppes, Department of Geography & Earth Sciences, University of North Carolina at Charlotte; email: [email protected]; and Russell Keanini, Department of Mechanical Engineering and Engineering Science, University of North Carolina at Charlotte