Cracking up: emerging evidence for the importance of the sun in the mechanical weathering of rocks
Shortcut URL: https://serc.carleton.edu/60237
Location
Continent: North America & Asia
Country: United States, Mongolia
State/Province:CA, NM, AZ
City/Town:
UTM coordinates and datum: none
Setting
Climate Setting: Semi-Arid
Tectonic setting:
Type: Process
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Description
Introduction
Physical weathering, through the breakdown of rock into sediment, plays a fundamental role in the rock cycle and potentially a key role in landscape evolution. In addition, the physical breakdown of natural and synthetic materials continuously costs humans lives, dollars and time.
The first unequivocal stage of physical weathering is the formation of an incipient crack. While certain cracks can be linked to tectonics or to forest fires, surface boulders and cobbles universally exhibit vertical cracks of unknown origins (hereafter: "cracks"; Figure 1).
A large body of literature describes mechanisms for the growth and expansion of such cracks, but the mechanism for their initiation has been disputed for over a century. Processes such as freeze-thaw and salt shattering are commonly thought of as key drivers for rock cracking; however, all such processes require a crack to be present so that water or salts can enter, expand and propagate the fracture.
The potential importance of sun and associated thermal stresses that might result in the breakdown of rocks was recognized long ago (e.g. Bartlett, 1832). Certain experimental studies in the early 1900's however (e.g. Griggs 1936; Blackwelder, 1933) cast lasting doubt on the notion that thermal stresses produced by insolation are important in rock cracking. In these studies, small rock tablets were put in ovens and heated and cooled for the equivalent of hundreds of years of daily cycles and no cracks formed. Since these studies, the majority of textbooks and workers cite the sun as a possible, but not important source of cracking in surface rocks.
New data, however, have begun to provide evidence that the sun may play a key role in the physical weathering of rocks at the Earth's surface. The following information is derived from two papers (McFadden et al., 2005; Eppes et al., 2010) that explore a new hypothesis for the role of the sun in cracking rocks.
A new hypothesis
In thinking of the role of the sun in cracking rocks, we can start by making some simple observations. In the experiments by Griggs (1936), small cut stone tablets were put in ovens which would uniformly heat them up and then cool them off. In nature however, and particularly in deserts, rocks are not uniformly heated and cooled. Instead they are heated on the east in the morning, and in the west in the afternoon. Based on these observations, it can be hypothesized that perhaps it is this directional heating and cooling that results in the important stresses which might crack rocks (Figure 2). From a simplified physics stand point, we can go further and hypothesize that the differential thermal expansion of a rock that is caused by directional heating during certain times of day causes sufficient stress on the rock to exceed its tensional strength. When the tensile strength of the rock is exceeded, the material will fail and a crack will form. How could such a hypothesis be tested? Quite simply: if stresses are 'pulling' (tensional) in the east-west direction, then cracks that form should generally be oriented to the north-south.
The data
About 700 cracks on about 400 rocks ranging in size from ~20 - 80 cm were examined from deserts throughout the southwestern United States (Figure 3). When the orientations of all of these cracks are plotted, there is a notable north-south trend (Figure 4a). In and of itself, this preferred orientation of cracks in desert rocks is remarkable. It is difficult to imagine another forcing mechanism that could account for this preferred orientation of cracks in rocks other than a sun-related process. In examining all of these cracks, however, it is evident that the story is not a simple one.
The orientations of many cracks seem to be related to properties of the rocks that are cracked (Figure 5). As you might expect, the orientations of many of the cracks are parallel to pre-existing fabric in the rocks such as bedding planes or foliation ('fabric cracks'). The orientation of many other cracks appear to be related to the shape of the rock. In particular, if rocks are longer than wide, then there are often cracks parallel to the long axis ( 'longitudinal cracks'). Also, if the rock contains a large flat face, there are often cracks that are oriented parallel to that surface ('surface cracks'). The majority of cracks studied, however, fell in none of these categories. When the orientations of these leftover cracks were plotted on a rose diagram, there the clear north-south trend becomes even more visible (Figure 4b).
Another interesting feature of the rose diagram of crack orientations is that there is not a single main trend or mode to the data right at north-south. Instead, there are 2 primary modes a bit to the east and to the west of north. What might explain these two separate modes? At ~35 degrees North latitude where these data were collected, the sun does not rise and set perfectly in the east and west, but varies according to the time of year from northeast/northwest in the summer to southeast/southwest in the winter. Thus it might be hypothesized that cracks are forming due to sun-related stresses that arise at certain times of the day and/or year in order to produce these different modes of cracking.
To test this latitude-dependant hypothesis, another 100 cracks were examined in the Gobi desert of Mongolia (Figure 3). The orientations of these cracks is remarkably unimodal (Figure 6), with an orientation farther to the east than that of the cracks from 35 degrees latitude. At this location (45 degrees latitude) the sun will rise farther in the north and south on the summer and winter solstices respectively. If we assume that cracks are forming perpendicular to the stresses that arise as a result of this directional heating and cooling, then such a shift in crack orientations as a function of latitude would be expected. The unimodality of the Gobi data might be attributable to the importance of the winter months in cracking rocks under cold-dry climatic conditions, as opposed to the hot-dry conditions of US deserts.
Conclusions
Physical weathering, through the breakdown of rock into sediment, plays a fundamental role in the rock cycle and potentially a key role in landscape evolution. In addition, the physical breakdown of natural and synthetic materials continuously costs humans lives, dollars and time.
The first unequivocal stage of physical weathering is the formation of an incipient crack. While certain cracks can be linked to tectonics or to forest fires, surface boulders and cobbles universally exhibit vertical cracks of unknown origins (hereafter: "cracks"; Figure 1).
A large body of literature describes mechanisms for the growth and expansion of such cracks, but the mechanism for their initiation has been disputed for over a century. Processes such as freeze-thaw and salt shattering are commonly thought of as key drivers for rock cracking; however, all such processes require a crack to be present so that water or salts can enter, expand and propagate the fracture.
The potential importance of sun and associated thermal stresses that might result in the breakdown of rocks was recognized long ago (e.g. Bartlett, 1832). Certain experimental studies in the early 1900's however (e.g. Griggs 1936; Blackwelder, 1933) cast lasting doubt on the notion that thermal stresses produced by insolation are important in rock cracking. In these studies, small rock tablets were put in ovens and heated and cooled for the equivalent of hundreds of years of daily cycles and no cracks formed. Since these studies, the majority of textbooks and workers cite the sun as a possible, but not important source of cracking in surface rocks.
New data, however, have begun to provide evidence that the sun may play a key role in the physical weathering of rocks at the Earth's surface. The following information is derived from two papers (McFadden et al., 2005; Eppes et al., 2010) that explore a new hypothesis for the role of the sun in cracking rocks.
A new hypothesis
In thinking of the role of the sun in cracking rocks, we can start by making some simple observations. In the experiments by Griggs (1936), small cut stone tablets were put in ovens which would uniformly heat them up and then cool them off. In nature however, and particularly in deserts, rocks are not uniformly heated and cooled. Instead they are heated on the east in the morning, and in the west in the afternoon. Based on these observations, it can be hypothesized that perhaps it is this directional heating and cooling that results in the important stresses which might crack rocks (Figure 2). From a simplified physics stand point, we can go further and hypothesize that the differential thermal expansion of a rock that is caused by directional heating during certain times of day causes sufficient stress on the rock to exceed its tensional strength. When the tensile strength of the rock is exceeded, the material will fail and a crack will form. How could such a hypothesis be tested? Quite simply: if stresses are 'pulling' (tensional) in the east-west direction, then cracks that form should generally be oriented to the north-south.
The data
About 700 cracks on about 400 rocks ranging in size from ~20 - 80 cm were examined from deserts throughout the southwestern United States (Figure 3). When the orientations of all of these cracks are plotted, there is a notable north-south trend (Figure 4a). In and of itself, this preferred orientation of cracks in desert rocks is remarkable. It is difficult to imagine another forcing mechanism that could account for this preferred orientation of cracks in rocks other than a sun-related process. In examining all of these cracks, however, it is evident that the story is not a simple one.
The orientations of many cracks seem to be related to properties of the rocks that are cracked (Figure 5). As you might expect, the orientations of many of the cracks are parallel to pre-existing fabric in the rocks such as bedding planes or foliation ('fabric cracks'). The orientation of many other cracks appear to be related to the shape of the rock. In particular, if rocks are longer than wide, then there are often cracks parallel to the long axis ( 'longitudinal cracks'). Also, if the rock contains a large flat face, there are often cracks that are oriented parallel to that surface ('surface cracks'). The majority of cracks studied, however, fell in none of these categories. When the orientations of these leftover cracks were plotted on a rose diagram, there the clear north-south trend becomes even more visible (Figure 4b).
Another interesting feature of the rose diagram of crack orientations is that there is not a single main trend or mode to the data right at north-south. Instead, there are 2 primary modes a bit to the east and to the west of north. What might explain these two separate modes? At ~35 degrees North latitude where these data were collected, the sun does not rise and set perfectly in the east and west, but varies according to the time of year from northeast/northwest in the summer to southeast/southwest in the winter. Thus it might be hypothesized that cracks are forming due to sun-related stresses that arise at certain times of the day and/or year in order to produce these different modes of cracking.
To test this latitude-dependant hypothesis, another 100 cracks were examined in the Gobi desert of Mongolia (Figure 3). The orientations of these cracks is remarkably unimodal (Figure 6), with an orientation farther to the east than that of the cracks from 35 degrees latitude. At this location (45 degrees latitude) the sun will rise farther in the north and south on the summer and winter solstices respectively. If we assume that cracks are forming perpendicular to the stresses that arise as a result of this directional heating and cooling, then such a shift in crack orientations as a function of latitude would be expected. The unimodality of the Gobi data might be attributable to the importance of the winter months in cracking rocks under cold-dry climatic conditions, as opposed to the hot-dry conditions of US deserts.
Conclusions
Overall, these field data provide strong new evidence that the sun plays a key role in cracking surface rocks and also open doors to innumerable new hypotheses related to the processes that are important in physical weathering of rocks on Earth and on other planets.
Associated References
- Blackwelder, E.B., 1933, The insolation hypothesis of rock weathering: American Journal of Science, v. 226, p. 97–113.
- Eppes, Martha Cary, Griffing, David, 2009. Granular disintegration of marble in nature: A thermal-mechanical origin for a grus and corestone landscape, Geomorphology, V. 117 p. 170-180. doi: 10.1016/j.geomorph.2009.11.028
- Eppes, M.C., McFadden, L., Wegmann, K., Scuderi, L. 2010. Cracks in desert pavement rocks: further insights into mechanical weathering by directional solar heating. Geomorphology, v 123, p. 97-108.
- Griggs, D., 1936b, The factor of fatigue in rock exfoliation: Journal of Geology, v. 44, p. 783–796.
- Hall, K., 1999, The role of thermal stress fatigue in the breakdown of rock in cold regions: Geomorphology, v. 31, p. 47–63.
- McFadden, L., Eppes., M., Hallet, B., Gillespie, A., 2005. Physical weathering in arid landscapes due to diurnal variation in solar heating, Geological Society of America Bulletin, V. 117, p. 161-173.