A tale of two transitions: Linking the brittle–ductile transition to changing microphysical processes

Earth’s relatively rigid, outermost rocky layer, the lithosphere, encompasses Earth’s continental crust, oceanic crust, volcanic edifices, and earthquake-generating faults. The lithosphere is defined by its mechanical properties—in general, it is stronger than the more easily deformable rocks in Earth’s deeper, hotter interior. This definition has been quantified by myriad laboratory experiments seeking to measure the strength of rocks, and the underlying microphysical processes controlling rock strength, over a wide range of conditions (1). However, considerable uncertainty still surrounds the behavior of rocks at their strongest point, at which a variety of physical processes occur in concert. The article in this issue by O’Ghaffari et al. (2) takes a critical step in resolving this persistent challenge.

The general view, as depicted by the thick solid lines in Fig. 1, is that the strength of rocks increases with increasing depth due to increasing pressure until a tipping point, at which the strength of rocks decreases with increasing depth due to increasing temperature (3). At shallower depths, rocks are predicted to behave in a macroscopically brittle manner. Intact rocks fail, rapidly losing strength and localizing deformation into frictional faults. At greater depths, rocks are predicted to behave in a macroscopically ductile manner. Deformation in this regime is relatively stable and homogeneously distributed. Rocks are at their strongest at the transition between these two regimes, the brittle–ductile transition.

Fig. 1.

Schematic illustration of the strength of rocks as a function of depth into Earth’s interior. Bold lines indicate the limits often used in assessing the strength of Earth’s lithosphere. Dashed lines indicate a range of microphysical processes and their schematic limits in strength. O’Ghaffari et al. (2) provide a method for analyzing the distribution and interaction of these mechanisms in the brittle–ductile transition. The diagram assumes constant deformation rate and a single rock type.

This transition is an important area of inquiry since it is generally thought to be the nucleation site of large earthquakes (4), to influence the depth and extent of water infiltration in fault zones (5), and to moderate the energy potential of geothermal fluid circulation (6). Yet, despite its importance, rock strength in this region is poorly quantified, such that diagrams in both classic and recent analyses of the strength of rocks consistently leave question marks at depths corresponding to the brittle–ductile transition (1, 7). A typical approach (8) is to extrapolate the strength of rocks measured at low temperatures and pressures to greater depths, while extrapolating the strength of rocks measured at high temperatures and pressures to shallower depths. The intersection of the two extrapolations indicates a potential maximum strength of rocks. Unfortunately, this procedure tends to greatly overestimate the strength of rocks near the brittle–ductile transition (9), which can lead to major discrepancies between laboratory estimates of lithospheric strength and geophysical observations (10). These discrepancies highlight the need for a constitutive description that explicitly captures the behavior of rocks near the brittle–ductile transition.

The major roadblock to such a constitutive description of the brittle–ductile transition is a lack of insight into the microphysical processes controlling the macroscopic behavior. The key microphysical processes are relatively clear in the brittle regime, and models exist that describe crack nucleation and coalescence into localized faults in intact rocks (11) or that describe frictional deformation of fault rocks (12). The key microphysical processes are also relatively clear in the ductile regime. Here, models exist that describe the crystal plasticity (dislocation propagation, twin formation, and/or atomic diffusion) that leads to distributed deformation of rocks (13). These microphysical considerations help confirm and motivate the form of the constitutive equations typically used to describe that macroscopic behavior. Yet the manner in which these different microscopic processes combine and interact near the brittle–ductile transition is not clear. As recently emphasized by Meyer et al. (7), the dominant process may change throughout the deformation history of a rock. Microcracking may initiate from preexisting cracks (11), or as temperature increases, deformation may initiate with dislocations or twins that eventually create stress concentrations and nucleate new cracks (14). The sequence of these events and specific interactions among defects leads to contrasting constitutive models that must be distinguished by experiment.

O’Ghaffari et al. present a novel method for unpicking the transitions in microphysical processes that accompany the macroscopic brittle–ductile transition. Decades of work have sought to identify the microscopic processes at work near the brittle–ductile transition by examining defect populations within samples after deformation was imposed. However, these ex situ observations are often ambiguous in terms of the sequence of processes that occurred. At relatively mild conditions, in operando techniques can reveal crack nucleation or dislocation activity from, for example, sample volume changes (15) or X-ray peak broadening (16), respectively. Some recently developed in operando techniques allow imaging of both microcracking and crystal plasticity (17), although these techniques are hard won, tend to involve long data acquisition times and correspondingly poor time resolution, and are generally still at mild conditions not suitable for investigating geological materials. In contrast to existing methods, the approach presented by O’Ghaffari et al. (2) provides exceptional time resolution and is able to distinguish among microcracking, twin formation, and dislocation activity all in operando at the extreme conditions associated with rock deformation in deeper Earth’s lithosphere.

O’Ghaffari et al. provide the first evidence of dislocation avalanches in calcite, and also make the first observation of dislocation avalanches at high confining pressure.

The method of O’Ghaffari et al. (2) leverages subtle differences in the energy released during formation and motion of different crystal defects. During deformation, the authors record bursts of acoustic activity in the sample, which they then relate to the microscopic defects at play. The authors apply a variety of numerical methods to analyze and compare the waveforms of these acoustic emissions. Their primary observation is that the frequency of the waveforms tends to increase as the conditions are changed to promote ductility. Additional analyses identify groups of events with distinct waveform characteristics that correlate with macroscopic behavior. Based on the apparent size of the source of the acoustic emissions, the authors suggest that recorded events can be linked to microcracking, twinning, or dislocation motion, therefore providing an in operando method for detecting the activity of different defects.

This ability to distinguish among microphysical processes responsible for the acoustic emissions results from the distinct behavior of dislocations in calcite. Select materials exhibit acoustic emissions due to the catastrophic escape of dislocations as they collectively move past an obstacle, sometimes referred to as “dislocation avalanches” and often linked to jerky flow (18). Materials that produce dislocation avalanches generally exhibit high plastic anisotropy and include metals like zinc or tin (leading to the phenomenon of tin cry). Although many minerals have high plastic anisotropy, dislocation avalanches have only been observed in a couple of geological materials like NaCl (19) and water ice (20) because of the relative ease of conducting experiments on these materials in the ductile regime. O’Ghaffari et al. (2) provide the first evidence of dislocation avalanches in calcite and also make the first observation of dislocation avalanches at high confining pressure.

This major technical milestone provides new insight into the details of defect dynamics in rocks and opens the door for future in operando investigations linking transitions in microphysical processes to transitions in macroscopic behavior.