In the dynamic depths of the Earth, the process of phase transformation plays a pivotal yet elusive role in shaping the mechanical behavior of geological materials. These transformative events, where minerals reorganize their crystal structures under intense pressure and temperature, have long been hypothesized as key contributors to rheological weakening—essentially the softening and deformation of rocks deep within tectonic boundaries, subducting slabs, and convecting mantle regions. Despite decades of theoretical and indirect evidence, directly observing and quantifying such weakening during mineral phase transitions has remained an exceptional challenge, constraining our understanding of fundamental geodynamic processes.
A groundbreaking study has now pierced through this veil of uncertainty using advanced synchrotron-based experimental methods that capture transient weakening occurring during two critical mineral phase transitions: the polymorphic transformation of quartz to coesite (both silica polymorphs) and the olivine to ringwoodite transition in iron-bearing silicates. Remarkably, researchers recorded rheological weakening by up to three orders of magnitude during these transitions, providing the first direct, quantitative observation of this phenomenon. This discovery not only substantiates long-standing hypotheses but also reveals that such weakening is most prominent when the phase transformation progresses faster than mechanical deformation itself, illuminating a delicate competition between mineral physics and rock mechanics.
These findings have profound implications for geosciences, fundamentally altering how scientists conceptualize the deformation patterns within subduction zones, mantle convection systems, and continental collision zones. By demonstrating that transient, order-of-magnitude weakening can occur in situ during first-order phase transitions, this research invites a reevaluation of geodynamic models that have historically overlooked or underestimated the rheological consequences of mineralogical transformations. The experimental approach draws upon state-of-the-art synchrotron radiation, allowing for real-time measurements under conditions simulating those in the Earth’s interior, a technical feat that marks a significant advance over previous indirect or post-mortem studies.
The polymorphic quartz-to-coesite transition serves as a vital proxy for understanding silicate transformation mechanics due to its well-characterized structural rearrangement under high-pressure conditions. Similarly, the olivine-to-ringwoodite transformation, representing a major mantle phase change, is of particular interest because olivine-rich rocks populate the upper mantle and subsequently transform in the transition zone, approximately 410 to 660 kilometers deep. The experiments indicate that during these transitions, the crystal lattice instability and consequent volume changes facilitate deformation by transiently reducing rock strength, which in turn may promote localized strain accumulation.
Crucially, the magnitude of weakening was found to be contingent on the relative rates of transformation and deformation—when transformation proceeds rapidly compared to tectonic strain, the mineral structure does not have time to accommodate internal stresses elastically or plastically, resulting in a temporary softening that amplifies deformation. This transient weakening phenomenon challenges the canonical view that phase transformations uniformly harden rocks due to increased density and atomic packing in high-pressure forms. Instead, it suggests a two-stage process where initial weakening precedes eventual strengthening, dictated by kinetics and stress distribution during transformation.
Expanding beyond laboratory conditions, the study employs computational geodynamic models to assess the broader implications of these observations for subducting slabs—tectonic plates plunging into the mantle. The olivine-spinel (or olivine-ringwoodite) phase boundary is particularly susceptible to transient weakening under certain thermal and hydration states. Models reveal that cold and hydrous slabs, typical beneath the western Pacific, experience significant weakening that aligns with geophysical detections of slab stagnation or delayed penetration into the lower mantle transition zone. This insight offers a physical explanation for anomalous slab behaviors previously documented by seismic tomography but hitherto difficult to reconcile with mineral physics alone.
Moreover, the implications extend to understanding seismic anisotropy, mantle flow patterns, and the generation of deep-focus earthquakes. Transiently weakened rock volumes can localize strain and influence mantle viscosity gradients, which in turn affect convective circulation and tectonic loading. The detected weakening also suggests a mechanism by which phase transformation-induced microstructures and grain-scale heterogeneities can evolve during subduction, potentially facilitating episodic failure or shear localization—key processes implicated in seismicity and mantle faulting events.
The researchers emphasize that transformational weakening is likely a general characteristic of silicate minerals experiencing first-order phase changes under geophysically relevant conditions. Given that Earth’s mantle comprises a complex assemblage of such minerals, their results demand that this rheological complexity be integrated into next-generation geodynamic simulations. Including transformation-induced softening could enable more accurate predictions of mantle flow dynamics, slab morphology, and the mechanical coupling between tectonic plates and the underlying mantle.
Technically, the experimental setup leveraged synchrotron X-ray diffraction and imaging technologies capable of resolving microstructural evolution within samples subjected to pressures and temperatures paralleling those found deep within Earth. This achievement was facilitated by adaptive control over deformation rates and phase transition kinetics, enabling unprecedented insights into the interplay between mineral physics and rock mechanics. The detected weakening spans a remarkable range—up to three orders of magnitude—underscoring the dramatic and transient nature of the effect.
In addition, the coupling between mechanical deformation and phase transformation kinetics highlights the dynamic feedback mechanisms that govern material response in Earth’s interior. Rapid phase changes can temporarily overwhelm traditional deformation mechanisms such as dislocation glide or diffusion creep, effectively resetting the mechanical behavior of rocks. This nuanced understanding challenges static perspectives and calls for modeling frameworks that incorporate time-dependent transformations alongside evolving stress fields.
The study’s findings resonate with ongoing efforts in seismology and mineral physics to decode mantle heterogeneity and slab dynamics. The observation that hydrated, cold slabs are most vulnerable to transient weakening mirrors the distribution of stagnant slabs detected beneath regions like the western Pacific, where subducted material appears deflected or delayed at mantle transition zone depths. This correspondence between experimental quantification and geophysical observation provides a compelling link between microscale mineral behavior and macroscale tectonic patterns.
As geodynamics research increasingly converges with mineral physics, the role of phase transformation-induced weakening emerges as a critical control on the Earth’s rheology, arising from intrinsic material instabilities rather than external thermal or chemical variations alone. This paradigm shift not only advances fundamental geology but also offers potential pathways to improve earthquake hazard assessment, as localized weakening zones can serve as nucleation sites for deep seismic events.
Future directions prompted by this research include extending studies to other mineral systems and phase transitions prevalent in the mantle and crust, as well as refining models to simulate the spatial and temporal heterogeneity of transformational weakening under realistic geological conditions. Integrating these effects with fluid-rock interactions and grain size evolution may further illuminate the mechanisms by which Earth’s internal dynamics shape surface geology and tectonics.
Overall, this breakthrough work illuminates a critical, previously elusive mechanism controlling the mechanical behavior of Earth’s interior. By directly observing and quantifying transient rheological weakening during mineral phase transitions, it provides a foundational advance that bridges experimental mineral physics and large-scale geodynamics, setting a new standard for interpreting complex Earth processes. The scientific community now has robust tools and physical constraints to integrate transformational weakening into predictive models, promising more accurate reconstructions of tectonic evolution, mantle convection, and seismic phenomena.
This transformative perspective calls for a renewed focus on dynamic mineral transformations within Earth sciences and offers a powerful framework for resolving longstanding enigmas regarding the strength and behavior of rocks at immense depths and pressures. Harnessing the precision of synchrotron-based techniques alongside sophisticated geodynamic modelling has opened a novel window into the hidden mechanics of our planet, underscoring the intricate interdependence between microscopic mineral processes and the grand-scale forces driving Earth’s continuous evolution.
Subject of Research: Rheological weakening during mineral phase transformations in the Earth’s interior, specifically involving quartz-coesite and olivine-ringwoodite transitions under conditions relevant to subduction and mantle convection.
Article Title: Direct observations of transient weakening during phase transformations in quartz and olivine.
Article References:
Cross, A.J., Goddard, R.M., Kumamoto, K.M. et al. Direct observations of transient weakening during phase transformations in quartz and olivine.
Nat. Geosci. (2025). https://doi.org/10.1038/s41561-025-01703-6
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