The key difference between ductile and brittle deformation is that ductile deformation occurs at low strain rates, whereas brittle deformation occurs at high strain rates.
When increasing the stress applied on a particular rock, the rock passes through three types of successive stages of deformation. They are the elastic deformation, ductile deformation, and brittle deformation. Elastic deformation is a reversible deformation, ductile deformation is irreversible where brittle deformation causes the rock to break.
CONTENTS
1. Overview and Key Difference
2. What is Ductile Deformation
3. What is Brittle Deformation
4. Side by Side Comparison – Ductile vs Brittle Deformation in Tabular Form
5. Summary
What is Ductile Deformation?
Ductile deformation in Earth science is the production of large, open folds in the sediments or rocks in front of an advancing glacier which can develop into overfolds. This can cause the sediments or rocks to begin to undergo internal thrusting due to continued ice advance. This type of rock deformation is extremely rock-type dependent. This is because even very small variations of mineral composition in the rock can result in quite different ductile properties. Furthermore, the mechanism of ductile deformation is highly versatile.
Moreover, ductile deformation indicates the shape changes of a material through bending or flowing during which chemical bonds can become broken but subsequently reform into new bonds. This requires stress that surpasses the elastic threshold and a deformation rate that is slow enough to accommodate more strain without breaking down the material. A rock that experienced ductile deformation typically has properties such as fold, foliation, and lineation. However, foliation and lineation can also be observed after brittle deformation.
Several mechanisms are responsible for ductile deformation, including diffusion creep, dislocation creep, mechanical twinning/kinking, grain boundary sliding, and rigid body rotation. Diffusion creep occurs when deformation of solid crystals occurs via migration of atoms and vacancies throughout the crystal lattice. This process is driven by the chemical potential gradient exerted by external stresses.
Diffusion creep is the most common mechanism of diffusion creep; there are three subcategories of diffusion creep: Nabarro-Herring creep, Coble creep, and dissolution-precipitation creep. Among these three, the first two forms indicate the conservative diffusion of atoms and vacancies in a crystalline solid. The third method is also known as pressure solution creep or wet diffusion creep, and it requires a film of fluid that acts as a carrier of crystalline material. Here, the solute diffuses non-conservatively through the fluid from the sites of dissolution to precipitation along grain boundaries.
What is Brittle Deformation?
Brittle deformation is a type of deformation that occurs by fracturing and faulting. This term refers to the breaking of chemical bonds that are not subsequently undergoing any reforming. Therefore, the result of the brittle deformation is analogous to those observations in broken plates such as fractures. The brittle deformation of a particular rock depends on the rheology of the rock. The brittle deformation of a rock occurs at a high strain rate.
During brittle deformation, rocks usually exhibit a pseudoviscous effect prior to the failure which is reflected in a strength increase along with the increasing strain rate. We can easily find this effect in mining industry. We call this as static fatigue; a pillar or other load-bearing structure fails after some period of time under constant load.
What is the Difference Between Ductile and Brittle Deformation?
Three types of deformations can occur in rocks: elastic deformation, ductile deformation and brittle deformation. The key difference between ductile and brittle deformation is that ductile deformation occurs at low strain rates, whereas brittle deformation occurs at high strain rates. Furthermore, ductile deformation is irreversible but does not break the rock while brittle deformation is irreversible and also causes the breaking of the rock. Therefore, this is another significant difference between ductile and brittle deformation.
Below is a summary of the difference between ductile and brittle deformation in tabular form.
Summary – Ductile vs Brittle Deformation
Three types of deformations that can occur in rocks: elastic deformation, ductile deformation and brittle deformation. The key difference between ductile and brittle deformation is that ductile deformation occurs at low strain rates, whereas brittle deformation occurs at high strain rates.
Reference:
1.“Ductile Deformation.” ScienceDirect Topics, Available here.
Image Courtesy:
1. “Veins in a ductile shear zone” by Wenzhu Hou (CC BY 3.0) via Imaggeo
2. “Deformation mechanism” By Mangombe – Own work (CC BY-SA 4.0) via Commons Wikimedia
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