Intragranular movements in plastic deformation of rocks

Understanding Plastic Deformation & Intragranular Movements

Plastic deformation is a time-dependent process where rocks undergo permanent strain without fracturing. This is achieved through various mechanisms operating at the microscopic level, primarily involving the rearrangement of atoms within the mineral structure. Intragranular movements are the deformation mechanisms occurring within the individual grains of a polycrystalline rock.

Mechanisms of Intragranular Movement

1. Dislocation Glide

Dislocation glide is the most common mechanism of plastic deformation at relatively low temperatures. Dislocations are linear defects within the crystal lattice. Under stress, these dislocations move through the crystal structure, allowing atoms to shift positions without breaking bonds. This movement is analogous to moving an extra half-plane of atoms through the crystal. The ease of glide depends on the crystal structure and the presence of obstacles like other dislocations or impurities.

2. Dislocation Climb

At higher temperatures, dislocation climb becomes significant. This process involves the movement of dislocations out of their slip plane by the addition or removal of atoms. Climb requires the diffusion of vacancies (empty lattice sites) and is therefore temperature-dependent. It allows dislocations to overcome obstacles that would otherwise impede glide.

3. Twinning

Twinning is a deformation mechanism where a portion of the crystal lattice is mirrored across a twinning plane. This creates a symmetrical arrangement of atoms and results in a change in shape. Twinning is often observed in minerals with specific crystal structures, like quartz and calcite, and is particularly important at lower temperatures where dislocation glide is limited.

4. Grain Boundary Sliding

While technically not *strictly* intragranular, grain boundary sliding contributes significantly to overall plastic deformation. At elevated temperatures, grains can slide past each other along their boundaries. This is facilitated by the diffusion of atoms along the grain boundaries and is more prominent in fine-grained rocks. It often accompanies other intragranular mechanisms.

Factors Controlling Intragranular Movements

• Temperature: Higher temperatures promote dislocation climb and grain boundary sliding.
• Stress: Increased stress drives dislocation glide and twinning.
• Crystal Structure: Different crystal structures have varying numbers of slip systems (planes and directions along which dislocations can move), influencing the ease of deformation. For example, face-centered cubic (FCC) structures generally have more slip systems than body-centered cubic (BCC) structures.
• Water Content: The presence of water can enhance diffusion rates, promoting dislocation climb and grain boundary sliding.
• Grain Size: Smaller grain sizes generally lead to increased grain boundary area, promoting grain boundary sliding.

Examples in Geological Settings

The deformation of rocks in the Himalayas, formed by the collision of the Indian and Eurasian plates, provides a classic example of plastic deformation involving intragranular movements. Microscopic analysis of rocks from the Himalayas reveals evidence of extensive dislocation glide, climb, and twinning in minerals like quartz and feldspar. Similarly, the ductile shear zones found along fault lines demonstrate significant plastic deformation facilitated by these intragranular processes.

Deformation Mechanism	Temperature Regime	Dominant Factor	Mineral Example
Dislocation Glide	Low to Moderate	Stress	Halite
Dislocation Climb	Moderate to High	Temperature & Diffusion	Olivine
Twinning	Low	Crystal Structure	Quartz
Grain Boundary Sliding	High	Temperature & Grain Size	Ice (Glaciers)