Posted on August 11, 2018 GeoCon 3 Comments Pic of the day #16 (Plumose structure) Block diagram showing the various components of an ideal plumose structure on a joint. The face of joint 1 is exposed; joint 2 is within the rock (b) (c) Types of plumose structure. (a) Straight plume. (b) Curvy plume. (c) Plume with many arrest lines, suggesting that it opened repeatedly.
How it does form..?
When we talk about plumose structure : It form at a range of scales, depending on the grain size of the host rock. In very fine grained coal components of plumose structure tend to be much smaller than in relatively.
A plumose structure spreads outward from the joint origin represents the point at which the joint
started to grow.
Why Does Plumose Structure Form?
As we know Real joints are not perfectly smooth, because of two reasons
1. Real rocks are not perfectly isotropic and homogeneous, meaning that the material properties of a rock change from point to point in the rock. Inhomogeneities exist because not all grains in a rock have the same composition and because not all grains are in perfect contact with one another.
2. the stress field at the tip of a crack changes as the crack tip propagates. Thus, as the crack
grows, the stress intensity at the crack tip grows, up to a limiting value.
Now with these two conditions plumose can be explained
The dimple at the origin forms because the flaw at which the joint nucleated either was not perpendicular to the remote ?3, or caused a local change in the orientation of stress trajectories. The portion of the joint that formed in the immediate vicinity of the origin was, therefore, not
perpendicular to the remote ?3. As soon as the crack propagated away from the flaw, it curved into parallelism with the ?1-?2 principal plane (see the figure in the body).
In the mirror zone, the joint is still short, so the stress intensity, tensile stress magnitude,
and tip-propagation velocity are all relatively small. As a consequence of the low stress, only bonds in the plane exactly perpendicular to the local ?3 can break, so the joint surface that forms is very smooth.
In the mist zone, the joint moves faster, stress is higher, and stress at the joint tip is sufficiently large to break off-plane bonds, thereby forming microscopic off-plane cracks that make the surface rougher than in the mirror zone.
In the hackle zone, the joint tip is moving at its terminal velocity and stresses at the crack tip are so large that larger off-plane cracks propagate and the crack locally bifurcates at its tip to form microscopic splays that penetrate the joint walls.
Arrest lines on a joint surface represent places where the fracture tip pauses between successive increments of propagation. The visible ridge of the arrest line, in part, represents the contrast between the rough surface of the hackle and the relatively smooth surface of the mirror/mist zone formed as the fracture begins to propagate, and in part may be analogous
to the dimple formed at a crack origin. Thus, plumose structures form because of the twisting, tilting, and splitting occurring at the tip due to variations in local stress magnitude and orientation.