Development of plumose structures in brittle rocks has been investigated by analogy to fracturing experiments on glass and ceramic bodies. Plume morphology shows that structures commonly lumped as plumose are a composite of discrete features, formed at all scales, during fracture propagation.

Inclusion hackle forms when an advancing planar fracture front becomes locally distorted at an inhomogeneity. The planar fracture, locally split by the inclusion, does not rejoin in a single plane behind the inclusion. This causes the lagging fracture portion to curve into the leading one forming a steplike tail elongate in the propagation direction. Twist hackle forms when a fracture front abruptly encounters changed stress directions along an extended frontal section. The entire fracture front breaks into individually advancing en echelon twist-hackle faces, each face perpendicular to the new resultant principal tension. Faces diverge and are elongate in the propagation direction. The faces form hackle steps by curving into each other to complete separation. Velocity hackle, uncommo in rocks, forms at a limiting propagation velocity.

Plume axes mark areas of greatest tensile stress and lightest propagation velocities. Plume asymmetry indicates intrastratum fracturing stress distributions. Axes consistently at the top or bottom of each stratum in a layered sequence indicate overall downward and upward (perhaps basement induced) propagation directions respectively. Recognizing twist-hackle faces and steps as differently oriented planes produced by a single fracture event eliminates identification and misinterpretation of false fracture sets.

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