GEOLOGY 1 Lecture Notes - Lecture 19: Pore Water Pressure, Breccia, Differential Stress
Document Summary
Formation of joint relieves stress (elastic strain energy) for a critical distance to either side (stress shadow) Size of stress shadow is proportional to dimension of joint. Lithology - stiffer units in an extending sequence experience higher stresses and hence more joints. Overall spacing is about equal to bed thickness. Uplit and uprooting - as rock exhumed to surface, expands vertically and contracts horizontally - produces subvertical joint sets. Natural hydrofracturing associated with pore fluid pressure. Localized jointing associated with other structures e. g. regional warps. Behaviour of griffith cracks under confined compression is more complicated. Mode 2 + 3 fractures do not propagate in their own plane under confined compressive stress. Cannot grow as mode 2 or 3 fractures. As mode 2 and 3 cracks grow, they immediately curve and become mode 1 cracks. Mode 2 & 3 cracks are oriented at a high angle to 1 -> grow progressively. Cracks stop propagating after a short distance.
