Ribs on jointing in radiolarian chert, Peloponessos, Greece

&lt;p&gt;This rock slab may resemble bedding, but actually it is part of a joint face. &amp;nbsp;The curved features are known as 'ribs' and are expressions of joint propagation. &amp;nbsp;Jointing initiates at a nano-scale stress concentrator where stress has become intensified, and then the leading edge of the fracture travels through the rock at a velocity of approximately half the speed of sound. &amp;nbsp;There are times when the energy of propagation declines radically, and when this happens the propagation of the front of the fracture slows (or stops). &amp;nbsp;In such intervals the local orientation of tension direction may rotate, and thus the fracture surface (always oriented perpendicular to greatest tension) will torque. &amp;nbsp;This shift is what makes the ribs apparent in outcrops. &amp;nbsp;Resumed propagation at full velocity once again creates a fracture surface parallel to the dominant direction of the joint face.&lt;/p&gt;

Whereabouts

I collected this rock during the summer of 2008 in the Pindos fold and thrust belt of the Peloponessos near the village of Lykaeos. I like this rock because it is like collecting a sound wave; I can imagine the sound emanating as the fracture ripped open the rock.

Age & Formation

Radiolarian chert from within Chert Series Beds of Jurassic age, ~180 Ma, Peloponessus, Greece.