During placement of no3 wire (1st piece of gear), 2.5m from base of route, into small crack (5cm long) to the right of arête, the crack opened to 30cm, running to base of arête and caused movement of approx 5mm outward in a sliding motion. I have left the no3 wire in place to help mark the issue as am concerned that this part of the arête is now quiet fragile and could cause injury if falls. Also concerned that if this piece goes then it could destabilise arête rock above which looks reliant upon the base piece. Left the crag when the rain came in, no other climbers on this crag at the time.