There would be massive global pressure on Europe to handle the exit in a grown-up fashion, with backstops in place to stabilize Greece. The IMF would step in.
The German finance ministry is already drawing up such plans, and quite correctly so (unfortunately roping in the British too to spread the losses, which is a thorny subject).
Needless to say, the real danger is contagion to Portugal, Ireland, Spain, Italy, Belgium, France, and the deadly linkages between €15 trillion in public and private debt in these countries and the €27 trillion European banking nexus.