  The rapid shape change in Zr isotopes near neutron number $N$=60 is
identified to be caused by type II shell evolution associated with massive
proton excitations to its $0g_{9/2}$ orbit, and is shown to be a quantum phase
transition. Monte Carlo shell-model calculations are carried out for Zr
isotopes of $N$=50-70 with many configurations spanned by eight proton orbits
and eight neutron orbits. Energy levels and B(E2) values are obtained within a
single framework in a good agreement with experiments, depicting various shapes
in going from $N$=50 to 70. Novel coexistence of prolate and triaxial shapes is
suggested.
