Experiments on Mg1.8Fe0.2SiO4 olivine at 18 to 20 gigapascals and 1000 degrees to 1400 degrees C show that transformation to the high-pressure polymorphs wadsleyite and ringwoodite involves two competing nucleation mechanisms when the grain size of the olivine is large : (i) incoherent nucleation on olivine grain boundaries and (ii) coherent intracrystalline nucleation of ringwoodite lamellae on shear-induced stacking faults in olivine un the (100) planes, followed by the nucleation of wadsleyite on the ringwoodite. The large amount of Intracrystalline transformation, which occurs under low differential stress under the pressure and temperature conditions of the mantle transition zone, suggests that shear-induced intracrystalline transformation of olivine to wadsleyite and ringwoodite is important in subducting slabs.