  In this paper we propose a new mechanism describing the initial spike of
giant flares in the framework of the starquake model. We investigate the
evolution of a plasma on a closed magnetic flux tube in the magnetosphere of a
magnetar in the case of a sudden energy release and discuss the relationship
with observations of giant flares. We perform one-dimensional numerical
simulations of the relativistic magnetohydrodynamics in Schwarzschild geometry.
We assume energy is injected at the footpoints of the loop by a hot star
surface containing random perturbations of the transverse velocity. Alfv\'en
waves are generated and propagate upward, accompanying very hot plasma that is
also continuously heated by nonlinearly generated compressive waves. We find
that the front edges of the fireball regions collide at the top of the tube
with their symmetrically launched counterparts. This collision results in an
energy release which can describe the light curve of initial spikes of giant
flares.
