  We study the simultaneous effects of the symmetry energy and temperature on
the crust-core transition of a magnetar. The dynamical and the thermodynamical
spinodals are used to calculate the transition region within a relativistic
mean-field approach for the equation of state. Quantizing magnetic fields with
intensities in the range of $ 2\times 10^{15}<B<5\times 10 ^{16}$G are
considered. Under these strong magnetic fields, the crust extension is very
sensitive to the density dependence of the symmetry energy, and the properties
that depend on the crust thickness could set a constraint on the equation of
state. It is shown that the effect on the extension of the crust-core
transition is washed out for temperatures above $10^{9}$ K. However, for
temperatures below that value, a noticeable effect exists that grows as the
temperature decreases and which should be taken into account when the evolution
of magnetars is studied.
