  Our aim is to explain instances in which the value of the logarithmic Mahler
measure of a polynomial can be written in an unexpectedly neat manner. To this
end we examine polynomials defining rational curves, which allows their
zero-locus to be parametrized. In these instances the unusual simplification is
a consequence of the Galois descent property for Bloch groups. This principle
enables one to explain why the arguments of the dilogarithm function depend
only on the points where the rational curve intersects the torus |x|=|y|=1. In
the process we also present a general method for computing the Mahler measure
of any parametrizable polynomial.
