  Metal abundance and its evolution are studied for Mg II quasar absorption
line systems from their weak, unsaturated spectral lines using stacked spectra
from the archived data of Sloan Digital Sky Survey. They show an abundance
pattern that resembles that of Galactic halo or Small Magellanic Cloud, with
metallicity [Z/H] showing an evolution from redshift z=2 to 0.5: metallicity
becomes approximately solar or even larger at z~0. We show that the evolution
of the metal abundance traces the cumulative amount of the hydrogen fuel
consumed in star formation in galaxies. With the aid of a spectroscopic
simulation code, we infer the median gas density of the cloud to be roughly 0.3
$\rm cm^{-3}$, with which the elemental abundance in various ionization stages,
in particular C I, is consistently explained. This gas density implies that the
size of the Mg II clouds is of the order of 0.03 kpc, which suggests that
individual Mg II clouds around a galaxy are of a baryonic mass typically $\rm
10^{3} \, M_{\odot}$. This means that Mg II clouds are numerous and `foamy',
rather than the large entity that covers a sizable fraction of galaxies with a
single cloud.
