  We present an extensive study of vortex dynamics in a high-quality single
crystal of HgBa$_2$CuO$_{4+\delta}$ (Hg1201), a highly anisotropic
superconductor that is a model system for studying the effects of anisotropy.
From magnetization $M$ measurements over a wide range of temperatures $T$ and
fields $H$, we construct a detailed vortex phase diagram. We find that the
temperature-dependent vortex penetration field $H_p(T)$, second magnetization
peak $H_{smp}(T)$, and irreversibility field $H_{irr}(T)$ all decay
exponentially at low temperatures and exhibit an abrupt change in behavior at
high temperatures $T/T_c \gtrsim 0.5$. By measuring the rates of thermally
activated vortex motion (creep) $S(T,H)=|d \ln M(T,H) / d \ln t|$, we reveal
glassy behavior involving collective creep of bundles of 2D pancake vortices as
well as temperature- and time-tuned crossovers from elastic (collective)
dynamics to plastic flow. Based on the creep results, we show that the second
magnetization peak coincides with the elastic-to-plastic crossover at low $T$,
yet the mechanism changes at higher temperatures.
