1. R-process: Ultra-rapid neutron absorption without sufficient time for decay between neutron impacts.
2. S-process: Repeated absorption of neutrons with time between impacts for some decays to occur.
3. F-process: While energy cannot be liberated from fusion of iron and more massive elements, such fusions can and do occur. Thermodynamics favors such endothermic fusions at sufficiently high temperatures. Such fusions can cool the core of a star, accelerating collapse. Such fusions produce high mass nuclei, which quickly decay into more ordinary elements.
4. I-process: Inverse fission caused when heavy nuclei collide (as in F-process) in the presence of very high neutron fluxes during supernova explosions. This process is approximately the reverse of ordinary nuclear fission.
5. N-process: when outer layers of tentative neutronium are bounced off of cores in supernovae, and are ejected from the stars, then spall into ultra-massive nuclei that quickly decay into more stable ordinary nuclei.
Not all of the above processes are distinct, but rather grade into one another.
Furthermore, the initial rarity of the heavy but relatively non-reactive elements is exacerbated by the process of the formation of the Earth's core. These elements do not readily form oxides, but remain in elemental form. As such, they are soluble in the molten iron that makes up most of the Earth's core. They were therefore dissolved out of the original molten mass of metal and rock and descended to Earth's core. More reactive metals, like lead, for instance, DO readily form oxides. Hence, although lead's cosmic abundance is far lower than that of gold, it is far more common in the crust where oxides prevail.