  Young cavities in the X-ray emitting hot gas in galaxy clusters are often
filled with radio synchrotron emission and it is widely thought that the
cavities are inflated by these cosmic rays. At a later stage of its evolution,
when the cavity becomes buoyant, the converging flow of gas beneath the cavity
results in a filament of thermal gas, a cavity jet, that moves radially outward
at large subsonic velocities. As the cavity jet forms, the cosmic ray electrons
may diffuse through the cavity walls, filling a large volume surrounding the
cavity jet, as observed in M87/Virgo and elsewhere, sometimes referred to as
relic radio sources. We compute the combined evolution of cosmic rays, cavities
and cavity jets. The observed pattern in M87/Virgo can be reached in 100 Myrs,
matching the synchrotron age of the extended radio source. A 20-30 kpc long
cavity jet is surrounded by a quasi-spherical radio lobe 40 kpc in diameter,
but the initial cavity has disappeared. At later times the cavity jet will fall
back to the origin, leaving only the extended radio source. The combined
jet-lobe evolution in M87/Virgo requires a total cosmic ray energy that is more
than 10 times larger than that usually assumed, 4PV.
