  To understand the formation mechanism of large plumes in solar prominences,
we investigate the formation process of two such phenomena. We studied the
dynamic and thermal properties of two large plumes using observations from New
Vacuum Solar Telescope, the Solar Dynamic Observatory, and the Solar
Terrestrial Relations Observatory-Ahead. We find that two large plumes observed
with high-resolution data are quite different from previously studied
small-scale plumes. They are born at the top of a prominence bubble with a
large projected area of 10-20 Mm^2 . Before the occurrence of each large plume,
the bubble expands and takes on a quasi-semicircular appearance. Meanwhile, the
emission intensity of extreme-ultra-violet (EUV) bands increases in the bubble.
A small-scale filament is found to erupt in the bubble during the second large
plume. At the point at which the height of the bubble is comparable with half
the width of the bubble, the bubble becomes unstable and generates the plumes.
During the formation of plumes, two side edges of the top of the bubble, which
are dominated by opposite Doppler signals, approach each other. The large plume
then emerges and keeps rising up with a constant speed of about 13-15 km/s.
These two large plumes have temperatures of 1.3 x 10^6 Kelvin and densities of
2.0 x 10^9 cm^-3, two orders hotter and one order less dense than the typical
prominence. We also find that the bubble is a hot, low-density volume instead
of a void region beneath the cold and dense prominence. Therefore, we conclude
that these two large plumes are the result of the breakup of the prominence
bubble triggered by an enhancement of thermal pressure; they separate from the
bubble, most likely by magnetic reconnection.
