What started and ended the Neoproterozoic Snowball Earth glaciations?
In plain words
Around 700 and 640 million years ago ice seems to have reached the equator, covering nearly the whole planet. It is unclear what pushed Earth into this state and how it escaped, since white ice reflects most sunlight.
Precise statement
For the Sturtian (about 717 to 660 Myr ago) and Marinoan (ending about 635 Myr ago) glaciations, determine the trigger for crossing the ice-albedo bifurcation (CO2 drawdown by weathering of low-latitude basalt provinces, volcanic aerosols, or other) and the deglaciation pathway, including the $pCO2$ at which a globally glaciated state with dust-darkened ice becomes unstable. An answer is a quantified trigger and exit $pCO2$ consistent with the geochronology of glacial durations of order $10^7$ yr and with cap-carbonate geochemistry.
What would settle it
Coupled ice-climate-carbon cycle models reproducing the measured onset timing and multi-million-year durations with independently constrained weathering and outgassing rates.