The main edifice is around 500,000 years old and is composed of mostly lavas which have built up over time. It is unclear when Rainier last event minor event was with different souces stating different things! According to the GVP an event occurred back in 1894 which involved minor activity at the summit. It has been a while since any major activity has been seen, with up to VEI 4 events occuring in the past as is shown on the GVP. With a volcano such as Rainier any future eruption it is highly probable that it would be preceeded by warning signs such as deformation, hydrothermal activity and degassing increases. Despite the location of Rainier, there is a lot more work to be done on the volcano to understand its history and hence its potential future!
I am afraid that posts will be a little more sparse and irregular over the coming months due to an extremely busy summer (getting married and a dissertation to complete!), however I should get back to regular posting by the end of July!
Global Volcanism Program
Sisson, T.W., 1995. History and Hazards of Mount Rainier, Washington. USGS. Open-File Report 95-642.