More recently introduced terms such as 'annually laminated' are synonymous with varve. Of the many rhythmites in the geological record, varves are one of the most important and illuminating in studies of past climate change. Varves are amongst the smallest-scale events recognised in stratigraphy.
PCs are designed to shut down automatically when heat endangers the hardware. To find out if this is your problem, download and install Speedfan. It's small and free. Although Speedfan's basic job is controlling the speed of your PC's fans (you probably guessed that by the name), it also provides the CPU temperature.
I was just turning off my PC, he replied innocently. Sometimes I forget that some of the stuff I take for granted isn't common knowledge. So in case you've been committing this same heinous shutdown crime, allow me to enlighten you. That is not, repeat, not the proper way to shut down a PC. The proper way is to click Start, Shut down. The only time you should press and hold the power button is if your computer is locked up and otherwise unresponsive.