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.
If Windows inexpicably closes and your PC shuts down, it's almost certainly overheated. The PC, which monitors the temperature of the CPU, is protecting it's delicate circuits from the ravages of heat. (On the other hand, if you PC frequently shuts off without closing Windows first, causing an error message when you next boot, it's likley time to replace the power supply.)