Why can volcanic eruptions help to begin an ice age?
In 1815 (Mount Tambora) and again in 1883 (Krakatoa), unusual volcanic activity from the Dutch East Indies (modern Indonesia) disrupted weather patterns. 1816 was known as the "Year Without Summer" and weather patterns after Krakatoa didn't return to normal until 1888. These eruptions were unusually large, but neither caused an Ice Age; in fact, the Mount Tambora eruption occurred at the tail end of a "Little Ice Age" (roughly 1300-1850 AD). The smoke and ash from these eruptions blocked a significant amount of sunlight and killed several seasons' worth of crops in much of the world.
A supervolcano--an apocalyptic, VEI 7 or 8, could possibly trigger an Ice Age. There hasn't been a volcano of this magnitude in over 26,000 years, though one is said to be percolating and ready to blow under Yellowstone.