Krakatoa in the South Pacific erupted in August 1883 with enough ferocity to obliterate 2/3 of the island. The explosion and tsunamis killed 36K to 120K. The summer temperature across the globe dropped 0.72°F and the resultant severe winter is known not so affectionately in North America as the "Starvation Winter." The tsunamis created by the eruption traveled around the globe ... three times! The second giant eruption was Mount Katmai in Alaska, 1916. Similar to Krakatoa, the entire mountain was obliterated in 45 minutes. Pyroclastic material filled the downstream river valley to depths of 1500 ft. Fortunately there was apparently only one casualty, an old native on Kodiak Island that, as I recall, died from complications of breathing ash and preexisting lung condition. The natives in the interior recognized the odd behavior of animals and evacuated via canoes to King Salmon in time. Your great grandfather could have lived to see environmental side effects of both volcanoes, though he would only have been eight years old when Krakatoa erupted.
