The largest volcanic eruption in the 1900s happened very early in the century--
but (thankfully) far from large cities and population centers.
Novarupta (which means “new eruption”) is located at the base of the Alaska Peninsula. It is almost 300 miles from Anchorage (Alaska's largest city) and about 750 miles to Juneau (Alaska's capital city). When it began to erupt on this day in 1912, nobody knew it for a while. Alaska was not yet a U.S. state, population density was even lower then than now, few scientists were studying volcanoes, and there was not yet a worldwide seismic network in place. People as far away as Juneau did hear the sound of later blasts.
For the next sixty hours, Novarupta continued to erupt, shooting gas, rocks, and ash high into the air. Compared to other famous volcanoes, Novarupta emitted 30 times more “ejecta” than did the 1980 eruption of Mount St Helens. Yet the latter volcano, even with modern monitoring and plenty of warning, killed 57 people. There were NO human deaths attributed to Novarupta! (Unfortunately, many, many plants and animals died.)
(Sadly, the largest volcanoes in the 1800s were super-deadly. In 1815 Tambora killed many thousands of Indonesians directly and was the cause of even more deaths because so many crops and so much forest was destroyed, and in 1883 Krakatoa killed from 36 to 120 thousand Indonesians. Obviously these volcanic eruptions were in more densely populated areas, although they were more powerful, as well.)
To learn more about volcanoes, check out this earlier post.