Instrumental observations have been made for almost 50 years. Geologists, volcanologists, geochemists, and geophysicists now study numerous different aspects of eruptions. Through these detailed studies a clear picture emerges about how volcanoes erupt. Volcanologists use this knowledge to predict eruptions and to produce hazard maps for volcanoes. Have a look at How to become a Volcanologist and Working on Hawaiian Volcanoes.
Volcanoes can erupt for long periods.
Kilauea erupted for nearly a hundred years from the early 1800s to 1924. The current eruption of Kilauea Volcano started in 1983 and is going strong. Erta Ale and Erebus are other volcanoes that have been erupting for a long time.