Short answers, pulled from the story.
Galileo Galilei observed the moon Io on the 8th of January 1610 while looking through his telescope. He published this discovery in Sidereus Nuncius three months later, marking the first recorded sighting of these Jovian satellites.
Simon Marius proposed names for the moons based on lovers of Zeus including Io the priestess who became one of Jupiter's many partners. The International Astronomical Union adopted his naming scheme in the mid-20th century after it gained traction only centuries after his death.
Voyager 1 arrived at Jupiter on the 5th of March 1979 and returned images showing a young surface dotted with colorful plains. Navigation engineer Linda A. Morabito spotted a plume in those images proving the moon was volcanically active for the first time.
Tidal forces acting on Io are roughly 20,000 times stronger than Earth experiences from our Moon due to Laplace resonance. Friction generated within the interior by constant stretching produces heat up to 200 times greater than radioactive decay alone which melts portions of the mantle and drives extreme volcanism.
Loki Patera stands out as the largest feature on Io contributing 25% of global heat output on average. It is classified as a patera which is a flat-floored depression bounded by steep walls averaging 100 kilometers in diameter.