Tectonics

The Hawaiian shield volcanoes are the largest mountains on Earth.
The eruptive history of Mount St. Helens (Washington) began about 40,000 years ago with dacitic volcanism, which continued intermittently until about 2,500 years ago.
The principal volcanoes of the Hawaii Islands (from northwest (oldest) to southeast (youngest)) are Niihau, Kauai, Oahu, Molokai, Lanai, Maui, Kahoolawe, and Hawaii (Big Island).
It isn't that simple. There is not one magnitude above which damage will occur.
There are a few historic examples of simultaneous eruptions from volcanoes or vents located within about 10 km of each other, but it's very difficult to determine whether one might have caused the other.
On May 18, 1980, at Mount St.
Yes. Crater lakes atop volcanoes are typically the most acid, with pH values as low as 0.1 (very strong acid). Normal lake waters, in contrast, have relatively neutral pH values near 7.0.
On January 19, 1968, a thermonuclear test, codenamed Faultless, took place in the Central Nevada Supplemental Test Area. The codename turned out to be a poor choice of words because a fresh fault rupture some 1200 meters long was produced.
Yes. Encounters between aircraft and clouds of volcanic ash are a serious concern.
The violent separation of gas from lava may produce rock froth called pumice.