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The book “The Jupiter Effect” was a hell of a nice piece of quackery, but one thing I remember from it, that still intrigues, is that there’s a cycle of sunspot activity that corresponds to the number of years it takes Jupiter to complete one orbit. OTOH, Jupiter’s mass to that of the Sun is about the same as Earth’s to Jupiter.

Various possible external forces. For example it appears distinctly possible that the huge Deccan Traps were influenced by the Yucatan meteor 65 mya, and possibly an even larger Shiva meteor just off shore. The Siberian Traps are a huge area, and perhaps a meteor crashed into the center where it started. The 3 great Yellowstone eruptions cover a 2 million year period, but there is a string of other calderas stretching westward and covering around 16 million years, so I doubt meteors had anything to do with that ongoing phenomena. This string starts at the Oregon/Nevada border and can be seen on a map at this link.
http://www.youngzine.org/article/yellowstone-fury-beneath-forest

Thomas Jaggar an early modern volcanologist established the first US volcanic observatory in Hawaii. I have a book of his in which it a report appears that magma levels may be influenced by low or high levels of sun spots. The great Toba eruption left a caldera 16 by 65 miles and it was only recently recognized the Lake Toba was actually a caldera lake. I doubt that a meteor cause that one. At any rate Jaggar was an important figure. Check him out.
https://en.wikipedia.org/wiki/Thomas_Jaggar
http://www.nps.gov/havo/planyourvisit/jaggar_museum.htm

Several times I have noticed that big earthquakes have occurred after severe droughts. I have theorized that either the reduction of weight above a fault, or cracking allowing sudden influx of ground water might be triggering factors for earthquakes, but perhaps also for some volcanoes.