Shakespeare includes the theme of weather (especially thunder) to show that something bad is happening, and to paint a gloomy and dark setting for the audience. Also, the thunder is only described right before Macbeth goes to meet the witches, which represents that the witches are doing evil and unnatural things that go against the norm. Similar to how thunder goes against the normal sunshine and happy weather.
The Physics of the Plasma Universe by Anthony Peratt describes magma as a plasma, a medium containing moving charges. So we should expect volcanoes not only to exhibit electrical behavior but to have that behavior connected with the larger plasma environment, that is, to be elements in a larger electrical circuit.