"A still—Volcano—Life" describes precisely the type of life that Emily Dickinson herself led: one of enormous power contained beneath a quiet surface.
Dickinson lived in 19th-century New England, in a society that expected women to be passive and obedient. Hardly anyone knew that Dickinson spent her nights writing some of the most important poetry in the English language. Like a volcano, then, Dickinson's life was one of surface calm masking an interior world of energy, creative power, and destructive potential. The same could be said of many women living under patriarchal norms at the time.
The "Volcano" the poem describes works as a metaphor for something potentially powerful but apparently calm. The speaker uses the word "volcano" as an adjective here, describing a kind of "Life"; indeed, an earlier draft used "Volcanic" rather than "Volcano." Take a look at Dickinson's caesurae here:
A still— || Volcano— || Life—
Those intense pauses emphasize the word "Volcano," making it seem all the more strange and powerful.
This "still—Volcano—Life" flickers during the night, but it's hardly noticed—an image that also suggests this poem is a self-portrait. Dickinson usually wrote at night, and the volcano's "flicker[ing]" calls to mind the light of a candle.
Lines 3 and 4 present some interpretive difficulties:
When it was dark enough to do
Without erasing sight—
These words might refer to a time of night when the conditions are just right for composing poetry. Or darkness might suggest sleep and "sight" the insight of the imagination, the volcanic, creative, below-the-surface work taking place during the dark hours.
The words "erasing sight" could also apply to the "Volcano—Life" itself—suggesting this life can be so bright that it becomes invisible or impossible to look at. Perhaps this hints at a potentially destructive power in the volcano life's hidden creativity.