The wiki article is worth a read, too:
El Hierro - Wikipedia, the free encyclopedia
especially this part:
"Between 4.15 and 4.20am on 10 October 2011 the earthquake swarm changed behavior
and produced a
harmonic tremor. Harmonic tremor is produced by
magma movements
and can indicate that an eruption has begun. It is thought that a small
submarine eruption may have begun, 7km south of
La Restinga. "