A magma burst

Magma flows into the magma chamber from below.  The ground
rises for months as magma accumulates and the chamber expands.
Earthquakes occur in its roof.

The ground rifts and deflates rapidly.  Magma flows underground
along the fissures to the north or south.  Its movements can be
traced with seismometers.  Deflation may take only 2 days.
(The line drawings are by Decker & Decker and are based on
similar events taking place in Hawaii).


References:

Páll Einarsson & Bryndís Brandsdóttir 1980. Seismological evidence for lateral magma intrusion during the July 1978 deflation of the Krafla volcano in NE-Iceland.  J. Geophys. 47: 160-165.