1. To become active and spew forth lava and rocks; said of volcanoes; SYN. belch.
2. To appear or intensify suddenly; SYN. flare up, flare, break open, burst out.
3. To emerge suddenly; SYN. come out, break through, push through.
4. To start abruptly; SYN. break out.
5. To manifest skin eruptions; SYN. break out.