Complex sentence: They can be formedat water depths of 500 m (Heiken and Wohletz 1985), where hydrostatic pressure is high enough to inhibit vesiculation in basaltic magma.
Split into simpler sentences: They can be formedat water depths of 500 m Heiken, G. & Wohletz, K.1985. University of California Press, Berkeley, where hydrostatic pressure is high enough to inhibit vesiculation in basaltic magma.