WHAT’S IN A PULSE?

The single-serving growth model of magma chambers is quickly becoming outdated. The more systems that are studied under detailed scrutiny, the more obvious it becomes that magma chamber growth is complex and that evolution of the chamber occurs over time. This is true over the gamut of igneous system volumes; from huge batholiths (e.g. the Tuolumne Intrusive Suite) all the way down to smallish laccoliths (e.g. Trachyte Mesa). Pulsed growth may be the rule. However, in the literature, the word pulse is used without definition. A pulse is a pulse is a pulse. But what is a pulse? In a series of scaled analog experiments that produce laccoliths of molten paraffin in a layered gelatin host, I have found that pulses are likely not so simple. In the experiments, I inject molten paraffin into competent gelatin. The magma analog forms its own dike network, and upon reaching an interface between gelatin layers, the dike is captured and diverted into a concordant intrusion. Despite being fed from a pressure vessel at constant overpressure, as a single injection (or pulse), the growth of the resulting laccolith is pulse-like in style. The lateral progression of the laccolith is herky-jerky, and the intrusion undergoes periods of inflation and deflation. This is clear pulse-like behavior that is occurring within a single pulse. This observed fractal nature of pulsing suggests that we need more specificity with our language. Furthermore, these observations raise several interesting questions. If an intrusion grows by a single pulse, but that single pulse is pulse-like, what intrusions, if any, grow without multiple pulses? How would these smaller scale flow-pulses be identified in the rock record and differentiated from other types of pulses? And if pulses occur on multiple size and time scales, how deep does the rabbit hole go?