Understanding the origin of shocked quartz found at the K/T boundary is critical to models of exogenic versus endogenic cause for K/T boundary constituents. While the association between impacts and shocked quartz is well established, an endogenic origin for shocked quartz at the K/T boundary requires that impulsive pressures >60 kbars have to be generated during explosive volcanism.

It is canonical that the most catastrophic explosive volcanic eruptions are events of sustained decompression, not detonations. They can have durations of several hours or days, and result in magmas—which have often undergone a protracted cooling and crystallization history—being erupted by systematic evacuation of large crustal magma chambers. Such eruptions are driven by an overpressure (>lithostatic) developed in the magma chamber. Overpressures may develop due to two processes: (1) Buoyant pressure of the magma, and (2) exsolution of volatiles during cooling and crystallization. These result in expansion of the chamber and fracture of the confining rock to form a conduit through which the vesiculating magma erupts. The limiting factor to these overpressures is the tensile strength of the rock surrounding the magma chamber, which limits maximum overpressures to approximately 200 bars; this value agrees with estimates of between 100 and 400 bars obtained from direct observations and modeling of deposit characteristics. The maximum pressures associated with explosive volcanism are therefore orders of magnitude lower than those required to shock quartz (>60 kbar). This is corroborated by the lack of field or petrographic evidence for shock metamorphism of volcanic ejecta or country rock.

We show that models for a volcanic origin for shocked quartz are seriously flawed, and in the light of the well-established association between shocked quartz and impact sites, an exogenic origin is preferred.