THE ROOM NON-PROBLEM

A long-standing enigma in petrology and structural geology has been the “room problem.” Making space for a magma body the size of a large pluton is indeed a problem, but this reflects the venerable “big tank” model in which a pluton represents crystallization of a magma body of the same size. We argue that there generally is no room problem because most plutons are intruded in small increments over durations long enough that space is made by familiar processes at uniformitarian rates.

A large and growing body of geochronological, field, geochemical, geophysical, and experimental results indicates that most plutons are composite and were assembled over durations on the order of 10⁶ years. This time scale accords with InSAR-determined rates of non-eruptive volcanic inflation (~10⁶-10⁷ m³/yr) and with the ~10⁷ year life spans of major volcanic centers. Geophysical surveys have failed to image big magma chambers under active volcanic centers. Although large ignimbrite eruptions indicate that big tanks do form, they reflect transient periods of high power input relative to heat loss. Field relations traditionally interpreted in terms of huge, bottomless, molar-shaped plutons are equally consistent with incremental pluton growth by crack inflation. Crack-seal pluton growth accords with volcano geodesy which records short-lived pulses of rapid surface uplift (crack inflation) and subsidence (increment cooling). It accounts for the fact that wall-rock inclusions--regardless of size and density--are generally rare, unsorted, and concentrated near pluton margins; such xenoliths were likely never engulfed in or sank through magma. Sharp, discordant pluton roofs are typically the upper limits of crack systems into which numerous magma increments intruded to form the pluton. Incremental pluton growth by crack-seal eliminates major inconsistencies between conventional interpretations of volcanic and plutonic phenomena and renders the “room problem” largely irrelevant.