Large, shallow silicic magma chambers can apparently generate their own potential to initiate caldera-producing eruptions. Differentiation of an initially water-poor siliceous magma yields a stably graded zone of extremely fractionated low-density liquids of low temperature, low crystallinity, and high water content at the chamber top. After protracted evolution of such a system, the uppermost magma in the chamber will become oversatured in water, generating an excess pressure. By equating the magma and chamber volumes at some pressure above the stable lithostatic value, this excess magmatic pressure can be evaluated as a function of the magma's compressibility, water content and solubility, lithostatic pressure (chamber depth), and some assumed measure of the chamber's ability to expand. Critical overpressure between 5 and 25 MPa are believed to be sufficient to rupture most chambers, implying that for rhyolite magma at 850¿C, the critical water contents are in the range 3--6 wt% for chambers buried at depths of 1.5--7 km. The vesicularities of the magma in the oversatured layer are calculated to be ≤1% but may reach ~10% in cases where the oversaturated layer is a small (less than a few percent) part of the chamber and when the chamber can dilate during the increase in pressure. It follows from this study that fractionated magmas evolving at shallow depth cannot attain water contents much above 6 or 7 wt%, since in so doing they become oversatured and are ultimately erupted.