**1. Introduction**

... *we speculate that hot springs were relatively active during wet periods in contrast to the present situation, and that the sinter and travertine along the fault zone at the east edge of the horst on which the rhyolite lies were deposited during one or more pluvial periods associated with glacial stages. The present climate sustains no thermal springs other than an ephemeral flow at Coso Hot Springs following local precipitation.*

In the quotation above Du ffield et al. [1] noted that climate and long-term precipitation trends may a ffect the activity of thermal springs, in this case at the Plio-Pleistocene Coso volcanic field in eastern California. Thermal springs are a major conduit by which heat is removed from geothermal systems [2], and so increased thermal spring activity means more rapid cooling of the underlying magmatic system. The purpose of this paper is to show that such interactions between magma and climate may play a role in whether magma intruded into the crust accumulates fast enough to form a large magma body rather than an incrementally emplaced pluton, and thus whether climate can a ffect the development of caldera-forming eruptions.

A parcel of magma in the crust moving upward through a fracture has several possible fates. It may (1) reach the surface, contributing to a volcanic eruption; (2) freeze en route, showing up in the geologic record as a dike; (3) freeze where earlier parcels did, forming an incrementally emplaced pluton; or (4) reach a site of persistent magma accumulation, contributing to a magma body. The geologic record contains evidence for all of these scenarios.

It was long thought fates 3 and 4 were essentially the same in that plutons, with volumes on the order of 103–104 km3, are just the frozen remains of large magma chambers, but a growing body of evidence shows that many plutons were assembled incrementally over timescales of 105–106 years and never existed as large bodies of magma [3,4]. However, ignimbrite eruptions with volumes on the order of 1000 km<sup>3</sup> are proof that large magma bodies can exist in the crust, if only ephemerally.

In this paper I examine the conditions that di fferentiate fates 3 and 4; specifically, factors that tip the balance in favor of incremental pluton assembly versus development of a large magma chamber.

The critical balance is whether magma supply to the bottom of the system is su fficiently rapid to outpace loss of heat out the top. The former is presumably governed by magma supply rate or tectonic control on magma supply from the mantle or lower crust, and the latter by heat-transfer processes in the crust above the site of magma accumulation. I use simple physical arguments to show that long-term climate—specifically, precipitation—can play a role in the ultimate fate of upper-crustal magma accumulations.
