Considering a geyser system as described above and applying what is known about the behavior of water and steam, we can understand what causes a natural thermal eruption. Figure 50 shows diagrammatically the succession of events believed to occur during the typical eruptive cycle of a geyser such as Old Faithful. [Illustration: A GEYSER IN ACTION. Photographs of successive stages in the eruption of Old Faithful illustrate what probably happens during a natural geyser eruption. The underground plumbing is diagrammatic and does not reflect any specific knowledge of Old Faithful’s system. Direction of flow of water is shown by arrows. (Based on information supplied by D. E. White, L. J. P. Muffler, R. O. Fournier, and A. H. Truesdell.) (Fig. 50) Stage 1 (Recovery or recharge stage). After an eruption, the partly emptied geyser tubes and chambers fill again with water. Hot water enters through a feeder conduit from below, and cooler water percolates in from side channels nearer the surface. Steam bubbles (with some other gases such as carbon dioxide and hydrogen sulfide) start to form in upflowing currents, as a decrease in pressure causes a corresponding decrease in boiling temperature. At first the bubbles condense in the cooler, near-surface water that is not yet at boiling temperature, but eventually all water is heated enough that the bubbles will no longer condense or “dissolve.”] [Illustration: Stage 2 (Preliminary eruption stage). As the rising gas bubbles grow in size and number, they tend to clog certain parts of the geyser tube, perhaps at some narrow or constricted point such as at A. When this happens, the expanding steam abruptly forces its way upward through the system and causes some of the water to discharge from the surface vent in preliminary spurts. The deeper part of the system, however, is not yet quite hot enough for “triggering.”] [Illustration: Stage 3 (Full eruption stage). Finally, a preliminary spurt “unloads” enough water (with resulting reduction in pressure) to start a chain reaction deeper in the system. Larger amounts of water in the side chambers and pore spaces begin to flash into steam, and the geyser rapidly surges into full eruption.] [Illustration: Stage 4 (Steam stage). When most of the extra energy is spent, and the geyser tubes and chambers are nearly empty, the eruption ceases. Some water remains in local pockets and pore spaces, continuing to make steam for a short while. Thereafter the system begins to fill again, and the eruptive cycle starts anew.]