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Volcanoes do not remain active forever. Eventually there is some shift in the Earth's crust, the focus of the intense heat moves away slightly, and the eruptions come to an end. But if water should percolate down through the rocks and reach the magma chamber it will become super-heated and rise up like steam in a kettle. On the way, the water may dissolve minerals from the rocks through which it passes. As the water emerges from hot springs, the minerals are deposited in terraces like the ones in Roturua in New Zealand. In some places the super-heated steam has dissolved the softer rocks through which it passes and brings them up as boiling mud. Elsewhere the boiling water shoots up as huge fountains and the whole area is wreathed in steam. These are all signs of an area where volcanoes are on the wane. The famous hot springs of Yellowstone are also heated by a vast chamber of molten rock beneath the surface. The water that wells up from the pools here are so hot that no creature can live in them. But when they trickle over the brim of the pools they begin to cool, at which point rich colonies of bacteria and algae begin to grow. In places, they thrive so well that they break the surface and divert the flow of water so that there are areas that are cool enough for brine flies to settle. The flies come to feed on the algae and mate. They then lay their eggs directly in the warm mat of algae, each egg having a long white thread to its case. But the eggs are food for mites that clamber about on the algae, while the flies are food for prowling spiders. Another visitor is a larger fly that comes to eat the grubs of the brine flies. In the end, increasing amounts of grubs begin to eat away at the algae and the algae mat weakens. Eventually it gives way and lethally hot water gushes down and kills an entire generation of grubs and many of the hunters that lived on them. Now the process must start all over again.