In June we set off again on our second visit to the great continent of North America, led by that dedicated trio of Bristol University geologists who had bravely planned another marvellous tour for us.

Our trip this time was to be very different from the red rock country of the sunbaked South West which we visited two years ago: this was to be a trip of spectacular mountain scenery, pine forests and lava plateaux.

Our 19 day tour took us on a 3500 mile long journey, amid some of the most stunning scenery imaginable, from the Pacific coast of Vancouver Island to the rain forests of Washington State, the strange volcanic landscape of Craters of the Moon to the snow capped peaks of Canada’s Rocky Mountains. But it was Yellowstone National Park in Wyoming which really held us spell- bound, a 3450 square mile wilderness of pine forests, mountain chains, turbulent rivers, steaming geysers and boiling mudpots. Nowhere else in the world is there so rich a concentration of hydrothermal features, which spring to life in many sections of the Park, with dramatic effect upon the landscape.

Yellowstone Lake in the heart of the Park occupies a spot where some 600,000 years ago a giant volcanic explosion occurred, blasting out an estimated 100 cubic miles of Rhyolite lava and ash, and leaving a caldera 40 miles long by 30 miles wide; the largest known caldera in the world.

Although volcanic activity ceased in Yellowstone some 70,000 years ago, there is still a giant chamber of magma within a few miles of the surface, and this is the heat source for the Park’s rich collec- tion of hydrothermal features. Ground water, which is able to percolate deep down through ring fractures surrounding the caldera, becomes superheated, and convec- tion causes it to rise, emerging on the surface at temperatures a degree or two below boiling point. If there are cavities or restric- tions in the outflow channel, then a geyser will result, with an intermittent flow since the cavi- ties must fill again after each eruption. If there is a free passage to the surface, a hot