Читать книгу The Pacific Crest Trail. Hiking the PCT from Mexico to Canada онлайн

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Pumice is formed when frothy, escaping lava solidifies, trapping lots of bubbles. Pumice is very light and will float on water until it becomes waterlogged. In violent volcanic eruptions, escaping steam often tears magma and the solid rock surrounding the vent into small particles, resulting in volcanic ash.

In places along the PCT you will see polygonal basalt lava columns, the best-known being the Devil’s Postpile at Reds Meadow. The Postpile was created about 100,000 years ago when a lava flow was impounded by a moraine and reached a thickness of 400ft. Because of its great thickness, much of the pooled lava mass cooled slowly and evenly, producing long, symmetrical columns. The mainly hexagonal joints developed when the lava contracted during the cooling process.

More recently, ice ages have eroded the volcanoes of the Cascade Mountains. Glaciers remain on many volcanoes, particularly Mount Rainier and Glacier Peak. You can gain an idea of the comparative age of volcanoes from their appearance. Younger ones, including many of the small volcanoes that you see, tend to have the classical conic shape, whereas older volcanoes have been eroded during the ice ages, leaving only the towering crags of their more resistant cores.