Geysers!

I went to Yellowstone National Park for the first time this past summer (July '08) and I was absolutely amazed. The natural beauty of the park is stunning, and I was also impressed on a deeper level with the geology and geothermal features I saw.

Of course I made it a point to see Old Faithful and waited with hundreds of other tourists for the eruption to occur. The supervolcano that underlies the Yellowstone area is the cause of the amazing heights the geyser reaches, as precipitation seeps through the porous ground rock and comes into contact with rocks heated by the magma, which then rises through a series of cracks and fissures within the Earth. Pressure builds beneath the Earth due to the high temperatures, and pushes the water in the upper layers of the Earth's surface out and causes an overflow. This overflow relieves the pressure below and causes it to vaporize into steam, and the force of this vaporization causes the water to shoot into the air.

Yellowstone has the world's densest concentration of geysers, with over 200 within 1.5 square miles. One of my favorite areas was the Black Sand Basin, as I've always had an affinity for obsidian, and weathered volcanic glass makes up the black sand of its eponym.