Craters of the Moon National Monument, Idaho -

Dubbed the "strangest 75 square miles of North America" by an early visitor, the Craters of the Moon National Monument offers one of the west's most unusual displays of volcanic phenomena. The parks covers a large valley pockmarked with volcanic cones, lava flows, and other bizarre volcanic features. Rather than one large volcano there are many small craters and fissures.

The valley sits atop a large fissure in the earth's crust known as the Great Rift Zone. The craters run in a line above the fissure, roughly southeast to northwest. The lava flows run from most of the craters. Shown left is a lava flow through a field of small cones. The park features two type of basalt lava flows: "Aa" and "Pahoehoe" (Hawaiian terms). Pahoehoe lava emerges in a very liquid state and hardens very smoothly as in the photo left. It contains dissolved gas and is associated with spectacular "lava fountain" displays during contemporary eruptions in such places as Hawaii or Italy. Aa lava emerges very thick and viscous during eruption resulting in short, jagged flows, drying into bizarre jagged shapes. Aa is razor sharp, easily cutting hands and even boots.

There are several fascinating caves which may be explored within the park. The caves are actually "lava tubes". The tubes are formed during large flows of Pahoehoe lava. This liquid form of lava cools very quickly when exposed to the air and develops a crust. The interior lava is insulated and flows within the crust creating tunnels. Since the lava is also gaseous, explosions collapsed the tunnels in places, creating openings through which today's visitors may enter.

The Great Rift last became active about 15,000 years ago. Most activity ceased here about 2,000 years ago.

The park has a network of fairly easy trails accessing the craters, lava flows, and the caves. Most of the park is also a designated wilderness area.