The Big Lava Bed is a volcanic feature, but not a classic volcanic cone. Rather, it is a 20-square-mile basalt flow that oozed out of fissures and cracks some 9000 years ago from a source vent in the north-central part of the bed, just south of Goose Lake on the lower flanks of Mount Adams in the southern Indian Heaven area. The Pacific Crest Trail skirts the west side of the basalt field.
Spindly pines cover much of the Big Lava Bed, giving it the appearance of a gentle, young forest from a distance. Get near the lava, though, and the truth comes out: There is nothing gentle about this landscape. The terrain in the Big Lava Bed is a jumble of rock. Huge blocks of lava, towering up like black cotton-candy tufts, dot the landscape. Many of these are pahoehoe formations – wrinkled masses of rock formed by fast-moving lava. Pahoehoe is easy to identify: just look for the rocks that are a solid mass of ropey folds and wrinkles. Between the great tufts of lava are fissures, crevices, pressure ridges, and fields of sharp, jagged black basalt.
Step off the trail, and the landscape is nearly impassable. The lava is abrasive, brittle, and very unstable. Try to scramble to the top of one of the short lava knobs scattered around the area, and you’ll find your leather boots not just scuffed but slashed and cut to ribbons. Better to enjoy the rugged beauty of the lava flow from the relative safety of the trail. – Source: Washington Trails Association