Boulders created by a large dense flow of lava are at the top of a canyon slope and will eventually tumble or slide to the bottom of the canyon. Photo by Robert Dryja

By ROBERT DRYJA
Los Alamos

We previously reviewed how the composition of lava by itself contributes to the resulting rocks seen in fields. Lava that is predominately liquid to start can become dense boulders after it has cooled. Lava that is powdery or porous when created can become sandy with small stones.

A second influence is concerned with the slopes coming down from the rim of a volcanic crater. The angles of slopes are a major influence. Are they steep or gentle? The angle of a slope relates to the volcanic crater itself. Steeper slopes are created when close to a crater rim. Land around a crater is pushed upward as part of the eruption throwing lava out. Lava then spreads outward from a crater rim for miles and became more level. For example, lava flowed sixteen miles from the Valles Caldera crater rim to the Rio Grande River.

Erosion from rain and melting snow has cut canyons into the otherwise flat sloping planes that spread out from the Valles Caldera crater. The canyons seen today may have steep or gently sloping sides depending on how erosion progressed over thousands of years. Gently sloping canyon sides have been cut into the fields made of softer, sandier lava soils. More steeply sloping canyon walls have occurred where denser layers of lava eroded. Moving water can more easily break up and wash away soft rock.

The sequence of denser or softer layers of lava laying on top of one another influences the kind of slope made in a field or canyon. The steep upper rim of a crater or canyon is created when layers of denser lava erode. Dense lava also breaks into large boulders. In contrast softer layers of lava erode to create the gentler slopes leading down from a rim. Boulders created at rims eventually may slide or roll down to the more level fields or the bottom of the canyon (See pictures 1, 2 and 3).

A large boulder has split in half while coming down the slope of a canyon side. Photo by Robert Dryja

A set of large boulders in a row were created from a single thick lava layer. They eventually settled on the same level field, creating the row seen in the background. Photo by Robert Dryja