By ROBERT DRYJA
Los Alamos

We have reviewed how the composition of lava rock influences the size of boulders. We also have considered how the slope of the land influences where boulders may eventually settle.

Three interesting kinds of rock formations may occur:

• • • • • (1) vertical columns next to one another;
        • (2) a single column; or
        • (3) a field of rock.

The rim of a canyon may be made of thick layers of dense lava. A thick layer would cool sooner from its outside since it take time for heat within to move outward to the surface.

Cooling lava contracts and so cracks penetrate from the surface downward into the lava. The upper rim of a canyon wall therefore may be made of vertical columns. Softer layers of lava may occur beneath denser layers.

These lower layers would have been older than the layers lying on top of them. Slopes into the canyon are created from the softer rock eroding and pieces of vertical columns breaking away, (see Picture 1).

Picture 1:  Vertical columns were created by dense lava rock splitting apart as it cooled.  It then became exposed on the canyon side. Photo by Robert Dryja

A single dense column of rock may be created as part of an eruption. Other eruptions then may surround this column with softer rock. The softer rock erodes away more quickly and the column of dense rock remains. Picture 2 shows such a column that is 30 feet tall and 20 feet wide. An even taller column may emerge as the softer soil and rock around it continues to erode away.

Picture 2:  A vertical column of dense lava was surrounded by softer lava that has eroded away more quickly. Photo by Robert Dryja

Another variation is based on the type of rock created toward a volcano’s rim and steepness of its slope.  Dense lava has resulted in a large number of boulders and large rocks being created at the rim of volcano. These then have slid and tumbled down a steep slope over a long period of time. Individual rocks have enough space between them such that rain or melting snow has washed potential soil down below and between them. Plants have not grown and a long barren slope has resulted down the steep mountainside. The barren slope only ends when it has reached a relatively level area. Soil now has accumulated around the rocks and plants can grow, (see Picture 3). This kind of slope is called a felsenmeer.

Picture 3: Dense lava rocks gather down a steep slope. The slope does not hold water or soil for plant growth. Growth occurs only when the slope become level toward the bottom. Photo by Robert Dryja

The rocks and slopes that create canyon walls can be influenced by a factor that has nothing to do with geology. This factor is sunlight. A canyon that goes east/west has sides that face south or north. A division of direct sunlight occurs between the two sides. A canyon that goes north/south in contrast can have direct sunlight expose both of its sides equally.

The erosion of a wall can be affected by how much sunlight reaches it. A south facing canyon wall receives more sunlight directly. The sunlight can warm and dry the wall so that little moisture remains. More water can flow in a shorter period of time from melting snow. A steeper slope can be eroded toward the bottom of the canyon wall, (see Picture 4).

Picture 4: A south facing vertical canyon wall has less plant growth. The slope toward its bottom is steeper due to erosion. Courtesy/LANL

A north facing wall in contrast is shaded from direct sunlight and can retain moisture. More trees and shrubs can grow as a result.They can reduce the amount of erosion by absorbing or blocking the free flow of water down a canyon slope. A broader, gentler slope may be created. This gentler slope may be directly across from the steeper slope of south facing canyon wall, (see Picture 5).

Picture 5: A north facing side of the canyon is forested and has a gentle slope. Plant growth is reducing erosion on this side. The south facing side in contrast has a vertical wall with a steep slope mostly of rock eroded toward its bottom. Photo by Kelly Michals