By ROBERT DRYJA
Los Alamos 

Trees were covered with golden leaves in October. These have fallen and blown away with the arrival of November. A new view of the landscape has become visible. A variety of fields among the mountain sides of a volcano now may be easily seen in a forest of tree trunks. Some fields may be level without rocks or boulders on their surface. Others may have an occasional boulder sitting on them. Still other may boulders lying about regularly.

What contributes to theses differences from field to field? Simply look at the landscape to see two factors. One factor relates to the creation of the boulders themselves. A second factor relates to how level or angled a landscape may be. Both of these factors are a result of volcanic activity.

The eruption in 1980 of Mount St. Helens in Washington State provides a starting point in relation to the first factor. Many pictures of it are available. It is estimated to have ejected initially 0.46 cubic miles of lava and ash as part of a lateral eruption from its crater. Another 0.26 cubic miles erupted vertically. Together, an estimated ¾ cubic mile of lava and ash were ejected. The crater rim fell in elevation by 1,300 feet. The resulting crater was somewhat oval in shape and was about 1.8 miles across at its widest. More volcanic activity has occurred through 2008, or for 28 years. Earthquake activity continues to occur, or 46 years after the initial major eruptions.

The Valles Caldera provides another example. It erupted 1.25 million years ago. An estimated 75 cubic miles of lava and ash was ejected. This is 100 times more than what was ejected at Mount St. Helens. The resulting oval crater was up to 14 miles across. Fifteen more eruptions have occurred since this major eruption. The last one occurred 40,000 years ago. Volcanic ash from the Valles Caldera has been found throughout the western half of the United States. The Mount St. Helens eruption was like a fire cracker in comparison.

The resulting lava and ash flows from Mount St. Helens and the Valles Caldera eruptions followed a similar pattern even if they occurred on vastly different scales. Repeated eruptions created layers of lava and ash lying on top of one another as they flowed from a crater. A layer may have been many feet deep or only few inches. It was composed of lava that varied from liquid to powdery. The lava at the bottom of layer also could have been highly compressed by the weight of rock above it. A variety of rocks resulted when the lava cooled.

The rock that was created from compressed liquid compressed lava was very dense and solid. This kind of rock would become the boulders seen today. A thick layer of lava could break apart into boulders. Rain water in contrast could penetrate into powdery lava, called tuff. Water expanded when it froze in the winter, pushing rock particles apart. This contributed to the smaller rocks, sand, and gravel-like material now making up the tree covered slopes and fields seen today.

Denser layers of lava have broken apart into rocks. These rocks have washed down a temporary stream that was created during a spring snow melt or summer rain flash flood. Photo by Robert Dryja                                                               Lava in this area was much more porous. It has become much more powdery with small stones in this temporary stream bed. Photo by Robert Dryja                                                                                                                                    A single boulder has come to rest on a level field toward the bottom of a canyon. Photo by Robert Dryja