Soil Erosion and the Formation of Canyons at Mount St. Helens after the 1980 Eruption - Report Example

Soil Erosion and the Formation of Canyons at Mount St. Helens after the 1980 Eruption

Introduction

Mount St. Helens has been a source of much attention for both professional and amateur scientists over the years. Of greater interest is its eruption on May 11, 1980, which came after 123 years of dormancy. The area around Mount St. Helens has since become a natural laboratory where visiting scientists study geologic deposits, continuing volcanic processes, processes to recover the landscape, among others (Major et al., 2009). Of interest here is the long-term impact that the 1980 eruption had on the mountain itself as well as the landscape. The eruption saw a huge landslide; many pyroclastics flow, an explosive lateral blast, mudflows and devastating flows of volcanic debris (Major et al., 2009; Austin, 2009, 2014). These elements contributed toward major erosion during the eruption, and which have continued since.

Rapid Erosion during the Eruption and Since

• Brief Background

Mount St. Helens eruption on May 13, 1980, ranks among the United States’ (US) most significant natural disasters. It achieved significant feats concerning the various aspects associated with volcanic eruptions: landslide, lateral blasts, pyroclastic flows, debris flows and mudflows, and ash clouds, among others. For example, it was characterized by such a tremendous lateral blast that had never been witnessed or recorded before, and its debris avalanche constituted what is historically the largest recorded landslide (Major et al., 2009).

Various elements may have contributed to the erosion in the area at the time of the eruption and in the years since. It is important to note that St. Helens continued to experience other smaller eruptions after the May 13 (major) eruption, including several distinct eruptive episodes between the late 1980 and 1986 (Major et al., 2009). Erosion results from turbulence and abrasion, and the curvature of the path, which creates velocity (Waite&Moran, 2009; Domokos et al., 2014). All these factors, among others, were and have been at play at and around Mount St. Helens.

• The Erosion and the Formation of Canyons

Before Mount St. Helens became a natural laboratory for scientists, there had been a consensus among scientists and geologists that erosion of rivers and creeks, and the formation of deep canyons took great time periods. However, rapid erosion at Mount St. Helens after the eruption has since helped debunk that notion. The area has since experienced erosion with new features and concentrated mainly in the blast zone. Every single place of the blast zone has been reached and affected by some erosion. Scientists have since discovered many processes of erosion in the area as well as different features that have formed in that time (Major, 2009; Austin, 2009, 2014).

A significant amount of erosion at Mount St. Helens occurred during the eruption. This resulted from scouring caused by the steam blast, water waves, landslides and mudflows, and the flows of hot pumice ash (or pyroclastic flows) (Major et al., 2009; Austin, 2009, 2014 ). Direct blast (an equivalent of 20-megaton TNT) was characterized by hot gas and rock fragments that abraded the mountains’ slopes. Pyroclastic flows are erosive blasts of volcanic ash and steam. The eruption also generated big water waves in Spirit Lake, which went on to cause severe erosion on the slopes around the lake. Also related to this were overland floodwater flows, which resulted in gully and rill patterns.

Erosion also resulted from abrasion caused by the debris avalanche (that is, the movement of great ice, rock and debris masses) that resulted from the eruption (Austin, 2014). To create a picture of the devastation caused, the eruption caused the longest debris avalanche ever observed and recorded (Major et al., 2009; Austin, 2014). “Two-thirds cubic mile of landslide and eruption debris from May 18, 1980, occupies 23 square miles of the North Fork of the Toutle River north and West of the crater” (Austin, 2009, p.13). The resultant debris deposit covered the entire width of the valley that ran alongside Toutle River’s North Fork, about 16 miles, and it averaged 150 feet in thickness.

Jetting steam from buried glacier ice formed explosion pits, which also caused further erosion. Mass wasting caused significant changes to slopes that were already unstable, particularly around areas that were already sculpted by other erosion agents. This left behind a varied landscape that was more vulnerable to erosion. However, the biggest erosion agent during the eruption is believed to have been the mudflows. Essentially, these are vicious mud streams gouged out of deposits of soft volcanic ash, which over time went on to form new canyons (Austin, 2009; 2014).

Many features and events after the major eruption continued to play a significant role in erosion. Some of these caused other mudslides. For example, for about two years after the major eruption, the debris avalanche’s upper drainage area remained unconnected to the Pacific Ocean. At the same time, as the debris blocked the valley, Spirit Lake basin’s water, as well as the volcano's crater, remained unconnected to the Toutle River. Then another explosive eruption of St. Helens on March 19, 1982 caused a thick snowpack in the crater to melt. This went on to create a major sheet-like flood, and this evolved into a major mudflow. This filled the big pit of steam explosion with mud, and the mud overflowed the pit’s west rim cutting a deep ravine into the deposits left by the 1980 explosion (Austin, 2009, 2014). The flow also “formed channels over much of the hummocky rockslide debris, allowing cataracts to erode headward” (Austin, 2014, p. 23).

Erosion has continued to occur in the areas since. This is especially evident in the number of deep canyons that have emerged over that time. Bedrock, for example, has since been eroded to at least 600 feet deep, and from it, on the volcano’s north flank, Loowit Canyon and Step Canyon have formed. Several other individual canyons have also emerged in the area. There are some on the debris avalanche deposit, which is as deep as 140 feet. In the wake of the erosion, there are now elevated plateaus. These are on both the south and north of a great breach. The result resembles Colorado River’s rims to the north and south. There are also “gully-headed and amphitheater side canyons in the breach which resemble the side canyons of the Grand Canyon” (Austin, 2014, p. 17). Over a long period, it seems that the small creeks that make their way through the Toutle River headwaters have continued to carve the canyons. However, the evident erosion has occurred in unprecedentedly short time.

• Other Possible Factors: The Environment

Other than the elements directly associated with the volcano, some of the erosion agents had to do with the environment – that is, topographical elements. For one, being a mountain, the terrain was steep (Waite & Moran, 2009). The area also generally lacked vegetation. Moreover, the area until then only enjoyed an average annual precipitation of 130 inches, which is not much. All these combinations of factors could have contributed to the erosion (Majoretal, 2009).

Conclusion

No doubt, the major eruption of May 13, 1980, and the smaller eruptions since, set in motion significant events that slow and ultimately had a significant impact on the geology of the area. The eruption itself shook the earth making it loose and thereby enabling erosion to happen in the process. Other materials were also added to the landscape. The ash, for example, and all the material that constituted the debris avalanche ultimately affected the content of the rocks in the area. The flow of the rivers in the area must also have had an impact on the quality of rocks in the area. The result was, among others, a rapid stratification formation. The hitherto recorded erosion, therefore, has not only been the result of the eruption but also of the landscape's response to the disruptive impact since. The erosion is not over yet and only time will tell just how disruptive that 1980 eruption was to not only Mount St. Helens but also the rest of the surrounding landscape.