Revegetation Following the May 1980 Eruption of Mount St. Helens

Irene Nester
ES 771 Remote Sensing
Emporia State University
8 May 2009

The andesitic volcano Mount St. Helens, part of the Cascade Range, is located in southwestern Washington state. Historically, Mount St. Helens has had a significant eruption approximately every 100 to 200 years. The last major eruption occurred on 18 May 1980. This eruption was triggered by a moderate earthquake which caused a landslide on the northern side of the volcano. This was followed by several explosive releases of steam, ash and rock fragments both vertically, and laterally to the north. The main blast removed a large portion of the northern face of the stratocone and reduced its overall height, previously about 2975 m, by approximately 400 m. The lower slopes and surrounding areas were covered by coniferous forest prior to the eruption. The force of the blast and pyroclastic flows destroyed all structures and vegetation in a fan-shaped zone extending approximately 12 km to the north. The flows partially filled the north fork of the Toutle River to the northwest and Spirit Lake to the northeast. Between approximately 12 and 19 km from the crater, standing trees were flattened and stripped of bark. For several kilometers farther out, trees were left standing but were seared and killed by heat from the blast. Lahars (mud flows) moved downslope into local drainage channels. Several smaller subsequent eruptions occurred in May, June, July, August, and October of 1980 (NASA, 2008).

Detailed descriptions of the 18 May 1980 eruption are found on the NASA Geomorphology Web Site and on the Seattle Times Web Site. A map of the area around Mount St. Helens is also available on the Seattle Times site.

The first plant species to colonize the area blanketed by volcanic ash and debris was Pacific Lupine (Lupinus lepidus), a perennial herb. A study of vegetation communities performed in 2005 identified seven distinct plant community types comprised of 26 different relatively common species (del Moral, 2005). The series of six images below show various stages of revegetation during the 25 years following the eruption. Each image covers an area approximately 50 km wide. Click on the images to enlarge them and to see the NDVI legend.

Mount St. Helens in July 2008

Acquisition Date: 21 July 2008
Platform/Sensor: Landsat 5 TM
Image: Composite of Bands 3, 4 and 5

1979

Acquisition Date: 9 October 1979
Platform/Sensor: Landsat 3 MSS
Left Image: Composite of Bands 5, 6 and 7
Right Image: NDVI Ratio of Bands 5 and 7

This image was acquired approximately seven months before the 18 May 1980 eruption. The symmetrical volcanic cone of Mount St. Helens is prominent near the lower center of the image. In the false color composite, vegetation is yellow to brown, barren areas are shades of gray to blue, and water bodies appear black. In the NDVI vegetation index image, active vegetation is dark bluish green to light green, and barren areas and water bodies are shades of pale yellow to dark orange (large NDVI image with legend). Spirit Lake is visible to the northeast of Mount St. Helens.

1980

Acquisition Date: 31 July 1980
Platform/Sensor: Landsat 2 MSS
Left Image: Composite of Bands 5, 6 and 7
Right Image: NDVI Ratio of Bands 5 and 7

This image was acquired approximately ten weeks after the 18 May 1980 eruption, and after several smaller eruptions that occurred in May, June and July 1980. The enlarged crater (approximately 2 by 3 km wide) of Mount St. Helens is visible in the lower center of the image. The zone of total destruction (approximately 20 by 30 km wide) spreads from the northeast to the northwest away from the volcano. Mud flows are visible in the drainage channels of the north and south forks of the Toutle River near the center right of the image. Spirit Lake is partially covered with debris.

1984

Acquisition Date: 17 June 1984
Platform/Sensor: Landsat 5 TM
Left Image: Composite of Bands 3, 4 and 5
Right Image: NDVI Ratio of Bands 3 and 4

In the false color composite, vegetation is light to dark green, barren areas are shades of mauve to lavender, water bodies appear black, and snow is light blue. Four years after the eruption, vegetation has advanced onto the edges of the zone covered by volcanic deposits. The most significant increase in vegetation is apparent to the northwest of the volcanic crater. Small drainage channels are visble in the mud-filled Toutle River channels.

1990

Acquisition Date: 22 September 1990
Platform/Sensor: Landsat 5 TM
Left Image: Composite of Bands 3, 4 and 5
Right Image: NDVI Ratio of Bands 3 and 4

In this scene, acquired more than ten years after the eruption, vegetation extends farther into the zone of destruction compared to the scene from 1984. The north face of the volcano and the area immediately below it still has little or no vegetation.

1999

Acquisition Date: 7 September 1999
Platform/Sensor: Landsat 7 ETM+
Left Image: Composite of Bands 3, 4 and 5
Right Image: NDVI Ratio of Bands 3 and 4

In this scene, acquired more than 19 years after the eruption, vegetation has continued to advance toward the volcanic crater. Although vegetation coverage increased in comparison with the previous scene from 1990, the north face of the volcano and the area below it still has relatively little vegetation.

2005

Acquisition Date: 21 July 2005
Platform/Sensor: Landsat 7 ETM+
Left Image: Composite of Bands 3, 4 and 5
Right Image: NDVI Ratio of Bands 3 and 4

There is relatively little difference between this image and the image acquired in 1999. This scene appears to show less active vegetation in the vicinity of Mount St. Helens, possibly due to annual or seasonal variations.

References

del Morel, R., and Lacher, I. L. 2005. Vegetation Patterns 25 Years after the Eruption of Mount St. Helens, Washington, USA. American Journal of Botany 92(12): 1948-1956.

National Aeronautics and Space Administration (NASA). 2008. Geomorphology from Space, Volcanic, Chapter 3: Plate V-8, Mount St. Helens. http://disc.sci.gsfc.nasa.gov/geomorphology/GEO_3/GEO_PLATE_V-8.shtml.

NASA. 2008. Mount St. Helens Rebirth. Visible Earth Web Site: http://visibleearth.nasa.gov/view_rec.php?id=189.

The Seattle Times. 2009. 1980 Eruption of Mount St. Helens. http://seattletimes.nwsource.com/news/local/links/sthelens/eruption.html.