Journal of Creation - 2018 Volume 32 Issue 1

interacted with the pre-disturbance landscape to form a mosaic pattern of multiple disturbance zones. 7, 8 A gradient of disturbance was established, extending from areas near the mountain, where intense processes eliminated all pre-eruption organisms, to distant sites, where limited disturbance allowed survival of most organisms. Five major disturbance processes and the zones of disturbance they formed are as follows (figure 1):

1. Debris avalanche: The eruption began with a 5. 1 Richter magnitude earthquake, which triggered a massive debris avalanche composed of the mountain’s summit and north flank. 9 Part of this landslide travelled through Spirit Lake, generating a giant oscillating wave extending 260 m up onto adjacent mountains. The wave brought thousands of logs and other forest materials back into Spirit Lake, forming a giant floating log mat occupying about 40% of the lake’s surface. 10 Most of the remaining avalanche material entered the North Fork Toutle River Valley where it buried 60 km2 of land with deposits having a mean depth of 45 m.

2. Directed (lateral) blast: The debris avalanche exposed magma inside the volcano, along with superheated water, resulting in a north-directed, lateral, ground-hugging blast, which devastated 600 km2 of forest within less than 10 minutes. 11 In most of this area, trees were flattened (blowdown zone), except at the periphery, where they were heat-killed but left standing (scorch zone). The landscape impacted by the directed blast is now known as the ‘blast zone’.

3. Vertical eruption: Following the lateral blast, Mount St
Helens produced a vertical eruption plume for 9 hours,
resulting in a continuous rain of tephra (ash and pumice
fragments) on the blast zone and extending hundreds of
kilometres to the east (figure 2). 12 The erupted material was
composed of a silica-rich, nutrient-poor volcanic rock
called ‘dacite’.
4. Pyroclastic flows: In the afternoon of May 18, incinerating
pyroclastic flows of pumice spilled from the crater onto debris
avalanche deposits north of Mount St Helens, forming the 15
km2 Pumice Plain. 13
5. Mudflows (lahars): Heat from the eruption rapidly melted the
mountain’s winter snowpack and glaciers. The runoff mixed
with volcanic debris, forming mudflows, which travelled
down streams draining Mount St Helens, leaving deposits. 14

The complex landscape produced by these processes forms the stage on which the drama of biological recovery is playing.

Figure 1. This map of the blast zone shows disturbance zones, including: debris avalanche deposit, forest blowdown zone, scorch zone, Pumice Plain, and mudflow deposits. The arrow represents a gradient of disturbance, which depicts a progressive decline in disturbance intensity between the Pumice Plain and the intact forest.

Figure 2. On 18 May 1980, Mount St Helens erupted a plume of ash extending upward over 20 km and lasting 9 hours. Ash reached the Atlantic coast in 3 days and circled the earth in 2 weeks.