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.
Crater Walls Pyroclastic Flo w
Scorched Forest Lava Dome
Blown Down Forest Debris Avalanche
Lakes Streams Mud;ow
KM
0 5 10
N
Mount
St. Helens
Spirit
Lake
Lake
Castle
Lake
NorthForkToutle River
South Fork Toutle River
GreenRiver Washington