Geoscience Reference
In-Depth Information
Boundary
approximate
Phases
Runout
P
roxi
mal
Medial
Distal
Crater
2500
2000
Rapid change in
flow behaviour
1500
Location of
whaleback bars
1000
0.5 -1.0 mm
Incised
canyons
on cone
0.5 -1.0 mm
1.3 mm
500
1.3 mm
2-5 mm
Broad alluvial channel,
previously glaciated,
with bedrock valley side
5 -20 mm
4 -10 mm
Toutle River
0
Confined bedrock channel
Expanding alluvial
channel with bedrock
valley sides
Bedrock
gorge
Alluvial
channel
0
10
20
30
40
50
60
Channel distance from crater, in kilometres
Fig. 2.13
Schematic diagram showing the changing dominance of different sediment transfer processes with distance from the crater
following the 1980 eruption of Mount St Helens. Variation in dominant grain-size (mm) of the deposits with distance away from the
crater is also shown. (Source: Scott 1988; Courtesy of U.S. Geological Survey.)
10
1
Fig. 2.14
Relationship between
travel distance (
L
) and vertical drop
(
H
) of large landslides. The value
0.62 is the friction coefficient
applicable to sliding of a rigid block.
(Reprinted from
Journal of
Volcanology and Geothermal
Research
, Siebert, L. (1984) Large
volcanic debris avalanches:
characteristics of source areas,
deposits and associated eruptions,
22
(304), 163-97.)
Non-volcanic landslides
V
olcanic debris avalanches
Pyroclastic flows
Lahars
0.1
1
10
100
500
Travel distance (km)
Along active continental margins earthquake
and volcanic activity can result in greatly enhanced
sediment loads. At Mount St Helens in the four
months following the eruption the Cowlitz River
(a tributary of the Columbia River, see Case Study
Fig. 2.2) had a sediment load of 140
estimated normal annual load of the Columbia
of 10
10
6
t. In the years following the eruption
the Columbia had an annual sediment discharge
of 35
×
10
6
t (Meade & Parker 1985). Dramatic
post-eruption changes in channel morphology and
reworking of volcanic debris resulted in sediment
yields that are two orders of magnitude greater
×
10
6
t,
an amount that needs to be compared with the
×
