Download Case Study: Extrusive Landforms and their impact on the

Case Study: Extrusive Landforms and their impact on the landscape
Igneous Extrusion in the Cascade Range
The Cascade Range
The Cascade Mountain range lies along the
western coast of the United States, extending
from Northern California, through Oregon and
Washington in to British Columbia, Canada. This
area is seismically active and also has a long
history of volcanic activity. The Cascade Range
is a continental mountain arc formed due to the
subduction of an offshore oceanic plate beneath
the continental crust. Here the Juan de Fuca plate
is being subducted under the North American
Plate. As the Juan de Fuca plate descends it
begins to melt due to heat and pressure produced
by friction at the subduction zone. This melting
of the plate produces molten rock (magma) which
is less dense than the surrounding material and
therefore rises to the surface forming volcanic
mountains. The Cascade range has built up over a
period of volcanic activity that began almost
40,000 years ago. Figure 1 (opposite) shows the
major volcanoes in the Cascade Range. The
highest point in the Cascades is Mount Rainier
(4392m) in Washington. The volcanoes within
the Cascade range include a range of extrusive
landforms including: lava plateau; cinder cones;
shield volcanoes; calderas; lava domes and
composite (stratovolcanoes).
Lava Plateau
Lava plateaus result from fissure
eruptions in which basaltic lava
flows out of a crack in the earth’s
surface. Basalt lavas are very
‘runny’ and take a long time to
cool. Flood basalts rapidly flow
away from the fissure and build up
a plain, successive flows result in
the growth of a lava plateau. This
form of volcanism in areas within
plates is attributed to mantle plumes
or hot spots. This type of volcanism
was
responsible
for
the
development of the Hawaiian island
chain. The Columbia River lava
plateau dates back 12-17 million
years ago. It covers 130,000km2 of
the states of Washington and
Oregon in the NW USA and is
composed of hundreds of separate
basalt flows, which in places reach
a total thickness of 2000m.
Shield Volcanoes
Like lava plateaus, shield
volcanoes are built almost
entirely of fluid lava flows,
however rather than fissure
eruptions, the lava erupts
from a central vent, flowing
in all directions. The cone is
built up slowly by successive
lava flows that travel wide
distances around the volcano
and cool, forming a volcanic
‘cone’ with gently sloping
sides. There are various
shield volcanoes along the
Cascade Range in Oregon,
including Belknap, ThreeFingered Jack, Mount Washington and Mount Bachelor. Belknap Shield Volcano (above
photo) and its surrounding lava flows cover 98 square kilometres. The initial eruptive phase
began over 2900 years ago, and Belknap is one of the Cascades youngest shield volcanoes
with lava flows as young as 1,400 years.
Cinder Cones
Cinder cones form
from the build up of
pyroclastic materials
ejected from a single
vent. The profile of
the cone will depend
on the maximum
angle at which the
pyroclastic
debris
remains
stable.
Cinder cones rarely
rise in height much
above 1,000ft. The
steepest
slopes
around the summit
are formed from the
largest
materials,
which fall first, and the base of the cone has gentle slopes formed by the finer material which
is carried further away from the vent. Cinder cones often form on the flanks of larger
volcanoes. Lava Butte Cinder Cone (see photograph) in Oregon rises 500ft above the ground,
and has view over the Cascade Range. The cone formed 7000 years ago covering nine miles
with lava.
Calderas
Calderas are large, steep
walled
basin
shaped
depressions that range in
size from a few km to
greater than 50km in
diameter.
Calderas form when violent volcanic eruptions empty the
magma chamber. Due to lack of support the roof of the
chamber collapses and the summit of the volcano collapses
inwards creating the depression known as a caldera. The
caldera may fill with water forming a lake. Crater Lake
Caldera (see photograph) is one such example in the Cascade
range. This caldera formed around 6,600 years ago and is the
remnants of a high volcano called Mount Mazama, which
literally ‘lost its top’ as the lava beneath the mountain drained
out in violent explosions and the top collapsed inwards. A
small cinder cone (Wizard Islands) rising out of the lake that
now occupies the caldera, was formed by small eruptions.
Caldera’s are often characterised by geothermal activity, such
as geysers and hot springs, and earthquakes are sometimes
common. Yellowstone National Park, Wyoming is actually
situated within a caldera, accounting for its many geothermal
features. Seismic unrest in the Park has recently been detected by scientists, however whether
or not it is signs of a future eruption or simply isolated events in a long period of calm, is
unknown.
Composite (Stratovolcanoes) Volcanoes and Lava Domes: The Case of Mount St Helens.
Composite or Stratovolcanoes are
extrusive landforms often related to
volcanic activity at subduction zones
such as that where the Nazca plate is
being subducted under the North
American plate. These landforms are
steep-sided usually symmetrical
cones. The cone is built up of
alternating layers of lava flows and
pyroclastic debris such as ash,
cinders, blocks and bombs. These
volcanoes are characterised by a conduit system (see diagram) with a crater at the summit
containing a central vent bringing magma up from a magma chamber below. Smaller volcanic
forms often form on the flanks of composite volcanoes where lava may flow from fissures
forming secondary or parasitic cones. The conduit system helps to provide an internal
structure to support the cone, strengthening of the cone in this way may for example occur
where magma solidifies in dykes (vertical intrusions cutting across bedding planes).
Mount St Helens, Washington (part of the
Cascade Range) is one example of an
imposing stratovolcano. The volcano is
40,000 years old and 2,549m high and still
remains potentially active volcano even
though it is now fairly quiet. A major
eruption of the volcano in 1980 marked the
end of a period of dormancy since 1857.
During the five-month period following the
1980 eruption, there were five small
explosive eruptions which along with 16
further small eruptions in 1986 created a
lava dome in the crater formed during the
1980 eruption (the photograph shows the dome in 1981). Lava Domes are rounded steep sided lava
masses formed by felsic lavas that are viscous and therefore do not spread laterally,
accumulating around the vent instead. Domes often plug vents and trap gases leading to the
build up of pressure and a subsequent explosive eruption shattering the dome. This actually
occurred in the 1980 eruption of Mount St Helens.
The photograph below shows the present lava dome at Mount St Helens sitting within the
crater formed during the 1980 eruption.
Build up to the 1980 eruption of Mt St Helens
The eruption was one of the most closely observed and well documented. The north face of
the mountain began to bulge more than a month before the eruption due to the pressure of the
magma chamber, indicating that magma was clearly rising. On May the 18th an earthquake
triggered the collapse of the bulging slope, resulting in a landslide and setting up a chain of
events which included a lateral blast, superheated pyroclastic debris flows and lahars.
Effects of the eruption
 the volcanic ash combined with ice and snow which had been converted to
superheated steam, and formed lahars (volcanic mudflows) which rushed down the
sides of the mountain picking up cars, trucks and trees and destroying houses before
flowing into rivers at the base of the mountain;
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gas blast from the exposed magma (which had been contained at pressure) devastated
an area 30km wide and extending 20km northwards, flattening trees within the
surrounding zone.
A vertical ash cloud was sent 25km into the atmosphere causing periods of darkness
and circled around the earth in just over two weeks.
Pyroclastic flows travelling at 130km/hr rushed down the flanks of the volcano
(temperatures reaching 500oc) burning everything in their path.
The volcano itself was reduced in height by 390metres and a 3km long crater was
formed on the north-facing slope (see photograph above).
Ash from the eruption ruined crops in the area and disrupted communications. It also
raised water temperatures in rivers and streams, killing all wildlife.
Despite warnings of the pending eruption, 61 deaths were still reported, mainly
caused by the release of poisonous gases accompanying the blast waves.
For More Information:
The photographs are courtesy of USGS and CVO (Cascades Volcano Observatory) and be
found on their website: http://www.vulcan.wr.usgs.gov/Volcanoes. This site is well worth a
look as it gives lots of examples of extrusive landforms, with clear descriptions and super
photographs and graphics – a very valuable resource!
Sources
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Press, F. and Siever, R. (2001) Understanding Earth (3rd Edition), Freeman.
Van Rose, S. and Mercer, I.F (1999) Volcanoes, The Natural History Museum.
Waugh, D. (2000) Geography: An Integrated Approach, Nelson Thornes
Daniels, G.G (ed) (1982) Planet Earth, Volcanoes, Time-Life Books.
Figure on Caldera Formation – taken from Bunnett (1973) Physical Geography in
Diagrams p.30, Longman.