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 130,000km 2 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, Three-
Fingered 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 18 th 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;

 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.
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A vertical ash cloud was sent 25km into the atmosphere causing periods of darkness and circled around the earth in just over two weeks.
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Pyroclastic flows travelling at 130km/hr rushed down the flanks of the volcano
(temperatures reaching 500 o c) burning everything in their path.
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The volcano itself was reduced in height by 390metres and a 3km long crater was formed on the north-facing slope (see photograph above).
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Ash from the eruption ruined crops in the area and disrupted communications. It also raised water temperatures in rivers and streams, killing all wildlife.
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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

Press, F. and Siever, R. (2001) Understanding Earth (3 rd 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.