Geog Practice Question 3

Impact of Volcanic Hazards in Developed Countries

The May 18th, 1980 eruption of Mount St. Helens in the United States is an example of a Plinian eruption. Describe the volcanic hazards and their effects that resulted from this eruption. (9)

• Plinian eruption
  -Characterised by large eruptive columns that are powered upward partly by the thrust of expanding gases, and by convective forces with exit velocities of several hundred meters per second.
  - Large amount of ash and pumice and produced during the eruption.
  - The eruption may be accompanied by pyroclastic flows.
  - The ejection of large volumes of ash and magma may result in a caldera collapse.
  - Many stratovolcano tend to erupt in this manner.
  
  
• Volcanic Hazards and Effects
  - Ash Fall
  - After the explosive lateral blast of the volcano, a strong and vertically directed explosion of ash and steam occurred, giving rise to a column of ash that was almost 20 kilometres high.
  - By the time the eruption stopped, the ash cloud had already spread to the central United States.
  -The ash particles in the atmosphere gave rise to lightning, causing many forest fires as a result.
  - During ash falls, visibility was reduced and roads had to be closed to traffics for up to weeks.
  - Air transportation was disrupted as several airports in eastern Washington shut down due to ash accumulation and poor visibility. Over a thousand commercial flights were also cancelled as a result.
  - The ash infiltrated many spaces and was highly abrasive, leading to the contamination of oil systems, the clogging of air filters and the abrasion of moving surfaces. Thus, many machinery were damaged as a result.
  - In areas where there was high ash accumulation, many crops were killed. On the other hand, crops survived in fields where ash accumulation was much lower. Output of these agricultural fields even increased as a result of higher summer precipitation brought on by the ash falls.
  - The ash falls added on to the economic cost of the volcanic eruption. US$2.2 million were spent to remove the ash and the entire process took up to 10 weeks in some areas.
  
  
• Pyroclastic flows
  - A few hours after the start of the lateral blast, pyroclastic flows began to form and continued to occur during the 5 hours of intense eruptive activity. At least 17 pyroclastic flows were observed during the eruption.
  - These flows were deadly since the full volume and force of pyroclastic materials were directed sideways by the lateral eruption of Mount St. Helens. This resulted in flows with high velocities and high temperatures with the ability to destroy everything in their paths.
  - Steam-blast explosions were observed when the hot pyroclastic flows entered water bodies like the Spirit Lake. The explosions formed many pits on the sides of the pyroclastic flow deposits. Plumes of ash and steam as high as 1.9 kilometres were observed as well.
  - When the pyroclastic flows encountered the North Fork of the Toutle River, the debris were deposited in the river. The accumulation and eventual overflow of the debris collected in the river was to lead to the formation of destructive lahars.
  
  
• Lahars
  - In the North Fork of the Toutle River, the water-saturated parts of the massive debris avalanche deposits began to collect and eventually overflow, giving rise to the largest and most destructive lahar.
  - 0.01 cubic kilometres of water, mud and debris were deposited into the Swift Reservoir when lahars moved along the south-eastern flank of the volcano and along the drainages of the Swift Creek, Pine Creek, and Muddy River.
  - The impact of the lahars’ flow front are so strong that lahars can even crush or carry away buildings. The forest around Mount St. Helens were destroyed. More than 200 homes and over 300 kilometres of road were destroyed by these lahars, rendering many people homeless.
  - Valuable land were buried by the layers of rock and mud from the lahars, lowering their value and rendering them useless for agriculture. This added on to the economic cost of the volcanic eruptions.
  
  
• Combined effects of the volcanic hazards
  - The devastation on the people and their property were widespread. The death toll was 57 and many others were injured during the eruption.
  - The fauna of the forest around Mount St. Helens was affected as well. An estimated number of 7000 big animals like bears and deer died during the eruption along with birds and most small mammals.
  - Immediately after the eruption, tourism, an important industry in Washington, was badly affected. Conventions, meetings, and social gatherings also were canceled or postponed at cities and resorts elsewhere in Washington.
  - Official figures estimate the cost of the destruction wrecked by the eruption to be US$1.1 billion. The actual cost remain difficult to determine.
  - A few months after the eruption, there were reports of residents, who had coped successfully during the eruption, suffering from stress and emotional problems. Funding for mental health programs for these residents were requested.
  - The May 18th eruption also resulted in the collapse of the summit of Mount St. Helens, forming a caldera instead.