You should spend about 20 minutes on Questions 1—14 which are based on Reading Passage 1
MT. ST. HELENS ERUPTION (1980)

Mt. St. Helen's was known as one of the most picturesque volcanoes in the Cascade Range before its violent eruption on May 18, 1980. The eruption generated a massive lateral blast that devastated the northern flank of the volcano, flattening millions of mature Douglas fir trees over a fan-shaped area of 600 square kilometers. The blast zone was further subjected to a huge debris avalanche.

The event occurred along the boundary of two of the moving plates that make up the Earth’s crust. They meet at the junction of the North American continent and the Pacific Ocean. One edge of the continental North American plate overrides the oceanic Juan de Fuca micro-plate, producing the volcanic Cascade range that includes fifteen currently active volcanoes. Before 1980, the last one to erupt was California's Mt. Lassen, from 1914 to 1917. In the mid-1970's scientists were concerned that Mt. Baker, in northern Washington, might be the site of the next volcanic eruption, based on increased fumarolic activity on the volcano. However, in 1978, volcanologists Dwight Crandell and Don Millineaux suggested that Mt. St. Helens was potentially the most likely volcano to erupt in the twentieth century. They based their evidence on the relatively young age of the volcano, and its frequent historic eruptions. Before the 1980 eruption, it had been 130 years since Mt. St. Helens last erupted. The style of historic eruptions at Mt. St. Helens was also worrisome. Mt. St. Helens typically generates explosive pyroclastic eruptions, in contrast to many other Cascade volcanoes, such as Mt. Rainier which typically generates relatively non-explosive eruptions of lava.

In 1980, the University of Washington had just completed the establishment of a system of seismometers to help monitor the Cascade volcanoes. The computer feeds for the station went into operation on March 1. The first indications of a major problem came on March 20, when a 4.2 magnitude earthquake was recorded beneath Mt. St. Helens. Three days later another 4.0 M was recorded, and that evening the earthquakes began occurring in swarms centered directly beneath the volcano, at a rate of about 15 per hour. By March 25, magnitude 4 events were shaking Mt. St. Helens at a rate of about 3 per hour.

In early afternoon on March 27, a loud explosion was heard from the direction of Mt. St. Helens. Although the volcano was shrouded in clouds, a summit eruption was verified by a news team from the Vancouver Columbian. As they circled the summit in an airplane, they spotted a dense column of ash rising through the clouds to a height of about 2000 m. As the weather cleared later in the day, a new crater was visible, The summit eruption on March 27 was typical of several small eruptions that would occur through April and early May. None of these eruptions were magmatic in character, but instead they were steam eruptions generated by the heating of groundwater above a rising plug of magma that had invaded the central conduit of the volcano.

The March 27 eruption generated a huge east-trending fissure high on the north side of the summit. It extended down both sides of the volcano over a distance of about 1500 m. Another, less extensive fracture system had developed farther down the north flank of the volcano, parallel to the higher fracture. Measurements showed that the region between the two fractures had expanded outward to produce a huge north-flank bulge. The bulge was verified by a US Forest Service aerial spotter who reported seeing both fractures open and close as the north flank bulged upward during the hours immediately following the steam eruption.

Several more steam eruptions occurred on March 28, many of which were only a few minutes or an hour in duration. Harmonic tremors and minor eruptions occurred through March and April, with the eruption rate declining from about one per hour in March to about one per day in April. By mid-April, the eruptions had reamed out a new crater, with a diameter of about 400 m. Although the eruptions had declined somewhat in April, earthquake activity continued at impressive rates. The epicenters were largely confined to a shallow area beneath the north-flank bulge. The bulge was continually monitored and it got larger and more conspicuous with time. A week before the climactic blast, it had expanded outward by over 100 m and the rate of expansion was about 2 m/day!

The eruption began during a relatively quiet period in which no steam explosions had occurred for four days. On May 18, at 8:32 a.m., a 5.0-M earthquake triggered a very rapid series of events. The entire northern slope above the bulge failed and the north flank of the volcano began to slide downward from almost the exact site of the east-west fracture at the summit. This gigantic landslide released a tremendous mass from above the hydrothermal system that had driven the precursor steam eruptions. The abrupt loss of confining pressure above the heated groundwater caused a massive flashing to steam, which initiated a hydrothermal blast that was directed laterally through the landslide scarp. The lateral hydrothermal blast rapidly overtook the avalanche and devastated a fan-shaped area to the north, which was nearly 30 km wide over a distance of 20 kilometers. Trees were blown down like matchsticks.

The 1980 eruption of Mt. St. Helens is the most studied volcanic eruption of the twentieth century. Because geologists had been expecting the event, they were able to amass vast amounts of technical data when it happened. Study of atmospheric particles formed as a result of the explosion showed that droplets of sulphuric acid acting as a screen between the Sun and the Earth’s surface, caused a distinct drop in temperature. There is no doubt that the activity of Mt. St. Helens has influenced our climate. The volcano is still alive. Lava domes have formed inside the new crater, and have periodically burst. The threat of Mt. St. Helens lives on.