GL Jan Q1 Volcanic Hazards St Helens

1. Table 1 is an account of the events leading up to the major eruption of Mount St. Helens in Figure 1shows profiles of Mount St. Helens together with earthquake data associated with the May 18theruption. Volcanic activity at Mount St. Helens began on 27th March, 1980 with explosive ash and steam eruptions. “At 8:32 A.M. on May 18, Mount St. Helens was shaken by a magnitude 5 earthquake, centred 1 to 2 kilometres beneath its north flank. Geologists Keith and Dorothy Stoffel, who were flying in a light plane about 400 metres above the summit at just that moment, saw several icefalls start down the steep crater walls. About fifteen seconds later they were the closest witnesses to the onset of the largest landslide in recorded history, closely followed by a huge volcanic eruption. ‘The whole north side of the summit crater began to move instantaneously as one gigantic mass’, Dorothy recalled. Seconds later a massive explosion shook the mountain.”

Refer to Table 1andFigure 1. (a) Give a name for Feature X on Figure 1(12thMay profile) and explain its origin. [2] Feature X Origin (b) Explain how the distribution and type of seismic activity in Figure 1might indicate a possible volcanic eruption. [3]

(c) (i) Suggest two factors most likely to have been responsible for triggering the landslide. [2] Factor Factor (ii) Using all the data provided, account for the shape of the post-eruption profile.[3] (d) Explain the use of one technique by which ground deformation of the northern bulge might have been monitored. [2]

Refer to Table 1andFigure 1. (a) Give a name for Feature X on Figure 1(12thMay profile) and explain its origin. [2] Feature X Origin Crater/Vent Initial (steam/ash) explosive activity (b) Explain how the distribution and type of seismic activity in Figure 1might indicate a possible volcanic eruption. [3) Harmonic Tremors/long duration/low amplitude Confined pattern/footprint Large number Movement &/or intrusion of magma

(c) (i) Suggest two factors most likely to have been responsible for triggering the landslide. [2] Pressure from magma Over steep/critical angle of slope Vibration - Earthquakes (ii) Using all the data provided, account for the shape of the post-eruption profile.[3] Landslide Released pressure Triggering a lateral blast Debris flow to N New lava dome

(d) Explain the use of one technique by which ground deformation of the northern bulge might have been monitored. [2] Electronic distance measurement using lasers EDM Measurement of tilt electronically & by triangulation Standard levelling surveys to measure elevation changes Measuring opening of fissures Measuring water level changes in crater lake / boreholes