Interpreting Ground Displacement I (Grades 9-12)

In 1971, the feeder conduit to the Mauna Ulu eruption site became blocked. Magma continued to be supplied to the summit reservoir from the mantle and caused rapid summit inflation. Pressure reopened the conduit in February 1972, and the eruption continued. The period of rapid summit inflation was interrupted by two eruptions in 1971. On August 14, a 10-hour-long eruption occurred in Kilauea Caldera. On September 24, an eruption began in the caldera and, by the 29th, migrated 12 km down the southwest rift zone. Prior to the eruption in September, uplift occurred along the upper southwest rift zone. The pattern of uplift is shown in activity 12.8. The uplifted area is elongate and centered along the rift zone (Duffield and others, 1982).

Click here to see Activity 8. Students must study the diagram to answer the following questions.

  1. Is the center of ground displacement at Halemaumau Crater, the upper east rift zone, or the upper southwest rift zone? (circle one)
    The ground displacement is centered on the upper southwest rift zone.

  2. Does the ground displacement pattern indicate uplift or subsidence? (circle one)
    The ground displacement contours along the southwest rift zone are positive, indicating uplift. The tilt vectors also point radially outward, a pattern common for uplift. The single -2.5 mm contour for a small area northeast of the caldera indicates subsidence.

  3. What is the maximum amount of ground displacement?
    Maximum uplift is greater than 45 mm but less than 50 mm.

  4. What type of volcanic activity would you expect to be associated with this ground displacement pattern?
    The relatively large amounts of uplift indicate the intrusion of magma into the upper southwest rift zone and the potential for a fissure eruption. An eruption did occur along the axis of the uplift.

  5. Where is volcanic activity likely to occur?
    The eruption would be along the southwest rift zone.

  6. Why are the lines dashed for the southwest part of the pattern?
    No data was obtained for that area because it is rugged and difficult to survey.

  7. Are the tilt vectors useful for defining the center of ground displacement?
    Yes. Most of the vectors project to the center of the uplift. The two southwesternmost tilt vectors were influenced by the central uplift and the elongated uplift along the rift.

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