Westdahl Peak is a broad shield volcano in the central Aleutian arc, Alaska that produced moderate-sized eruptions in 1964, 1978-79, and 1991-92. Its frequent activity makes it an obvious choice for study with radar interferometry, and the first results lived up to expectations. An interferogram (image, upper left) revealed a pronounced area of uplift centered on the upper part of the volcano from October 1992 to October 1998. The deformation apparently started soon after the most recent eruption. This time period was not marked by any unusual seismicity that would have drawn attention to the volcano, so its reinflation would likely have gone unnoticed without the radar study.

The summit area of the volcano is covered by glaciers (uncolored areas in  images above), which makes the technique impossible to use for the summit region. The model used to generate the image in upper right shows that more than half of the deformation was probably localized in the ice-covered summit region. The model suggests that a magma body about 8 km (5 miles) beneath the center of the volcano expanded by about 50 million cubic meters (65 million cubic yards) from 1993 to 1998. If the model is correct, the summit area would have been uplifted about 17 cm.

Additional interferograms reveal that the inflation rate has not been steady. For example, more inflation occurred from 1993 to 1995 than from 1995 to 1998. Given the spacing between Westdahl's three most recent eruptions, another might be expected in the next 3-5 years, especially if the inflation continues.