Bulging of the North Flank
Before the May 18, 1980, Eruption

Introduction

Topographic and structural changes were evident at the summit of Mount St. Helens as early as March 27, when phreatic eruptive activity began. In addition to development of a new crater, the summit area was bisected by an east-trending fracture system, 2 km long, that defined an irregular graben. The area north of the graben appeared to be bulging outward and possibly upward. At first, the obscuring effects of heavy and continuing snow cover, interlayered with ash deposits from the phreatic eruptions, made it uncertain whether the graben and associated fractures involved the bedrock edifice or mainly reflected melting and increased movement on the summit ice cap. Frequent eruptions and poor weather also precluded direct ground examination of these features in early April, and initial attempts to obtain quantitative data on defonnation related to the volcanic activity were accordingly confined to tilt measurements on the lower flanks and beyond the base of the volcano.

By mid-April, the bulge had become ominous, and there was increasing concern for stability of the north flank of the volcano. Aided by the temporary cessation of eruptive activity and with arrival of the required equipment, we then established a network for ground defonnation measurements on the cone (fig. 83). Within a few days, these measurements yielded clear evidence of large-scale northwesterly movements of the bulge area, at rates of as much as 2.5 m/ day. Concurrently, the first detailed photogrammetric measurements revealed that the bulge had moved upward and outward as much as 100 m between August 1979 (the last preeruption vertical photography) and April 12, 1980 (Moore and Albee, this volume). The geodetic data, which provide details of defonnation rates, and the photogrammetric results, which better define the overall geometry of the defonnation, are complementary.

The growth of the bulge was inferred to record emplacement of magma into the volcanic edifice, and emplacement of a lava dome was anticipated because several other domes had been emplaced asymmetrically on the north half of the volcano in the last few thousand years (Mullineaux and Crandell, this volume). In late April and early May, results from the geodetic and photogrammetric studies confirmed that gravitational failure and landsliding of the bulge area were likely, and we hoped that changes in defonnation rates could be observed as indications of imminent failure. Unfortunately, no such changes took place. Nevertheless, these studies helped convince State and Federal officials to maintain closure of the north side of the volcano to public access (Miller and others, this volume).