REPORT:
Volcanic Studies at the U. S. Geological Survey's David A. Johnston Cascades Volcano Observatory, Vancouver, Washington

Introduction

Sixteen subsequent eruptions have taken place in the horseshoe-shaped crater formed by the May 18 eruption. No large explosive event has occurred since October 1980. The last 11 eruptions have built a lava dome to a height of 240 meters by the slow intrusion of magma into its interior and the extrusion of lava onto its surface; the most recent eruption of Marcy 29, 1984, added a new stubby lava flow to the top of the dome.

U.S. Geological Survey scientists at the David A. Johnston Cascades Volcano Observatory (CVO) in Vancouver, Washington, are studying the intermittent eruptive activity of Mount St. helens and hazards posed by the effects of the May 18 eruption. Many different monitoring techniques and instruments are used to record the "daily pulse" of the volcano to evaluate the change of future eruptive activity and to investigate fundamental volcanic processes. Studies also are made of the river basins and the new lakes around the volcano and of the massive debris avalanche that slid off the mountain on May 18, 1980. These studies provide data to asses water-related hazards to downstream communities from increased sedimentation, erosion, and flooding along the Toutle and Cowlitz Rivers. Such hydrologic hazards likely will persist long after the decline of eruptive activity at Mount St. Helens.

CVO is supported by the Geological Survey's Volcano Hazards Program, which is responsible for providing warnings of volcanic eruptions and related hazards in the United States. Information and advice are given to Federal, State, and local officials concerning specific volcanic hazards and the implementation of emergency-response plans. Activities designed to inform people of volcanic hazards include research on volcanic processes, monitoring active and potentially hazardous volcanoes, and mapping the types and the extent of volcanic deposits of past eruptions.

The goal of the monitoring and research activities of CVO is to give timely warnings of eruptive and related hydrologic hazards at Mount St. Helens. Three types of written public statements about volcanic activity are issued by CVO to provide hazard information to the public and to governmental agencies:

• A factual statement describes current conditions of the volcano but does not anticipate future events; such statements are revised when warranted by new developments.

• A forecast is a comparatively nonspecific statement about volcanic activity to occur weeks to decades in advance. A forecast is based on projections of past eruptive activity or is used when monitoring data are not well understood. This kind of statement is particularly useful for land use planning and development of emergency response plans.

• A prediction is a comparatively specific statement giving place, time, nature, and, ideally, size of an impending eruption. Correct predictions were made of all 14 eruptions at Mount St. Helens from June 1980 to the end of 1982.

Water-related hazard information is provided to the National Weather Service and other Federal, State, and local agencies involved in flood mitigation planning.

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