Monitoring:  | Gas | Ground Deformation | Hydrologic | Remote Sensing | Seismicity |

The Changing North Face of Mount St. Helens

Summit Fractures Form

27 March 1980 A small eruption early in the day blasted a crater 75 m across at the summit of Mount St. Helens and covered the snow with a layer of gray ash. Even more striking to scientists, however, were the two series of crack systems or faults cutting across the summit on both sides of the crater. The faults began forming during the height of intense earthquake activity on March 25 and grew to 1,500 m long within two days.

Summit Subsides Dramatically

29 March 1980 At least a dozen new eruptions blasted a second larger crater west of the first one. At the same time, the large area between the two faults cutting across the volcano subsided. Scientists also noticed that an area just left of the left fault was being pushed upward.

Ups and Downs of the Summit Area

7 April 1980 Airplane passes over the "deformed" summit of Mount St. Helens. The two craters have merged into one that is 400 m in diameter. By this time, the area between the two faults subsided 50 m. But just north of the faults (lower left), the ground was pushed up 70 m by magma rising into the volcano.

North Face Begins to Bulge

10 April 1980 In addition to the two large faults cutting across the summit area, a growing number of fractures appeared on the volcano's north face (center). As different parts of the summit were subsiding and rising, these new cracks indicated that a very large area on the north flank of the volcano was being shoved outward. Covering an area 1,800 m across, this area of ground deformation became known as "the bulge."

Landslide Warning Issued

30 April 1980 Along with hundreds of daily earthquakes and small eruptions of steam and ash, the ominous growth of the bulge became a serious concern to scientists. In addition to the question of whether a stronger eruption would occur, would the bulge suddenly slide away as a large avalanche. If so, how far might it go? Public statements about such a possibility were made by scientists who suggested that an avalanche might easily reach Spirit Lake and the North Fork Toutle River, about 8 km away.

Continued Growth of Bulge

12 May 1980 Fractured and covered with ash, the bulge is visible on the right in the photograph. The bulge grew outward 1.5 to 2 m a day for several weeks prior to May 12. The volcano was literally being wedged apart by magma rising into its cone. A high point north of the summit crater stood 150 m above the former surface of the volcano and parts of the bulge were about 100 m north of where they were before March 20.

Dramatic Lesson

So much magma had pushed its way into the cone of Mount St. Helens by May 17 that it was taking up the space equal to a sphere 600 m in diameter! After stretching and splitting apart the summit area, the magma expanded northward, uplifting and pushing aside the entire north side of the volcano. Even so, no magma had yet leaked to the surface. Besides showing how magma can deform a volcano so dramatically, the bulge of Mount St. Helens has become a very clear warning signal for scientists. When a similar bulge develops on another volcano, a large and very hazardous landslide could occur without any warning.

May 18, 1980 8:32 a.m. A huge landslide removed the bulge and much of the volcano's interior, triggering an enormous explosive eruption. In a matter of minutes, the volcano's tall cone slid away and a deep, horseshoe-shaped crater formed in its place. What happened to the bulge during the eruption on May 18, 1980?