VHP Photo Glossary: More photos of faults
Sketch by B. Myers
Mount St. Helens, Washington
Scientists measure the distance between two benchmarks spanning the fault scarp of a thrust fault on the crater floor of Mount St. Helens. This scarp developed on the crater floor in 1981 as magma rose into the lava dome (backgound) before erupting onto its surface. The pressure exerted by the rising magma against rocks surounding the conduit caused the crater floor to fracture along a plane gently inclined toward the dome.
Material above the fault (person kneeling on upper surface) was pushed over material below the fault (person in lower right); in this view the fault scarp advances toward the lower right; arrow indicates direction of movement. Scientists measured an increasing rate of movement of this thrust fault before two eruptions in June and October 1981, which helped them to predict both eruptions accurately.