REPORT:
Rockslide-debris Avalanche of May 18, 1980, Mount St. Helens Volcano, Washington

Plates, Figures, and Tables

PLATES

1. Isopach map of debris-avalanche deposit

2. Orientations of long axes of hummocks in the debris-avalanche deposit

3. Morphologic map of debris-avalanche deposit

4. Lithologic map of debris-avalanche deposit

5. Lithologic cross sections of debris-avalanche deposit

FIGURES

Frontispiece. Panorama of debris-avalanche deposit, July 31, 1980

1. Topographic map of volcanic edifice and crater formed on May 18, 1980

2. Cross sections of Mount St. Helens

3. Eruptive periods of Mount St. Helens that produced rocks in debris avalanche

4. Photograph of west wall of crater in summer 1980

5. Photographs of hand samples of rock from the old mountain

6. Thin sections of rock from the old mountain

7. Dikes of andesite and basalt in crater walls

8. Ternary diagram of orthopyroxene, clinopyroxene, and hornblende+opaques for modern dacite dome rocks

9. Photographs of juvenile blast dacite

10. Example of hummock on a topographic map

11. Hummock parameters versus distance from source

12. Hummock types

13. Photographs of types of hummocks

14. Hummock with preserved contact paralleling surface

15. Aerial photograph of distal end of debris-avalanche deposit

16. Typical hummock of North Fork unit

17. Levee of North Fork unit blocking mouth of a small stream

18. Aerial photograph of debris-avalanche deposit in area around Castle and Coldwater Lakes

19. Photograph showing ice fragment on surface of deposit, May 31, 1980

20. Photograph showing distal end of debris-avalanche deposit

21. Photograph showing post-depositional slip in hummocks

22. Oblique view showing ramp and terraces on Johnston Ridge

23. Conical hummock

24. Oblique aerial photograph of lobate deposits of marginal unit

25. Photograph showing hummocky proximal unit in crater before erosion

26. Photograph showing hummocks of proximal unit on north flank of Mount St. Helens

27. Photograph showing scratches resembling glacial striations on north flank of Mount St. Helens

28. Photograph showing jumbled hummocks of woody debris in distal unit

29. Photograph showing pavement fragment in distal material

30. Photograph showing dipping contact between distal and North Fork units

31. Photograph showing narrow and elongate volcanic dikes in debris-avalanche deposit cross cut by faults

32. Photograph showing volcanic dike and clastic dike in hummock of debris-avalanche deposit

33. Photograph showing lava-flow stratigraphy in debris-avalanche deposit

34. Photograph showing breakage pattern of modern dacite

35. Percentage of clasts of juvenile blast dacite versus distance from source

36. Oblique aerial photograph of debris-avalanche deposit showing break-in-slope

37. Photograph showing levees sloping down from break-in-slope

38. Photograph showing andesite and basalt unit overlying older dacite unit

39. Photograph showing undulating surface of blast deposit

40. Photograph showing blast deposit resting on fir needles and organic material

41. Timing of deposition of debris avalanche and blast currents

42. Intimate interconvolution of debris-avalanche and blast deposits

43. Ropy surface of lahar deposit

44. Oblique aerial photographs of moving lahars near North Fork Toutle River on afternoon of May 18, 1980

45. Scarp of a slump on debris-avalanche hummock

46. Hummock made of one debris-avalanche block in andesite and basalt unit

47. Aerial photograph showing enigmatic textural pattern on southern half of deposit

48. Volume of identified largest debris-avalanche blocks in 1-km-square areas versus distance from source

49. Photographs of typical 1-m-square windows

50. Line drawings of windows

51. Photograph and graph showing sand-cone test and density of debris-avalanche deposit versus distance from source

52. Largest clast found in debris-avalanche deposit

53. Histograms of grain-size analyses

54. Cumulative probability and Rosin's law related to grain size

55. Median diameter versus sorting coefficient

56. Triangular plot of percentages of gravel, sand, and mud

57. Graph showing computed size parameters versus distance from source

58. Graph showing size classifications versus distance from source

58. Graph showing percentage of size classifications versus distance from source

59. Graph showing sorting coefficient versus distance from source

60. Generalized lithologic map of debris-avalanche deposit, showing interpretation of areas of deposition of slide blocks

61. Summary of processes involved in transport of rockslide-debris avalanche

62. Oblique aerial photograph showing interpreted paths of slide blocks

63. Schematic cross sectional diagram showing flow of debris-avalanche blocks and production of matrix facies

64. Features suggesting first stages of turbulence

65. Mobility versus volume for volcanic and nonvolcanic debris avalanches

TABLES

1. Modal analyses of modern dacite rocks from Mount St. Helens

2. Slide blocks in each cross section and geologic units in each slide block

3. Lithologic counts of approximately 100 clasts >2 cm diameter in 1-m-square windows

4. Distance from source, density, grain-size, and sorting parameters for debris-avalanche and blast deposits

5. Grain-size data in phi units for debris-avalanche and blast deposits

6. Water budget of debris avalanche

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