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Ecent Eruptive History of Ount Hood, Oregon, Vd Potential Hazards from Jture Eruptions

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Ecent Eruptive History of Ount Hood, Oregon, Vd Potential Hazards from Jture Eruptions ECENT ERUPTIVE HISTORY OF OUNT HOOD, OREGON, VD POTENTIAL HAZARDS FROM JTURE ERUPTIONS &<ff'" **?''j**-' - - ~-~ r--± - . /&/'&'r: f _\ " ^rn*"- - "-"" /-.'-"I--:.-.-- ^fc: GEOLOGICAL SURVEY BULLETIN 1492 fv Recent Eruptive History of Mount Hood, Oregon, and Potential Hazards from Future Eruptions r SOUTH SIDE OF MOUNT HOOD. Dashed line outlines the fan of pyro- clastic-flow deposits and mudflows formed 1,500-1,800 years ago. In the center of Mount Hood's crater is a remnant of a dome (arrow) that was erupted between 200 and 300 years ago. Recent Eruptive History of Mount Hood, Oregon, and Potential Hazards from Future Eruptions By Dwight R. Crandell An assessment of expectable kinds of future eruptions and their possible effects on human lives and property based on Mount Hood's eruptive behavior during the last 15,000 years GEOLOGICAL SURVEY BULLETIN 1492 United States Department of the Interior CECIL D. ANDRUS, Secretary Geological Survey H. William Menard, Director Library of Congress catalog-card No. 79-22249 U.S. GOVERNMENT PRINTING OFFICE : 1980 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock No. 024-001-0333-6-4 IV CONTENTS Page Abstract ................................................... 1 Introduction Drainage .................................................. 3 Glaciation.................................................. 4 Volcanic rocks of Mount Hood ................................ 7 Deposits of old mudflows and pyroclastic flows from Mount Hood .. 8 Part I Recent eruptive periods and their products Polallie eruptive period ....................................... 10 Deposits of pyroclastic flows and mudflows .................. 11 Ash-cloud deposits and the Parkdale soils ................... 18 Age and origin of the Polallie deposits ...................... 25 Timberline eruptive period ................................... 26 Deposits of pyroclastic flows and mudflows .................. 27 Ash-cloud deposits ...................................... 31 Age and origin of the Timberline deposits ................... 34 Old Maid eruptive period ..................................... 35 Deposits in the Sandy River valley ......................... 35 Deposits in the White River valley ......................... 38 Crater Rock dome ....................................... 43 Ash-cloud(?)deposit..................................... 45 Recent flood deposits .................................... 47 Recent (historic?) pumice .................................... 48 V Part II Volcanic hazards assessment Page Potential volcanic hazards .................................... 50 Pyroclastic flows and associated ash clouds .................. 50 Explosive lateral blasts ................................... 52 Tephra ................................................ 52 Mudflows and floods .................................... 54 Volcanic gases .......................................... 55 Hazard zones ............................................... 56 Pyroclastic-flow, mudflow, and flood hazard zones PA and PB............................. ..................... 56 Mudflow and flood hazard zones MA, MB, and M ........... 57 Lateral-blast hazard zones LA and LB ...................... 58 Ash-cloud hazard zone ................................... 58 Discussion of hazard zones ................................ 60 Planning for the next eruption ................................. 63 Planning before an eruption occurs ......................... 63 Predicting the next eruption .............................. 63 Warning signs of an impending eruption .................... 63 What to do if there are signs of an impending eruption ......... 65 References cited ........................................... 66 Appendices A Terminology ............................................ 70 B Measured sections ....................................... 73 C Chemical analyses ....................................... 76 Index ..................................................... 77 ILLUSTRATIONS FRONTISPIECE. South side of Mount Hood. Pa«e PLATE 1. Areas of potential hazard from future pyroclastic flows, ash clouds, mudflows, floods, and lateral blasts at Mount Hood .................. In pocket FIGURE 1. Index map of the Mount Hood area.............. 2 2. Inferred extent of glaciers at Mount Hood during the Fraser Glaciation ......................... 5 3. Bouldery volcanic mudflows of Polallie age ........ 6 4. North side of Mount Hood ..................... 12 5. Valley of Newton Creek on southeast side of Mount Hood ................................... 14 6. West wall of White River canyon ................ 16 VI FIGURE 7. Ash-cloud deposits of Polallie and Timberline Page ages..................................... 20 8. Deposits of pyroclastic flows and mudflows on west side of Little Zigzag Canyon ................ 28 9. Fill terrace formed by mudflows of Timberline age ..................................... 29 10. Large prismatically jointed block of dacite ........ 30 11. Light-reddish-gray ash-cloud deposit of Timberline age ..................................... 32 12. Mudflow of Old Maid age overlying mudflows of Timberline age ........................... 37 13. Terrace along west side of White River about 1 km upstream from State Highway 35 ............ 39 14. Pyroclastic-flow deposit of Old Maid age .......... 40 15. Zone of reddish-gray to yellowish-brown oxidation at the top of the pyroclastic-flow deposit of Old Maid age in the White River valley .......... 41 16. View down the White River Canyon of a fill terrace formed by mudflows of Old Maid age ........ 43 17. Crater Rock in the center of Mount Hood's crater is the remnant of a dacite dome ............... 44 18. Map showing approximate isopachs of ash-cloud(?) deposits of Old Maid age ................... 47 19. Diagrammatic maps and cross sections of hypotheti­ cal mudflows ............................. 61 20. Approximate percentage of time, annually, that the wind blows toward various sectors at Salem, Oreg. ................................... 62 TABLE TABLE 1. Radiocarbon dates from samples of wood and charcoal in volcanic deposits in the Mount Hood area ......................... 22 VII To convert metrics Multiply by To obtain Millimeters (mm) 0.03937 Inches (in.) Centimeters (cm) .3937 Inches (in.) Meters (m) 3.28 Feet (ft) Kilometers (km) .62 (about 2/3) Miles (mi) Cubic meters (m3 ) 35.31 Cubic feet (ft3 ) Cubic kilometers (km3 ) .2399 (about 1/4) Cubic miles (mi3 ) Kilometers/hour (km/h) .62 Miles/hour (mi/h) h VIII RECENT ERUPTIVE HISTORY OF MOUNT HOOD, OREGON, AND POTENTIAL HAZARDS FROM FUTURE ERUPTIONS By DWIGHT R. CRANDELL ABSTRACT Each of three major eruptive periods at Mount Hood (12,000-15,000(7), 1,500-1,800, and 200-300 years ago) produced dacite domes, pyroclastic flows, and mudflows, but virtually no pumice. Most of the fine lithic ash that mantles the slopes of the volcano and the adjacent mountains fell from ash clouds that accompanied the pyroclastic flows. Widely scattered pumice lapilli that are present at the ground surface on the south, east, and north sides of Mount Hood may have been erupted during the mid-1800's, when the last known activity of the volcano occurred. The geologically recent history of Mount Hood suggests that the most likely eruptive event in the future will be the formation of another dome, probably within the present south-facing crater. The principal hazards that could accompany dome formation include pyroclastic flows and mudflows moving from the upper slopes of the volcano down the floors of valleys. Ash clouds which accompany pyroclastic flows may deposit as much as a meter of fine ash close to their source, and as much as 20 cm at a distance of 11 km downwind from the pyroclastic flows. Other hazards that could result from such eruptions include laterally directed explosive blasts that could propel rock fragments outward from the side of a dome at high speed, and toxic volcanic gases. The scarcity of pumiceous ash erupted during the last 15,000 years suggests that explosive pumice eruptions are not a major hazard at Mount Hood; thus, there seems to be little danger that such an eruption will significantly affect the Portland metropolitan area in the near future. 1 INTRODUCTION LOUNT HOOD volcano is situated at the crest of the Cascade Range 75 km east-southeast of Portland, Oreg., and 35 km south of the Columbia River (fig. 1). Because of the volcano's proximity to a large metropolitan area, future erup­ tions have been regarded as a potential threat by geologists and laypersons alike (McBirney, 1966, p. 25; Hammond, 1973; Harris, 1976, p. 264). The principal purpose of this report is to assess the nature and likelihood of this threat to communities around the volcano and to show areas of poten­ tial hazard when eruptions occur in the future. The volcanic- hazards map (pi. 1) that accompanies the report can be used for long-range land-use planning around the volcano and for preparing contingency plans for mitigating the effects of future eruptions. WASHINGTON 45°30' FIGURE 1. Index map of the Mount Hood area. Rectangular area is shown in detail on plate 1. If it is assumed that a volcano will continue to behave as it has in the past, a knowledge of its behavior pattern can pro­ vide a basis for predicting dangers that could result from future eruptions. Mount Hood lacks a well-documented record of historic eruptions. Thus, its past behavior must be analyzed by studying the geologic evidence of prehistoric eruptions.
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