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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />by these eruptions. <br />There have been no other naturally caused incidents that are within the scope of this plan. <br />4.1.4.8. Future Trends Ble <br />There is no evidence that typical volcanic activity levels among the volcanoes that pose an ash fall threat <br />to Hennepin County are either increasing or decreasing. These volcanic events happen in geologic time <br />in which eruption recurrence rates of hundreds, thousands or even hundreds of thousands of years are <br />possible. <br />Event Probabilities: The United States Geological Survey (USGS) has estimated the activity level and <br />eruption recurrence rate of each of the volcanoes in the western United States, Canada, and Alaska. <br />4.1.4.9. Indications and Forecasting <br />Volcanic forecasting is the responsibility of the United States Geological Survey and its Volcano <br />Observatories. USGS scientists categorize volcanoes and estimate their explosive potential based on <br />evidence of past eruptions. <br />4.1.4.10. Detection & Warning <br />USGS scientists monitor precursor activity and are often able to issue alerts of impending eruptions <br />months or weeks prior to the event. Ash clouds are tracked by the National Oceanic and Atmospheric <br />Administration. The Washington Volcano Ash Advisory Center (WVAAC) is responsible to provide alert and <br />warning services for aviation safety. The Minneapolis Air Route Traffic Control Center (ARTCC) is served <br />by the WVAAC. <br />4.1.4.11. Critical Values and Thresholds <br />4.1.4.11.1. Diameter: Ash particles are less than 2 millimeters in diameter down to very extremely <br />small particles of less than 0.001 millimeter. Volcanic ash is lofted high into the atmosphere and <br />can be blown thousands of miles away from the volcano. Larger and heavier particles will fall to <br />Earth much more quickly than smaller and lighter particles which may remain aloft for weeks or <br />longer. Extremely small particles suspended in the air can be invisible to the human eye, yet <br />present hazards to aviation. <br />4.1.4.11.2. Density: Ash particles have variable degrees of density (pumice, 700-1200 kg/m3; <br />glass, 2350-2450 kg/m3; crystals, 2700-3300 kg/m3; and rock particles, 2600-3200 kg/m3). The <br />high -density ash particles are hard (5 Mohs scale). Window glass and steel have a Mohs hardness <br />of 5.5, for example. Ash particles have sharp edges making them very abrasive. <br />31 <br />