Why Greenwater needs a Lahar Detection and Alerting System. [Archive]

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01-01-2008, 11:50 AM

The first article is an excellent and schollarly account of the overall risks posed by Mt Rainier. The article was developed by the National Park Service. In the excerpts from this article noted below selected details that show the risks to the White River valley and the areas below have been summarized. The document itself is leingthy and highly detailed. It is well worth a read. I've added emphasis to a few areas.

The document is named “Mount Rainier National Park Geologic Resource Evaluaiton Report (www.explore-greenwater.com/pix/Lahar/Mt Rainier National Park Geologic Resource Evaluation REport pdf.pdf)” The document is contained in a .pdf file which you can downloadby clicking on the title (above). Note that you will need Acrobat Reader to read this file. You can download the latest version of Acrobat Reader straight from the folks who make it at this site: http://www.adobe.com/products/acrobat/readstep2.html free of charge.

From the article:

The mountain’s great height and northerly location allowed glaciers to cut deeply into its volcanic deposits. Today, steam from the volcano creates ice caves near the summit of the volcano, and in the past, devastating debris flows and mudflows were triggered by lava and rock debris from Mount Rainier’s eruptions. The consistency of these mudflows on Mount Rainier is like wet cement. The collapse of unstable parts of the volcano has led to additional debris flows. At one time, the summit rose perhaps 2,000 ft (600 m) higher than it does today. About 5,700 years ago, an eruption took off the top of the mountain and left a depression 1.25 miles (2.01 km) in diameter. The mountain is a history of lava flows, lahars, mudflows, pyroclastic explosions, and ash falls mixed with glacial debris, glacial outwash floods, and rockfalls.

On clear days, about 2.5 million people of the greater Seattle- Tacoma metropolitan area can see Mount Rainier. About 150,000 live in areas swept by lahars and floods that emanated from the volcano during the last 6,000 years (Sisson et al., 2001). The large population at risk and the lack of information about Mount Rainier’s edifice geology, pre- Holocene history, and hydromagmatic system prompted its inclusion as one of 16 volcanoes worldwide targeted for intense research as part of the United Nations’ International Decade for Natural Disaster Reduction (IDNDR).

Decade Volcano designation led to new research and substantial progress in understanding hazards from Mount Rainier. In general, collapse hazards are greatest on the west flank and future edifice stability modeling promises to quantitatively assess collapse risks (Sisson et al., 2001). The likelihood of lahars formed by magma- ice interaction chiefly by pyroclastic flows is higher than was previously supposed. Such eruption-generated lahars threaten all valleys that radiate from the volcano. Mount Rainier has erupted more frequently than was previously known and the association between eruptions and lahars is stronger than previously thought. Although seismic and other eruption precursors serve to alert communities to increased risks, some sizeable lahars were shown to have no known eruptive triggers.

Volcanic hazards from Mount Rainier include those that occur during eruptions such as tephra falls, pyroclastic flows and surges,, volcanic projectiles, and lava flows and those that occur during dormant periods such as debris avalanches, lahars, and floods. The National Research Council (1994) included the following volcanic hazards or volcanic related events as potential threats to persons or property:

• Volcanic eruptions – the eruption of ash flows and tephra (ash or pumice)
• Edifice failure – the gravitational collapse of a portion of the volcano
• Glacial outburst floods – the sudden release of meltwater from glaciers and snowpack or from glacier dammed lakes on the edifice
• Lahars or debris flows, and debris avalanches – gravitational movement of commonly water- saturated volcanic debris down the steep slopes of the volcano and into nearby valleys. Although boundaries have been applied to these hazard zones at Mount Rainier, too many uncertainties exist about the source, size and mobility of future events to locate these zones with absolute certainly (Hoblitt et al., 1998).

Kinds of Volcanic Events include:

Seismicity
Earthquakes are precursors to volcanic eruptions although not every earthquake means an eruption is eminent. Mount Rainier is considered to be the secondmost seismically active volcano in the Cascades, second only to Mount St. Helens (Kiver and Harris, 1999). In a given month, an average of 1- 2 high- frequency volcanotectonic (VT) earthquakes occurs directly beneath the summit (Moran and Malone, 2000). Seismicity is concentrated both at the edifice and to the west in a broad north- south belt known as the west Rainier seismic zone (WRSZ) (Sisson et al., 2001). The frequent seismicity raises concerns that earthquakes might be produced that would be powerful enough to trigger edifice collapse.

Tephra (Volcanic Ash)
Explosive eruptions, like Mount St. Helens, produce vertical plumes of hot gases mixed with volcanic rock particles (tephra). If less dense than air, the mixture rises over the volcano’s vent until it reaches an altitude at which it ceases to be buoyant. Fine tephra or volcanic ash in the plume will be carried downwind and will fall to produce a deposit that covers a broad area. Tephra
thickness and particle size usually decrease with increasing distance from the volcano.

Volcanic Projectiles
Volcanic projectiles are particles thrown from the vent on ballistic arcs, like artillery shells. The range of these projectiles rarely exceeds 3 miles (5 km) from the vent (Hoblitt et al., 1998). Most projectiles are less than 3 feet (1 m) across. The primary hazard from volcanic projectiles is from direct impact. Because they may be quite hot when they land, the projectiles also may start fires if they land near combustible materials.

Pyroclastic Flows and Pyroclastic Surges
Pyroclastic flows are denser- than- air mixtures of hot rock fragments and gases whose down slope movement is controlled by topography. Pyroclastic flows are composed of particles and gas, but if the mixture is gasrich, it is called a pyroclastic surge. A pyroclastic surge is only weakly controlled by topography. The two often occur simultaneously.

Both pyroclastic flows and pyroclastic surges are extremely hazardous. Their speeds typically exceed 20 miles/hour (10 m/s) and sometimes exceed 200 miles/hour (100 m/s), making escape from their paths difficult or impossible (Hoblitt et al., 1998; NPS, 2001). Temperatures in pyroclastic flows are usually greater than 570o Fahrenheit (300o Celsius). Because of their high densities, high velocities, and high temperatures, pyroclastic flows can destroy all structures and kill all living things in their paths. Although they have lower densities and temperatures, pyroclastic surges may also be quite destructive and lethal. Animals may be killed by direct impact by rocks, severe burns, or suffocation.

Deposits of pyroclastic flows and surges exist at Mount Rainier, but not in abundance (Figure 4). Pyroclastic flow deposits about 2,500 years old are exposed in the South Puyallup River valley, about 7.5 miles (12 km) southwest of the volcano’s summit. A thin surge deposit about 1,000 years old was discovered in White River valley about 7 miles (11 km) northeast of the summit (Hoblitt et al., 1998). Pyroclastic flows that travel across glaciers, however, do not weld and so do not leave long- lasting deposits. The dearth of pyroclastic flows, therefore, may be the result of pyroclastic flows and surges passing over snow and ice and being converted to debris flows (Hoblitt et al., 1998; Sisson et al., 2001). Hot rock fragments melt snow and ice, mix with the meltwater, and form lahars. Because Mount Rainier supports glaciers on all its sides, pyroclastic flows and the lahars they produce threaten all the valleys that originate on the volcano.

The types of pyroclastic flows at Mount Rainier are termed “block- and- ash” pyroclastic flows that are generally the result of lava dome collapse. Since Mount Rainier has only one lava dome exposed, the pyroclastic flows probably derived from other processes such as vent clearing explosions, voluminous hydromagmatic eruptions, or the sudden failure of thick, viscous lavas
flowing over steep headwalls (Sisson et al., 2001).

Lava Flows
Andesite lava flows compose much of Mount Rainier. Because of the chemical composition of andesite, lavas composed of andesite tend to be viscous and rather slow moving. On gentle slopes, andesite lava flows more slowly than a person can walk. Lava flows will, however, destroy everything in their paths either by fire, impact, or burial. While the hazard to people from lava flows is low, a more serious hazard results when lava comes in contact with snow and ice. Flowing lava on the ice- covered slopes of Mount Rainier may break up, avalanche, and form much larger lahars (Sisson et al., 2001)

Volcanic Gases
Magma contains dissolved gasses that are released during and between eruptions. Andesitic volcanoes contain gases composed primarily of water vapor. Secondary gases are carbon dioxide and sulfur compounds. Minor amounts of carbon monoxide, chlorine, fluorine, boron compounds, ammonia, and several other compounds may be present, as well (Hoblitt et al., 1998). Volcanic gases are distributed by wind. They may be concentrated near a vent and then diffuse rapidly downwind. Injuries to eyes and lungs from acids, ammonia, and other compounds and suffocation by denser- than- air gases, such as carbon dioxide, are possible. Metals can be severely corroded by volcanic gases.

Debris Avalanches, Debris Flows, and Lahars
In 1980, at Mount St. Helens, rising magma created a bulge that broke away from the rest of the volcano and generated a rapidly moving landslide. Landslides caused by the failure of unstable slopes are called debris avalanches. A volcano’s slopes can also fail even if magma isn’t involved. Slopes may become unstable by melting of snow during periods of unusually high temperatures or unusually heavy rain in summer or early autumn, by glacial erosion, or as the strength of the rock is reduced by hydrothermal alteration. Hydrothermal alteration causes the rock to become weaker by chemically altering it to clay and other minerals. Eventually, the affected part of the volcano collapses under its own weight, generating a debris avalanche.

Non-magmatic debris avalanches are especially dangerous because they can happen without warning. Debris avalanches may be triggered by earthquakes, steam explosions, and intense rainstorms affecting weakened slopes in the park. Debris avalanches can travel tens of kilometers at speeds of tens to hundreds of kilometers per hour. Like pyroclastic flows, escape from a debris avalanche is difficult to impossible. Topography controls a debris avalanche, which will destroy everything in its path and leave a deposit that is usually a few meters to hundreds of meters thick (Hoblitt et al., 1998). Large debris avalanches may block the mouths of tributary valleys and cause lakes to form. When impounded water spills over the dam formed by the debris avalanche, it can quickly cut a channel and cause the lake to drain catastrophically.

Debris avalanches commonly contain enough water, snow, or ice to transform them into debris flows or lahars. Lahars are slurries of water and sediment (60 percent or more by volume) that resemble flowing cement. Lahars are sometimes called mudflows and can travel at speeds reaching a few tens of kilometers per hour along gently sloping distal valleys to more than 100 kilometers (60 miles) per hour on steep slopes near the volcano (Crandell, 1969A; Fiske et al., 1963, 1988; Scott et al., 1995; Hoblitt et al., 1998; Kiver and Harris, 1999). Water in reservoirs may be displaced by lahars and could cause floods farther downstream.

At least 60 lahars of various sized have flowed down valleys on Mount Rainier during the past 10,000 years (Figure 4). All of these lahars can be grouped into two general categories: cohesive and non- cohesive lahars. Cohesive lahars contain relatively large amounts of clay derived from chemically altered rocks. They form when debris avalanches originate from hydrothermally altered parts of the volcano. Non- cohesive lahars contain relatively little clay and are triggered whenever water mixes with loose rock debris. This can be caused by the mixing of pyroclastic flows or pyroclastic surges with snow or ice; relatively small debris avalanches; unusually heavy rain; or an abrupt release of glacier- stored water. The largest lahar at MORA in the last 10,000 years was a cohesive lahar known as the Osceola Mudflow.

The Osceola Mudflow occurred about 5,600 years ago and was at least 10 times larger than any other known lahar from Mount Rainier. Perhaps triggered as magma forced its way into the volcano, the mudflow was the product of a large debris avalanche composed mostly of hydrothermally- altered material. Extending at least as far as the [city] of Kent and to Commencement Bay (now the site of the Port of Tacoma), the Osceola Mudflow left deposits that cover an area of about 212 square miles (550 sq km) in the Puget Sound lowland (Hoblitt et al., 1998). The mudflow deposited more than 4.9 billion cubic yards (3.7 billion cu meters) of material (NPS, 2001). Remnants of the mudflow on the sides of the White River and West Fork valleys show that both valleys were temporarily filled with streams of mud more than 500 feet (150 m) thick (Crandell, 1969B). Today, a similar event would produce a mudflow in the reservoir behind Mud Mountain Dam where it would acquire more water and thus increase its mobility. The reservoir dam would be destroyed and the mudflow would easily inundate the towns of Enumclaw, Buckley, Kent, Auburn, Sumner, and Puyallup, a combined population of 125,000 people in 1999. [/B]If an emergency warning sounded at the time the mudflow began, most people would have less than 2 hours to evacuate their homes.

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