VOLCANIC HAZARDS FROM MT. RAINIER, WASHINGTON

Introduction

Mt. Rainier is the highest peak in the Casacde Range at 4393m (14, 410ft.) It has a larger amount of glacial ice than any other mountain in the conterminous United States, great topographic relief, and has had active episodes of volcanics within this millenium. The combination of these factors makes Mt. Rainier one of the most potentially dangerous volcanoes in the world. In fact, due to these dangers, Mt. Rainier was one of 16 volcanoes worldwide to be designated as "Decade Volcanoes". These volcanoes are the focus of an international initiative at better utilizing science and emergency management to reduce the severity of natural disasters.

The types of geologic phenomena that affect Mt. Rainier and its surrounding region include lava flows, volcanic gases, pyroclastic flows, tephra falls, and lahars. The first three of these hazards are usually reasonably localized phenomena, restrained to the flanks and low-lying regions surrounding the volcano. The final two hazards, tephra falls and lahars, are much more far-reaching and are a direct threat to the population of 1.5 million people in the out-lying area.

Volcanic Hazards

Lava Flows

Most of Mt. Rainer is composed of andesite lava flows. Lava flows are streams of molten rock than erupt from a volcano in an effusive, rather than eruptive manner. Then move downslope with gravity, but do not necessarily channel into stream beds and valleys due to the viscous, slow-moving nature of the flow. This means that the risk posed by lava flows on Mt. Rainier is not as high for people as it is for any property that get in the flow's path. Everything the flow comes in contact with is destroyed either by fire, impact or burial. As the lava moves downslope, it loses energy and heat, and eventually slows, cools, and solidifies.

The main danger to humans from lava flows on Nt. Rainier is when they come in contact with snow and ice. The result is a rapid melting, which in turn can initiate floods and lahars.

Volcanic Gases

Magma contains dissolved gases that are released before, during, and in-between volcanic eruptions. Andesitic magmas, as those found at Mt. Rainier, are primarily composed of water vapor, carbon dioxide, and sulfer compounds, with minor amounts of carbon monoxide, chlorine, fluorine, boron compounds, ammonia, and other compounds. Volcanic gases are distributed by the wind and tend to to have high concentrations near the volcanic vents with rapid dilution as you move away from the vent.

Humans and animals can sustain injuries to their eyes and lungs from the acid and ammonia in the gases. Another danger is suffocation, which can occur when denser-than-air gases such as carbon dioxide settle into the bottoms of closed valleys and effectively push out the oxygen. Other materials like metal can suffer severe corrosion from the gases as well.

Pyroclastic Flows

A pyroclastic flow is a mixture of hot gas and volcanic rock particles. It is produced by an explosive eruption and the cloud is denser than air. Therefore, unlike the tephra or ash fall which can rise many kilometers in the air, pyroclastic flows behave like a fluid and flow downslope over the ground surface. If the mixture is mostly rock particles with a smaller percentage of gasses, then the flow is controlled by topography and will be channelled into valleys. However, if the mixture has a high gas content, the flow is termed a pyroclastic surge. These surges are less dense because of the upwelling nature of the gas, and will separate from the denser pyroclastic flow at a turn in the valley, surging straight ahead.

These flows and surges are incredibly hazardous. The speeds of the flow range from 20-200 mi/hr and the temperatures are usually greater than 300 deg.C (570 deg.F). Due to their high density, high velocity, and high temperature, pyroclastic flows and surges destroy anything they encounter by either impact, burial, or incineration, leaving a swatch of complete destruction in their wake. At Mt. Rainier, the history of pyroclastic flows shows that they have occurred in the past (2500 & 1000 years ago), but are not abundant. Like lava flows, one of the most dangerous aspects of pyroclastic flows is its contribution to the initiation and density of lahars

Tephra Fall

Mt. Rainier is a volcano of andesitic composition. Volcanoes of this composition tend to have very explosive eruptions. (As compared to Hawaiian volcanoes, which are of basaltic composition, are tend to have effusive, lava flow type eruptions) When the volcano explodes through its top, vertical plumes of hot gases mixed with volcanic rock particles typically rise into the air above the volcano. The lighter-than-air mixture rises over the vent until it reaches an altitude at which it ceases to be buoyant. At the same time, as the plume is rising, high-altitude winds begin to move the tephra or ash cloud downwind. So as the ash becomes denser-than-air, it will fall from the cloud as ash or tepha fall over a broad area miles from the vent.

The ash cloud can block sunlight, restrict visibility for evacuators, rescue workers, and emergency personnel, cause building roof collapse due to the weight of the accumulated ash, instigate lightning storms, create or aggravate respiratory problems, ruin crops, short out power lines, damage mechanical devices, and potentially cut out aircraft engines. As you can see, ash fall is less immediately dangerous than the other volcanic hazards. However, even it has been shown that even thin tephra accumulations can seriously disrupt social and economic activity. And during and immediately after the eruption, tephra fall is a great hinderance to damage assessment and rescue operations. Compared to other Cascade volcanoes, Mt. Rainier is only a moderate tephra producer. In the past 10,000 years, only eleven eruptions have deposited layers of tephra.

Lahars

Lahars are slurries of water and sediment (60% sed. by volume) that look and behave like flowing concrete. Also called mudflows, lahars can travel at speeds of 10-60 mi/hr down the slopes of the volcano and along river valleys downstream. Their great velocity and density carries an enormous force and they can destroy or damage any structure in their path by impact or burial. Like the dense pyroclastic flows, lahars are strongly controlled by topography.

In the last 10,000 years at least 60 lahars of various sizes have originated from Mt. Rainier. The largest lahar, the Osceola Mudflow, occured 5600 years ago was 10 times larger than any other Mt. Rainier lahar. It was apparently triggered by a large debris avalanche and its deposits cover an area of about 550 sq. km (212 sq. miles), extending to the Seattle suburb of Kent. The Electron Mudflow, approx. 500 years agao, was another large lahar that racked the surrounding area. The deposits at Orting, approx. 15 miles away, were 18- 20 ft. thick.

Lahars do not have to initiated by a volcanic eruption. Events such as avalanches, earthquakes, and heavy rains have triggered lahars off Mt. Rainier as well. Circumstances conducive to future lahars, topographic relief, great volumes of ice, and the potential for renewed volcanism (and the earthquakes, over-steepening, and debris-avalanches associated with volcanism) are all present at Mt. Rainier. Therefore, lahars may be the greatest threat to communities downvalley from Mt. Rainier than any other volcanic hazard.