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Alaska Volcano Observatory

Here in the wilds of the Lake Clark area, it seems we are isolated from the rest of the world. From a human standpoint that may be so, yet we are intricately tied to the rest of the world through physical and geologic	A glacial erratic in a river bed above Telaquana Lake.
processes. The Lake Clark region is a classic showcase of past and present geologic events. The landscape tells the story of what has occurred many years ago, and today, we can see the forces that are creating our environment.

The theory of plate tectonics helps explain the distribution and occurrence of volcanoes and earthquakes around the world. The surface of the earth consists of eight major "plates" and about a dozen smaller ones. Each plate is about 50 miles thick and consists of a relatively shallow upper layer that deforms by either brittle breaking or elastic bending. A second deeper layer of the plate yields plastically, while an even lower layer is like a viscous fluid. It is on the lower viscous layer that the entire plate slides.

Similar to a piece of paper floating on water, the plate can move about on the surface without distorting. The earth's plates tend to be internally rigid and interact mostly at their edges. Most earthquake activity is a result of a difference in motion between the adjacent plate boundaries. The plates move relative to each other at rates that range from 1/2 inch up to about 5 inches per year. Although these rates are slow by human standards, they are extremely rapid by geologic standards. For example, a motion of 2 inches per year adds up to 30 miles in one million years. And some plates have been in continuous motion for 100 million years.

Deep within the oceans are a series of nearly continuous submarine mountain ranges. These great submarine ridges are marked by earthquakes and submarine volcanism. It is along the mid-ocean ridges that sea floor spreading occurs. Hot material from deep within the mantle rises up continually, adding new material to the earth's crust. The size of the earth is not expanding, so this new material must be consumed someplace else.

At trenches where plates collide, one plate is forced beneath the other in what is called a "subduction zone". As the subducted plate is forced to descend, it slips and slides, generating earthquakes. Tilting downward, the plate will plunge into the mantle to depths of 450 miles before the crustal material becomes molten. Being less dense than the mantle, the molten crustal material rises toward the earth's surface where much of it erupts as lava and builds up volcanic peaks. Typically, a belt of volcanoes lies above the inclined earthquake zone.

Mt. Redoubt's last eruption was in the spring of 1990.

The Aleutian Island subduction zone lies about 30 miles beneath the surface of the Kenai Peninsula, but abruptly dives to depths greater than 65 miles beneath the western edge of Cook Inlet, and to a depth greater than 100 miles beneath Redoubt and Iliamna volcanoes at the eastern end of the park. Here, the Pacific Ocean plate is being pushed beneath the North American Plate. The subduction along the Aleutian trench has been going on for the last three million years at a rate of 2.6 inches per year, and earthquakes and volcanoes are prevalent. Thirteen earthquakes of magnitude 5-6 on the Richter scale have occurred in the area since 1972, mostly at depths of 55-110 miles beneath Chinitna Bay and Tuxedni Bay. Strong earthquakes and volcanic eruptions can be expected to continue in the eastern part of the park as the Pacific plate continues to dive beneath the North American plate.

Alaska's volcanic belt is part of the Pacific "Ring of Fire" and contains 70 potentially active volcanoes. It extends from Mount Spurr near Lake Clark to Buldir Island in the western Aleutians.

Within the Lake Clark region itself there are four active (and three of the tallest) volcanoes. Mount Spurr, at 11,070 feet, lies just north of the park. Mount Redoubt, at 10,197 feet, and Mount Iliamna, at 10,016 feet, are both located in the park. To the south of the park lies Saint Augustine Island.

Mount Spurr erupted on July 9, 1953. That spectacular explosion sent a cloud of ash up 70,000 feet in just 40 minutes, according to U.S. Air Force pilots who were flying in the area when the eruption occurred. Ash dropped on Anchorage, only 80 miles east, with a total accumulation of 1/8 to 1/4 inch. The most recent eruptions took place on June 17, August 18, and September 16-17, 1992, with ash plumes reaching up to 30,000 feet, darkening the skies, and dusting Anchorage with ash once again.

The other volcanoes have also been active. Gases are frequently seen venting near the summit of Mount Iliamna, but there are no documented reports of recent eruptions, according to the USGS. Redoubt Volcano, just north of Iliamna, awakened December 14, 1989, dumping varying amounts of ash primarily north and west of the volcano and lightly dusting Anchorage and Kenai. Periodic eruptions continued throughout the week before Christmas, disrupting holiday air traffic. Eruptions continued until April 21, 1990. Until 1989, Redoubt had not erupted since 1966.

St. Augustine last erupted in 1986. Its eruption, too, sent ash several miles high and disrupted air traffic in south-central Alaska for several days.

Along with Lake Clark's volcanoes stand a frenzy of peaks called the Chigmit Mountains. John Kauffman once described the Chigmits "...as if colliding mountain waves had thrown up a sea of rock". In reality, the Chigmit Mountains were formed as a result of massive intrusions of granite coupled with the uplift of existing rock layers. The intruded rocks are moderately to highly deformed volcanic and sedimentary rocks. Today, we see a spectacular maze of jagged peaks and spires, and broad, U-shaped valleys carved out by glacial action.

At least three major glacial advances and retreats sent great masses of ice slowly but meticulously creeping down the mountains, carving and grinding out the valleys we see today. The last major advance of glaciers was during a period of geologic time called the Pleistocene, or Great Ice Age. During the Pleistocene, the overall temperature of the world was much cooler than it is today. Significant snow packs built up, creating great ice sheets that at one time covered over 50% of Alaska.

About 12,000 years ago, these great ice sheets and glaciers retreated as the world's climate experienced a warming trend. The awesome and spectacular landscapes we see throughout the park and preserve today are remnants of this period. In the western foothills, glaciers once pushed out onto the interior plains. Retreating at a later time, the glaciers filled their former beds with meltwater and created the remarkable jewel-like lakes that rank down the western side of the park and preserve. From north to south are Two Lakes, Telaquana Lake, Turquoise Lake, Twin Lakes, Lachbuna Lake, Lake Clark, Kontrashibuna Lake, and Tazimina Lakes.

Like rivers, glaciers move down slope under the influence of gravity and flow along the path of least resistance. Although glaciers in the park are now retreating, their ice movement is still down the mountain. A glacier is said to be retreating if the rate at which it melts is greater than the rate at which it moves down slope.

A glacier is a complex and dynamic system, continuously changing in response to fluctuations in temperature, precipitation, and other geologic processes. The future of glaciers and glaciation is, of course, unknown, but we do know that it takes only minor increases in precipitation and changes in temperature to cause significant build-up in the snow packs of existing glaciers.

For a glacier to exist, there has to be the right combination of temperature, elevation, moisture source, and area for accumulation of snow pack. The Chigmit Mountains provide the right combination of climatic and geographic features.

Warm air moving over the Pacific Ocean picks up moisture and carries it north. Eventually, the moist air is forced up the rugged Chigmit Mountains where temperatures are much cooler. As the marine air cools, it drops it's moisture as snowfall on the upper elevations, continually recharging the snow fields that feed the glaciers. As the weight and pressure on the buried layers builds, the snowflakes merge together to form ice. When over a period of years more snow and ice accumulates than is lost to melting, the glacier is said to be advancing.

Glacial ice appears blue because the crystal structure and physical characteristics of the water molecules absorb all of the light spectrum except blue. The ice reflects back that portion of the light spectrum which constitutes the blue color.

Many rivers start at the base of glaciers and flow through braided channels in the glaciers gravel debris.

At Lake Clark, glaciers are the dominant architects at work. Tremendously heavy and sharp glaciers tear, shear, and rip rock material away from the mountain and valley sides, transport the debris, and eventually dump it into piles called terminal moraines. Lateral moraines are piled up on each side of a glacier and are composed of plucked rock material from the valley walls, and rock that avalanches onto the ice surface. When two valley glaciers join, two lateral moraines merge to create a medial moraine in the middle of the combined glacier. At the foot of a glacier, a person can count the number of medial moraines and determine how many glacial valleys feed the main glacier.