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.

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 Lake Clark National Park and Preserve. 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.

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.

Like precarious stepping stones, the Aleutian Islands span the seas between the New and Old worlds - reaching westward from the Alaska Peninsula to within 500 miles of the Asian peninsula of Kamchatka. Situated between the Bering Sea and the Pacific Ocean, along the seam of the Pacific and American geologic plates, this 1,100 mile long archipelago has been, and continues to be, the focus of climatic and tectonic events. The Aleutian Chain's foundation of shifting geologic plates results in active volcanism and earthquakes - the birth processes of the islands themselves. The Aleutians betray their violent origins in their rugged landscape: mountainous terrain, precipitous coastlines, and black sand beaches. It is thought that at least twenty-six of the Chain's fifty-seven volcanoes have erupted within the past two centuries.

The 15 active volcanoes that line the Shelikof Strait make Katmai National Park and Preserve one of the world's most active volcanic centers today.  These Aleutian Range volcanoes are pipelines into the fiery cauldron that underlies Alaska's southern coast and extends down both Pacific Ocean shores--the so called Pacific Ring of Fire.  This Ring of Fire boasts more than four times more volcanic eruptions above sea level than any other region in historic times.

Nearly 10 percent of these more than 400 eruptions have occurred in Alaska; less than two percent in the rest of North America. The Ring of Fire marks edges where crustal plates bump against each other.  Superimposing a map of earthquake activity over a map of active volcanoes creates a massed record of violent earth changes ringing the Pacific Ocean from southern South America around through the Indonesian archipelago.

The Aniakchak Caldera is the result of a series of eruptions, the latest in 1931 that took place in Aniakchak National Monument and Preserve. Nearly six miles in diameter and covering some ten square miles, it is one of the finest examples of dry caldera in the world. Aniakchak’s' outer slopes are characterized as having sparse vegetation, barren ash flows, precipitous cliffs, and tilted rock strata. The interior of the caldera contains examples of almost every kind of volcanic feature: lava flows, areas of unusually high ground temperature, cinder cones, a lava plug, warm springs, explosion pits, and layers of volcanic and sedimentary rocks exposed by volcanic action. Vent Mountain, one of the cinder cones, is unusually high at 2,200 feet above the caldera floor. Cinder cones rarely exceed 1,000 feet in height. In the top of the Vent there is a crater about 2,000 feet in diameter. Other cinder cones in the caldera are over 200 feet high. The 1931 volcanic eruption, which probably took place in the southwestern section near Half Cone, added to the ash blanket in the vicinity of the volcano. Since 1931, the volcano has not been known to be active, though a U.S. Geological Survey researcher found areas of high-ground temperatures in the western portion of the caldera. This, plus the warm springs that are feeding Surprise Lake, indicate potential for future volcanic activity.

The event which heralded the doom of Mt. Mazama almost 7,000 years ago, and the beginning of Crater Lake, was the opening of a vent somewhere on the north side of the mountain. A column of ash and pumice was sent up by the volcano, creating a blanket of debris 20 feet thick in places. As the pressure of the underground magma grew, a series of other vents around the mountain opened up. Enormous quantities of pyroclastic, or molten rock composed of pumice, material were released. These lava flows traveled up to 25 miles beyond the base of the volcano. As the volcano emptied itself of molten rock, an empty chamber was left underground. The mass of the mountain collapsed in on this void within a matter of days after the eruption. What was left, a 4,000 foot deep caldera and a myriad of other geologic formations, have awed and inspired people for generations. Following the collapse of Mount Mazama, lava poured into the caldera even as the lake began to rise. Today, a small volcanic island, Wizard Island, appears on the west side of the lake. This cinder cone rises 760 feet (233 meters) above the lake and is surrounded by black volcanic lava blocks. A small crater, 300 feet (90 meters) across and 90 feet (27 meters) deep, rests on the summit. The crater is filled by snow during the winter months, but remains dry during the summer.

Mount St. Helens erupted at 8:32 Sunday morning, May 18, 1980. Shaken by an earthquake measuring 5.1 on the Richter scale, the north face of this tall symmetrical mountain collapsed in a massive rock debris avalanche. Nearly 150 square miles of forest was blown over or left dead and standing. At the same time a mushroom-shaped column of ash rose thousands of feet skyward and drifted downwind, turning day into night as dark, gray ash fell over eastern Washington and beyond-to Portland, OR 45 miles away, and 16 hours later, to central Colorado. The hot gas and magma melted the snow and ice that covered the volcano. The resulting floodwater mixed with the rock and debris to create concrete-like mudflows that scoured river valleys surrounding the mountain resulting in the largest landslide in recorded history. The eruption lasted 9 hours, but Mount St. Helens and the surrounding landscape were dramatically changed within moments.

The most isolated major island group on earth, the Hawaiian archipelago is 2400 miles (3862 km) from the nearest continent and has never had connection to any other land mass. They were formed as the Pacific Plate moved across a volcanic “hot spot” within the earth’s mantle. Hawaii Volcanoes National Park encompasses diverse environments that range from sea level to the summit of the earth's most massive volcano, Mauna Loa at 13,677 feet. Kilauea, the world's most active volcano, offers scientists insights on the birth of the Hawaiian Islands and visitors views of dramatic volcanic landscapes. Haleakala National Park is renowned for its inspiring volcanic landscapes. These amazing landscapes result from the constant clash of the constructive force of volcanism and the destructive forces of erosion. Haleakala is a shield volcano that has been above the ocean surface for about 1.5 million years. Haleakala is considered an active volcano and is monitored remotely through equipment which sends information to the Hawaii Volcanoes Observatory on the Island of Hawaii.