Alaska
The Aleutian Islands stretch in a 1200-mile-long arc from Russia’s Kamchatka Peninsula eastward to the Alaska Peninsula, marking the division between the Bering Sea to the north and the North Pacific Ocean to the south. The island arc is made up of fourteen large volcanic islands and fifty-five smaller ones, the product of ongoing plate tectonic collisions along the Pacific Ring of Fire (a string of volcanic and seismic activity around the Pacific Ocean rim). From the air, the easternmost islands, near the Alaska Peninsula, are clearly the largest, with the tallest volcanoes.
These islands were formed by the same geologic processes (along the interface between two major tectonic plates) that created the Aleutian Mountains on the Alaska Peninsula. The landscape here continues to be active, as the Pacific Plate dives under the North American Plate, forming a convergent plate boundary. The Aleutian Trench, which runs along this convergence, has been measured to depths over 25,000 feet. As the heavy, waterlogged Pacific Plate is subducted, increasing heat and pressure release the stored water in the downward-heading plate, partially melting the descending and overriding plates. This low-density melt rises to the surface and erupts along a chain of volcanoes on the North American side of the plate boundary. Earthquakes along this trench generate shaking up to magnitude 8.7 and tsunamis that travel as far as Hawaii and South America.
As a reminder that geology isn’t just ancient history, the Aleutian volcanoes continue to erupt and present significant threats, including ash clouds that can damage aircraft and cause jet engine failure. In 1988, the Alaska Volcano Observatory was formed in Anchorage to monitor the Aleutian Islands as well as the state’s other active volcanoes. In recent years, Mount Cleveland and Mount Pavlof have been the most active in the Aleutians, erupting every five to ten years.
The Aleutian Islands separate the North Pacific Ocean from the Bering Sea, following a convergent plate boundary. Shown here is the eastern portion of the island arc, off Alaska.
What is now the eastern end of the Aleutian Islands also helped set the scene for the first migration of humans to North America. During the last ice age—which reached its peak about 20,000 years ago and ended approximately 11,500 years ago—the Bering Land Bridge connected Russia and Alaska. Back then, huge quantities of the planet’s water were frozen in mammoth continental ice sheets, lowering sea levels and exposing the land bridge for a period of a few thousand years. Sometime between 20,000 and 15,000 years ago, the first people made their way into North America via this bridge, migrating from Asia into the New World. At that time, the modern-day Alaska Peninsula and eastern Aleutian Islands were part of the southern edge of this exposed passageway.
Look for a chain of mountainous volcanic islands curving to the southwest from the Alaska Peninsula on flights between the United States and Asia.
