Magma rising from a subduction zone between the North American and Pacific tectonic plates formed the Aleutian Islands, off the southwestern tip of Alaska. As the magma neared the surface, it cooled to form rock. The accumulation of cooled magma grew above the water's surface over time, forming the islands.
Where the Pacific and North American plates meet, the Pacific plate subducts, or travels below, the North American plate. The Aleutian Islands are just one formation created by this subduction zone. The Aleutian range of volcanoes is another by-product of this zone. The lower the plate goes, the more melting comes from the heat inside the earth.
The Pacific plate constantly moves in a northwest direction, while the North American plate travels southward. Subduction takes place at approximately 6 to 8 centimeters per year at the middle and northern intersection points of the plates. This means that more of the Pacific plate heads toward the earth's core each year, creating the possibility of increased magma flow.
The Aleutian Islands are part of the Ring of Fire, a series of fault lines and volcanic formations along the western coast of North America and the eastern coast of Asia.
Although the Aleutian volcanoes are active, the biggest volcanic explosion in recent history took place in 1911. Mount Katmai erupted on Alaska's southern peninsula, emptying its magma chamber and eventually creating a caldera, which is now the site of a warm crater lake.