The Volcanic Marianas Islands form west of the Marianas trench because plates are formed by underwater volcanoes which will move gently to the west.
They formed to the west of the trench because that is where the submerged mountain range is. The islands are volcanic due to the water the islands carry under the Mariana plate.
An ocean trench does not become a volcanic island, however volcanic islands often form near ocean trenches.
The Marianas trench .
The chain of volcanic islands that form parallel to a deep ocean trench is known as an island arc. These arcs are created by the subduction of one tectonic plate beneath another, leading to melting of the mantle and formation of magma that rises to the surface. Common examples include the Aleutian Islands in Alaska and the Japanese archipelago. Island arcs are characterized by their volcanic activity and often have a curved shape.
The Marianas trench .
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There are volcanoes along the floor of the ocean that form a trench. When these volcanoes erupt, it can cause tectonic movements.
The Aleutian Trench was formed through the process of subduction, where the Pacific Plate is being forced beneath the North American Plate. This creates a deep trench in the ocean floor, and can lead to the formation of volcanic islands along the subduction zone.
By volcanic activity,
Volcanic arcs form at plate subduction zones. Island arcs are volcanic islands that form over "hot spots" in the Earth's mantle. Because the islands are moving with the oceanic plate, they eventually are removed from the hot spot, forming a chain of islands in the direction of the plate movement.
Volcanic islands are often found at convergent plate boundaries where one tectonic plate is subducted beneath another. This subduction process can lead to the formation of volcanic arcs, such as the islands of Japan, Indonesia, and the Aleutian Islands.
A chain of 14 large volcanic islands and 55 smaller ones form the Aleutian Islands.