FORMATION of Volcanic Arcs 

volcanic arcs in SUBDUCTION ZONEs

Volcanic arcs
Volcanic arcs are volcanic mountain ranges that form in subduction zones. They form a belt of stratovolcanoes on the continental side of a subduction zone. The Three Sisters are stratovolcanoes in the Cascade Range.

Origins of the term supervolcano
The Three Sisters in the picture below was the subject of a debate in the early 20th Century.  A geologist suggested the Three Sisters and several other peaks were once a giant volcano that he named Mount Multnomah. 

Byers, a geologist,  in 1949 reviewing a book about the Oregon volcanoes first used the term supervolcano when referring to Mount Multnomah. Geologists have determined that each of the mountains grouped together as Mount Multnomah are separate volcanoes and not one large volcano that was destroyed in a climactic eruption.

Three Sisters part of the Cascade Volcanic Arc in Oregon

Three Sisters are part of the Cascade Volcanic Arc    Myrna Martin

Formation of stratovolcanoes

A volcanic chain of stratovolcanoes is created in subduction zone when a continental plate and an oceanic plate converge. The heavier oceanic plate bends and moves slowly into the mantle. The water soaked oceanic crust begins to melt as it descends beneath the ocean floor.

Molten rock above melting plates
The molten rock is hotter than surrounding rock in the lithosphere making it buoyant. Large plutons full of magma, molten rock, form above the melting plate. These plutons feed volcanoes on the continental side of the subduction zone.

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Volcanoes in Cascadia Subduction Zone

Cascadia Subduction Zone
The Cascade Range is the result of the Juan de Fuca Plate subducting beneath he North American Plate. The Cascadia Subduction Zone that has formed between the two plates is approximately 700 miles long. The fault line runs from British Columbia in the north to the Mendocino Triangle in northern California.

Cascade Range volcanoes
Mount Rainier, Mount Saint Helens, Mount Hood, and Mount Shasta are four of the stratovolcanoes that make up the Cascade Range. The mountain that have formed parallel to the Cascadia Subduction Zone on the continental side of the subduction zone.

Aleutian Islands are an arc of volcanoes

Aleutian Arc
The Aleutian Arc is located in a subduction zone with the Aleutian Trench as the boundary line between the North American Plate and the Pacific Plate.

Aleutian Islands
The Aleutian Islands formed on the north side of the Aleutian trench.The entire Aleutian Arc is 1500 miles long with over 100 volcanoes and extends from the Kamchatka Peninsula in Russia to Southeast Alaska.

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