island arc

An island arc is a curved linear chain of volcanic islands produced by the midocean collision of two crustal plates. One plate is gradually subducted, or pushed under, by the other, chiseling a deep trench as it plunges down. Friction and the heat of the Earth’s underlying mantle melt the leading edge of the subducted plate, and where molten rock Islands, extending 1,200 miles southwest from the Alaska Peninsula, make up a classic island arc, bordered by the deep (over 24,000 feet at some points) Aleutian Trench. In Basin and Range, John McPhee explains why the volcanic islands emerge in an arc by comparing an oceanic plate pushing under a continental plate to a knife slicing at a forty-five degree angle into an orange: “The incision becomes an arc on the surface of the orange. If the knife blade melts inside, little volcanoes will come up through the pores of the skin, and together they will form arcs, island arcs.”

Pamela Frierson