Pic of the day #6 (Island and Volcanic arc)


(a) An Andean-type continental arc, with a compressional backarc region. Here, the volcanic arc grows on continental crust, and compression has generated a fold-thrust belt and “Laramide-style” basement-cored uplifts. Large granitic plutons develop. This situation develops where the velocity of the overriding plate (VOR) is in the same direction and exceeds the rollback velocity (VRB).
(b) A Mariana-type island arc, with an extensional backarc. Here, the volcanic arc grows on oceanic crust, and a backarc basin develops in which there is seafloor spreading. A remnant arc, composed of a riftedoff fragment of the arc may occur in the basin. This situation develops when VOR < VRB, or is in the opposite direction to VRB. The island arc must move to keep up with the rollback.
(c) A Japan-type volcanic arc, in which the island arc has continental basement that had rifted off a continent when a backarc basin grew. Here, the backarc spreading has ceased, and the backarc is stable. This is because VOR = VRB. A strike-slip fault could develop in the backarc region.

The picture shows Island arc (oceanic and continental basement) and continental volcanic arc.

Different kinds of volcanic arcs and backarc regions.

Pic courtesy: Earth Structure – An Introduction to Structural Geology and Tectonics [Second Edition][W. W. Norton & Company][Ben A. van der Pluijm & Stephen Marshak][2004]


  1. In marine type island arch, if VRB and VOR is in opposite direction then why sea floor spreading occurred at back arc basin, why it is not at the trench?. If VRB and VOR is in the same direction then what is the reason of sea floor spreading ?.

    1. The Island arc part of the setting will always try to keep up with the trench (i.e. VRB). So if VOR < VRB or opposite to VRB then, the left hand side of the backarc basin (which is essentially the island arc part) will get pulled away (with velocity VRB) and there will be development of sea floor spreading center as right side is slower or opposite in direction.

      We hope this clears your doubt!

Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.