Backarc basins explained

What is a back-arc?

When two tectonic plates collide, the denser of the two plates (almost always an oceanic plate) is pushed under or "subducted" beneath the other along a linear region called a "subduction zone". When the subducted plate reaches a depth of 100-120km, it releases fluids into the overlying mantle, lowering the melting temperature of the mantle and producing melt. Because water is a major component of these fluids, this process is called "hydrous melting". Because the melt is less dense than the surrounding mantle, it rises to the surface and forms a line or "arc" of volcanoes, many of which are made of a rock called andesite. The region between the volcanic arc and the subduction zone is called the "fore-arc", while the region behind the volcanic arc away from the subduction zone is called the "back-arc".

What is a basin?

A basin is a bowl-shaped depression in the landscape, often associated with tectonic extension or stretching and thinning of the earth's crust (you can use chewing gum to model the relationship between stretching and thinning). At the center of many ocean basins, a "mid-ocean spreading center" marks the line where the oceanic crust is splitting apart. As the crust thins and moves outward from the spreading center, material from the Earth's mantle moves upward to fill the gap. Upward motion decreases the pressure on this material, causing it to melt. This process is called "decompression melting" and forms a different type of rock, called "basalt". We commonly find lots of basalt on the bottom of the ocean.

What is the Lau back-arc basin?

You can think of a back-arc basin as the combination of a subduction zone and a mid-ocean spreading center, where both hydrous melting and decompression melting can occur. 43 million years ago, the subduction zone separating the Australian and Pacific Plates started to retreat eastward in a process called "trench rollback". This movement opened up a triangle-shaped zone of extension in the Western Pacific. Around 5 million years ago, the Lau back-arc basin began to open up as the most recent area of extension. North of ABE vent field, farther from the arc, rocks are similar to the basalts formed at mid-ocean ridges by decompression melting. South of ABE, closer to the arc, rocks are more similar to those erupted at arc volcanoes, a result of the increasing effect of hydrous melting near the arc.
Contributed Guy Evans


  1. You said the subducted plate releases fluids thus lowering the melting temperature of the mantle. Is this the same as (or analogous to) the way a solute lowers the freezing point of water? Is the (mostly) water from the subducted plate the solute while the rock of the mantle is the solvent? That would be amazing. Or am I reading this wrong?
    M. Callaghan

  2. Sorry for the late reply. The addition of water disrupts the crystal structure of silicate minerals in the mantle, lowering the melting/freezing point. Likewise, the addition of a solute disrupts the crystal structure of ice and also lowers the melting/freezing point.