The Cameroon Volcanic Line (CVL) is an enigmatic structure that defies common dynamic models of melt generation and volcanic activity on Earth. There, magma generation and intrusion has been sustained for more than 70 Myr over a 1600 km long chain straddling the ocean–continent boundary, with no detectable spatial age progression. The chain is nearly perpendicular to the coastline and terminates in a Y-shaped structure that has not been affected by absolute plate motions, implying that the mantle upwelling that feeds magmatic activity is attached to the continent. We propose that this form of volcanism is due to a new type of instability that may develop within the subcontinental lithospheric mantle at the edge of a continent. Laboratory experiments document how lithosphere beneath a continental block of finite size can become unstable due to cooling from above. The instability pattern is made of linear upwellings and downwellings that converge radially towards the center of the continent in an outer region and an array of polygonal cells in a central region. The pattern is characterized by branching structures that are reminiscent of the strike and Y-shaped outline of the CVL. The instability develops over long timescales with small rates of upwelling and melting, and is attached to the continent by construction. Downwellings adjacent to upwellings induce compression in the crust, which may account for deformation in the Benue trough just before the onset of CVL magmatism.
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