The gravity and magnetic anomalies over the western Snake River Plain suggest that the plain is underlain by a layer of dense magnetic rock that may be the Miocene Columbia River Basalt Group and by a thin upper crust. The western Snake River Plain may have formed as a rift in Miocene time in response to northeast lateral spreading with the accumulation of more than a kilometre of basalt in the rift. The plain apparently is now in approximate isostatic balance with adjoining highlands.

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