The Mid-Tertiary ignimbrite flare-up,[a] was a dramatic period of volcanic eruptions in mid-Cenozoic time, approximately 25–40 million years ago, centered in the western United States.

The total volume includes 5x105 km3 of ash flow tuff and 5x106 km3 of intermediate and silicic lava.

The primary tectonic driving force behind this explosive volcanic activity is slab rollback.

When this stopped, the mantle wedge was opened up, and the result was the flare-up.

The specifics of this opening, including possible slab rollback,[5] slab windows,[6] or buckling of the plate,[7] can explain specific volcanic trends within the flare-up.