The oldest rocks in this region include silicic (rich in silica) lava domes from the late Miocene or early Pliocene, which lie near the province's eastern and southern borders, respectively. Volcanism in this area suggests a progression of silicic eruptions, known as the Newberry Trend, that moves in a northwest-trending direction from the Harney Basin to Newberry Volcano and the rest of the Cascade Range. Newberry Volcano's origins are somewhat controversial; some scientists think it originated from an independent hotspot, but overwhelming evidence suggests that it is part of the Cascades Arc and was produced by the subduction of the oceanic Juan de Fuca and Gorda tectonic plates under the continental North American Plate. However, Newberry Volcano has been transformed by tectonic processes, possibly related to subductive mechanisms that enhance melting of the Juan de Fuca tectonic plate. The High Lava Plains Trend, or the Newberry Trend, moves at an oblique angle to the underlying North American tectonic plate, for which subduction counterflow, gravitational flow along the lithosphere's base, faulting, and extension of the Basin and Range Province have all been proposed as possible mechanisms. At Newberry, the subducting plate has a depth that is shallower than elsewhere in the major crest of the Cascades,[b] accounting for its unique magmas. Newberry Volcano is likely fed by a magma chamber under the large, cauldron-like caldera at its summit. This caldera has dimensions of and formed about 75,000 years ago.