Very large impact structure, typically many tens to hundreds of kilometers in diameter, consisting of one or more concentric rings of peaks. Impact basins are proportionally shallower relative to their diameter compared with simple and complex craters. Basins are subdivided into Exposed core of uplifted rocks in center of a complex impact crater. Central peak material typically shows evidence of intense fracturing, faulting, and shock metamorphism., Central uplift characterized by a ring of peaks rather than a single peak. Peak rings are typical of larger terrestrial craters above ~50 km in diameter. Barton Crater on Venus. This 54-km (32-mi) diameter crater is the size at which craters on Venus begin to possess peak-rings instead of a and multi-ring basins. Mare Orientale on the Moon (photograph) is perhaps the archetypal multi-ring impact basin. Orientale contains four concentric rings with the inner ring bordering the mare Basalt is the most common extrusive igneous rock on the terrestrial planets. For example, more than 90% of all volcanic rock on Earth is basalt. The term basalt is applied to most low viscosity dark silicate lavas, regardless of composition. Basalt is a mafic, extrusive and fine grained igneous rock fill. The outer ring, known as the Cordillera Mountains, has a diameter of 920 km. The main two internal rings form the Rook and Inner Rook Mountains. The mountains in the rings rise up to 2–5 km above the surrounding plain.