The Caloris Basin. (NASA, JPL, Mariner 10, Planetary Photojournal)
Photomosaic of Caloris Basin area (the Basin is at the left), a gigantic impact structure bearing a strong resemblance to Mare Orientale, on the Moon. Violent shocks associated with the impact that formed the Basin created ringed structures all around it, and the sudden removal of large amounts of surface material allowed molten material from below to well up, and fill in the Basin, with still obvious lava flows. The large number of radial streaks bears testimony to the huge numbers of rocks, a mile or more in size, thrown outwards in all directions, gouging the surface where thrown horizontally, peppering it in all directions where thrown at higher angles.
The name of the Basin refers to its position on Mercury. As discussed in The Rotation of Mercury, Mercury has a very peculiar rotation, which is locked to the Sun not like our Moon, which always keeps one face to the Earth, but in such a way that every other orbit, one side or the exactly opposite side of Mercury faces the Sun. At perihelion, the Caloris Basin and the chaotic area opposite it (see the image below) are facing the Sun, and are heated to more than 800 Fahrenheit degrees; whereas at aphelion, areas exactly a quarter-way around the planet face the Sun, and are only heated to a little less than 600 degrees.
A false-color image of the Caloris Basin created from Mercury MESSENGER images. The lava fields filling the Basin are of clearly different material, than those in the surrounding regions. Dark blue areas within the Basin are moderately old craters, while bright white splashes are younger craters. Orange areas near the Basin's periphery are volcanic structures, probably associated with deep fissures produced by the gigantic impact which created the Basin. The Basin is estimated to be about 3.8 billion years old, and is thought to be the youngest large impact structure in the Solar System. (NASA, Carnegie Institution, APL, Mercury MESSENGER)
Chaotic terrain thought to be caused by severe shaking associated with the formation of the Caloris Basin. As the shock waves from the formation of the Basin passed through Mercury, the planet's relatively large core would have served as a lens, focusing the energy of the shock waves on an area directly opposite the basin -- which is exactly where this chaotic terrain is found. The image above is a closeup of the right central portion of the image below. (Mariner 10, NASA, Mariner 10 Image Archive)