mantle. Convection stopped when about 80% of the material had crystallized |- |colspan=2|Extrusion of the remaining molten material to form the crust, either as basaltic lavas in progressive eruptions, or possibly forming a short-lived magma ocean. |- |colspan=2|The deeper layers of the crust crystallize to form plutonic rocks, whereas older basalts are metamorphosed due to the pressure of newer surface layers. |- |colspan=2|Slow cooling of the interior |} Vesta is the only known intact asteroid that has been resurfaced in this manner. Because of this, some scientists refer to Vesta as a protoplanet. However, the presence of iron meteorites and achondritic meteorite classes without identified parent bodies indicates that there once were other differentiated planetesimals with igneous histories, which have since been shattered by impacts. {|class=wikitable |+Composition of the Vestan crust (by depth) |- |A lithified regolith, the source of howardites and brecciated eucrites. |- |Basaltic lava flows, a source of non-cumulate eucrites. |- |Plutonic rocks consisting of pyroxene, pigeonite and plagioclase, the source of cumulate eucrites. |- |Plutonic rocks rich in orthopyroxene with large grain sizes, the source of diogenites. |} On the basis of the sizes of V-type asteroids (thought to be pieces of Vesta's crust ejected during large impacts), and the depth of Rheasilvia crater (see below), the crust is thought to be roughly thick. Findings from the Dawn spacecraft have found evidence that the troughs that wrap around Vesta could be graben formed by impact-induced faulting (see Troughs section above), meaning that Vesta has more complex geology than other asteroids. Vesta's differentiated interior implies that it was in hydrostatic equilibrium and thus a dwarf planet in the past, but it is not today. The impacts that created the Rheasilvia and Veneneia craters occurred when Vesta was no longer warm and plastic enough to return to an equilibrium shape, distorting its once rounded shape and prohibiting it from being classified as a dwarf planet today. Regolith Vesta's surface is covered by regolith distinct from that found on the Moon or asteroids such as Itokawa. This is because space weathering acts differently. Vesta's surface shows no significant trace of nanophase iron because the impact speeds on Vesta are too low to make rock melting and vaporization an appreciable process. Instead, regolith evolution is dominated by brecciation and subsequent mixing of bright and dark components. The dark component is probably due to the infall of carbonaceous material, whereas the