Map of Martian volcanism and tectonics

Planets are born from tiny grains of dust, metal and ice. These form fluffy particles and grow gradually by sticking together. Once they reach a size of 300 km, they become hot enough to melt and turn spherical, like drops in weightlessness. They continue growing until the available material is used up. Their surface is then a magma ocean. Later, the cold of space allows a solid crust to form. Stress in this crust breaks the rock and creates cracks. The interior can remain molten for billions of years before cooling and solidifying. Volcanism stops when the mantle beneath the crust solidifies. Small planets reach this stage quickly; larger ones remain hot inside longer. The close moons of gas giants are heated by a different process: tides.

Tectonic processes release stresses in the solid crust and create fractures or faults, much as an object breaks when stressed. This happens during earthquakes. Crustal stress arises when the whole crust cools and contracts or when mantle currents act on it. Parts of the crust can be compressed and lifted along fractures, creating features such as wrinkle ridges, while elsewhere the surface subsides, as in the northern lowlands. Between two faults, the surface can drop to form a long, flat-bottomed trough called a graben.

Molten magma emerges through volcanoes; at the surface it is called lava. It may build cones or shields, or spread into lobate flows and extensive lava plains, depending on its fluidity and the slope. Lava cools quickly and becomes hard rock. Lava plains cover older surfaces and hide traces of their earlier history.

Kenneth Tanaka and colleagues mapped the faults and volcanic regions at the US Geological Survey (USGS).