The northern hemisphere is quite different. It has few craters and these appear far less eroded indicating they are much younger than those of the southern hemisphere. The northern hemisphere shows much geologic activity. The surface has been smoothed by repeated lava flows indicating a great deal of volcanic activity. There is also a lot of deformed uplifted crustal sections along with collapsed depressions.
Martian volcanoes are the shield type like the Hawaiian islands on Earth.
In these type, the lava flows freely and the lava flow can cover a tremendous area. The largest volcano on Mars is Olympus Mons. Mauna Loa is so heavy it has actually sunk into the Earth's crust and formed a moat around itself. Olympus Mons does not show any sign of sinking and leads to the conclusion that the Martian crust is much thicker than Earth's. The entire region around Olympus Mons appears to be an uplifted volcanic plain, called the Tharsis region.