THEMIS Image of the Day, August 22, 2018. Tyrrhenus Mons is one of the oldest martian volcanoes and Tyrrhena Fossae is the largest of the channels dissecting the volcano. As well as being one of the oldest volcanoes, it is made of layers that include softer volcanic ash rather than just basaltic flows.
This difference is evident in how the volcano is being eroded, creating broad intersecting sinuous channels. On Earth basaltic flows form broad shield volcanoes like Hawaii. Shield volcanoes can erupt from the central crater, as well as along the flanks.
Volcanoes with ash layers, called composite volcanoes, form steeper sides like Mt Rainier and Mt Fuji. The major amount of material erupts only from the central caldera.