There are many volcanoes on Mars. So many, that the planet is broken down into volcanic provinces for easier reference. Quite a few of them are very large because the planet has not had tectonic plate action for billions of years, so a single hotspot could flow unabated for millenia.
Olympus Mons is a shield volcano on Mars and it is the largest volcano in the Solar System. Located in the Tharsis region of the planet along with three other large volcanoes, Olympus Mons measures an incredible 27 km in height and is 3 times taller than Mount Everest. It is about 500 km in diameter. The mountain was formed from a single hotspot that flowed for thousands, if not millions, of years. The lack of plate tectonics that allowed this unhindered flow also prevented massive pressure buildups that would have blown the top off of the volcano, decreasing its overall height.
In the northern part of the Tharsis volcanic province is Alba Mons also known as Alba Patera. It is a unique volcanic structure for several reasons. The volcano features unnaturally low slopes formed by numerous and extensive lava flows. Its slopes are a mere 0.5 degrees. It has a double caldera feature with the central figure being 350 km wide and 1.5 km high. Flows from Alba Mons seem to extend 2,000 km north-south and 3,000 km east-west. The widespread flows make this one of the largest volcanoes in the Solar System by area. Some scientist point to the volcano’s antipodal location to the Hellas impact basin as a possible reason its formation. Seismic waves from the impact may have traveled through the planet causing a weakening of the crust at the point of origin for Alba Mons.
In the Elysium volcanic province there are three main volcanoes. The province covers an area that is about 2,000 km in diameter. The main volcanoes are Elysium Mons, Hecates Tholus, and Albor Tholus. The northwestern edge of the province is characterized by large channels that emerge from several valleys(grabens) on the flanks of Elysium Mons. The grabens may have formed from the subsurface release of large volumes of ground water. The channels are accompanied by associated sedimentary deposits possibly formed by mudflows. Elysium Mons is 375 km across and 14 km high. Hecates Tholus is 180 km across and 4.8 km high. Albor Tholus, the southern-most of the Elysium volcanoes, is 150 km in diameter and 4.1 km high.
There are many interesting volcanoes on Mars. The NASA source listed below will take you to a list of there Martian volcanoes and many details about each. Good luck with your research.
Here’s a cool slideshow of volcanoes on Mars, and more information about volcanism on Mars.
Finally, if you’d like to learn more about Mars in general, we have done several podcast episodes about the Red Planet at Astronomy Cast. Episode 52: Mars, and Episode 91: The Search for Water on Mars.