Arsia Mons: This shield volcano is part of an aligned trio known as the Tharsis Montes. Arsia Mons is rivaled only by Olympus Mons in terms of its volume. The summit of Arsia Mons is more than 9 kilometers (5.6 miles) higher than the surrounding plains. The caldera (crater )at the volcano summit is approximately 110 km (68 mi) across.