In the reduced size screen shot above, Olympus Mons is the large white volcano above and to the left of the three volcanoes in a row. It is the largest know volcano in our solar system at 374 miles in diameter (approximately the size of Arizona). It is 16 miles high with a caldera at the summit measuring 50 miles wide. NASA has more info on Olympus Mons here.
Google Earth, Olympus Mons closeup, NASA JPL