At the base of Olympus Mons... (edm - Natural Colors; credits: Lunar Explorer Italia)

The Ridge with 2 Knobs in the center of this edm is of special interest. The Lava Flows radiate away from these Knobs, indicating that the Flows were fed from these points. However, based on experience with the current eruption of Kilauea Volcano in Hawaii, it seems that these Vents were fed by a Lava Tube.
Thus the magma first reached the Surface some distance away, then was transported through a Tube to this location. A blockage in the Tube would force the lava to burst out and erupt onto the Surface.