Mauna Loa's summit is also the focal point for its two prominent rift zones, marked on the surface by well-preserved, relatively recent lava flows (easily seen in satellite imagery) and linearly arranged fracture lines intersected by cinder and splatter cones. These rift zones are deeply set structures, driven by dike intrusions along a decollement fault that is believed to reach down all the way to the volcano's base, deep. The first is a rift trending southwest from the caldera to the sea and a further underwater, with a prominent 40° directional change along its length; this rift zone is historically active across most of its length. The second, northeastern rift zone extends towards Hilo and is historically active across only the first of its length, with a nearly straight and, in its latter sections, poorly defined trend. The northeastern rift zone takes the form of a succession of cinder cones, the most prominent of which the high Puu Ulaula, or Red Hill. There is also a less definite northward rift zone that extends towards the Humuula Saddle marking the intersection of Mauna Loa and Mauna Kea.