Lava Butte, craning across the front seat, and looking out Dana's window. The rubble in the foreground is the aa aa lava flow that emerged from the south side of the cone during the latter stages of this eruption. As the magma works toward the surface, the more volatile-rich fraction tends to be toward the top, so the initial stage of the eruption blows off a lot more gas. Hence the cinders. Later, as less gas-rich magma becomes dominant, the lava erupted isn't blown out the vent, and will often push through the side of the cone as one or more flows. This is a very common pattern of behavior for cinder cones. Lava Butte is simply a very accessible and well-exposed example.