As lava is being extruded from the volcanic vent, the surface begins cooling immediately. When the viscous lava in the flow is pushed up through vents and cracks in the solidifying flow, the resultant formations are called "squeeze-ups." See Harold S. Colton and Charles F. Park, Jr., "Anosma or "Squeeze-Ups,"" Science (N.S._ Vol. LXXII (1930), p. 579. The above squeeze-up is visible in the middle of the collapsed lava tube. It is a slightly lighter color than the surrounding rock and has a small pine tree growing on the side.
Here is a closeup of the part of the picture with an arrow:
Squeeze-ups are common in lava flows throughout the world. Some of the formations are hollow due to the lava draining out of the structure shortly after the formation. At Sunset Crater, where the above pictures were taken, only about 25% of the magma was erupted as lava flows, the rest erupted explosively as scoria. The smaller particles built the cone shaped mound we call Sunset Crater.
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