Complex sentence: The eruptions from the Four Craters Lava Field were accompanied by a slight sinking of the older rock surface, forming a shallow, graben-like structure about 3 kilometers wide and extending to the south into an old lake basin.
Split into simpler sentences: The eruptions from the Four Craters Lava Field were accompanied by a slight sinking of the older rock surface. This shallow, graben-like sink is about 3 kilometers wide and extends to the south into an old lake basin.