The eruptions from the Four Craters Lava Field – four pyroclastic cone volcanoes formed during the Pleistocene or early Holocene –created a sinking of the older heavier rock, forming a shallow depression about 3 kilometers wide extending southward into an old lake basin.
“Crack-in-the-Ground” is a volcanic fissure over two miles long and up to 70 feet deep, marks the western edge of this small, volcano-tectonic depression.
The crack is the result of a tension fracture – a rock fracture at right angles to the direction of the most tension – the result of lava flows coming over the edge of a concealed fault zone.
