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.
It is located in the southwest corner of Four Craters Lava Field in the Deschutes National Forest. Hikers can walk the length of the main crack and explore its tributaries.