A Brief Guide to the Parapet View, Sinnott Memorial, Crater Lake National Park by John Campbell Merriam, 1938
View 5: Broken Edges of Layers on Crater Slopes Indicate Widening of Crater in All Directions by Breaking Away of Walls
If the activity of a volcano diminishes slowly, growth of the mountain may end in forming a symmetrical cone. If activity continues by spasmodic outbursts, explosions may blow away a considerable part of the peak. Other conditions may bring about undermining of the walls in such manner as to produce a wide cauldron-like crater, but without tremendous explosions.The edges of rock layers inside the crater wall are clearly exposed because they have been sharply broken around the entire rim region. This fracturing took place in course of widening the crater. Increase in size of the opening at the summit of the mountain may have been caused by tremendous explosions, or by collapse of the peak, or by combination of such activities. The precise nature of the story is not yet fully known.