After South Ice Cave, we moved onto two other close geological formations in this part of south central oregon. The first is known as Fort Rock, which is an example of a Tuff Ring, and the Second is called Hole in the Ground which is classed as a Maar or Explosion Crater. You will see from the photo's that while the names are not all that original they do fit well. Both are examples of where lava came to the surface and hit some body of water. Normally when lava hits the surface it forms a cinder cone like Pilot Butte in Bend. In cases where it hits water on the way to the surface, the reaction can be explosive.
Fort Rock
From Wikipedia (which sites the original source)
Fort Rock was created when basalt magma rose to the surface and encountered the wet muds of a lake bottom. Powered by a jet of steam, molten basalt was blown into the air, creating a fountain of hot lava particles and frothy ash. The pieces and blobs of hot lava and ash rained down around the vent and formed a saucer-shaped ring (called a tuff Ring) sitting like an island in the lake waters. Steam explosions also loosened angular chunks of black and red lava rock comprising the valley floor. These blocky inclusions are incorporated into the fine-grained tuff layers at Fort Rock. Waves from the lake waters eroded the outside of the ring, cutting the steep cliffs into terraces 66 feet (20 m) above the floor of Fort Rock Valley.
Hold in the Ground
So how was it formed? Basaltic magma intruding near the surface flashed ground water to steam, which blew out overlying rock and soil, along with some juvenile material. As material slid into the hole formed, it closed the vent and the process repeated, eventually forming the huge hole.Blocks as large as 26 ft in size were flung as far as 2.3 mi from the crater.
Watch the video. If you look closely you can see the two boys with red walking down to the center of the crater to give you an idea of the size.