In this report, I will be discussing the formations of tuff rings, tuff cones, and a variety of spectacular geologic features that can be seen in the Southern Oregon area (near Silver Lake); including Hole in the ground, Fort-Rock, and Table-Rock complexes. To begin, we will start with the background of how tuff rings and tuff cones are formed. Hydrovolcanic eruptions are some of the most violent spectacles, each generating hundreds, or even thousands, of explosions throughout the course of its eruption. Each eruption ejects a mixture of clasts, gas, and water droplets which either fall to the ground or evaporate into the atmosphere. These deposits build up rings of bedded tuff around the ...view middle of the document...
Towards the center of the basaltic basin, the features are primarily tuff rings and tuff cones, interpreted by geologists as the result of magma encountering shallow, standing water. More towards the edges of the prehistoric lake, the features are made up of maars and cinder cones, where the magma likely did not encounter any water.
The first major geologic feature that will be discussed is the Hole-in-the ground maar. This maar was formed as a result of the explosive interaction between basaltic magma and the prehistoric ground water that sat on the outskirts of the ancient lake bed. “Geologists date the eruption that formed hole in the ground maar between 13-18ka and estimated the depth of the magma/water interaction to be roughly 300-500m. The floor of the maar is between 112-156m below the ground surface and is rounded by a 35-65m high rim of bedded tuffs” (Brand, 2009). The hole in the ground formation has been studied by many geologists in their quest for more detail on the tuff ring and maars formations in the Southern Oregon area. There are no detailed topographic maps of this geologic feature so the elevations described are a mere estimate, however, the changing elevations and transported sediment is a good predictor of the geologic change occurring in the area. “The diameter of Big Hole measured at the rim varies from 2.1 to 2.4 km. However, the diameter is only 1.6 km at the level of the original surface, which is mapped by the top of older lavas in the maar wall” (Lorenz, 1970).
Since the big hole in the ground was formed, erosion has changed its shape considerably; therefore, when trying to get a sense for the changing formations surrounding this feature, geologists turn to the movement near the original surface, the slopes, stratigraphy, and make-up of the pyroclastic material that is left behind.
“A ledge on the southwestern half of the crater wall marks the original surface. It is formed by basaltic lavas of varying thickness, underlain by a sheet of ignimbrite. It is rather flat, and located in the southwestern part of the maar. It drops off sharply, possibly along a fault or flexure, toward the northeast part. The pyroclastic deposits around the maar are about 60 m thick on the eastern rim, and gradually diminish in thickness away from the crater. The deposits generally dip gently away from the crater except on the northeast wall and on the southwest slope, where inward dips reflect the pre-eruption morphology. North of the maar’s crater, younger lava from the Paulina Mountains, which surround Newberry Caldera, covers part of the Big Hole. This lava may be around 10,000 yrs old” (Lorenz, 1970)! The figure below provides a visual orientation to the variously aged basaltic materials that can be found surrounding this feature.
(Lorenz, 1970) 2
The next geologic feature that I will discuss and analyze is Fort-Rock State Park. Fort rock is a spectacular example of a tuff ring and an angular unconformity. The shallow, outer...