GSOC Journey to Maars
by Janet Rasmussen
Richard "Bart" Bartels and Bev Vogt arranged a spectacular field trip through the amazing geology of Central Oregon between LaPine and Lakeview, for the Geological Society of the Oregon Country (GSOC). We met in LaPine on Friday morning, June 27, 2008, and headed out. We were in for a hot weekend, with temperatures above 90 degrees. Most of us planned to camp the next two nights at primitive campgrounds in the area.
Our first stop was near milepost 30 on Hwy. 31 to collect specimens of a beautiful pinkish rock called Peyerl Tuff. It's made of glassy pink volcanic ash and pumice, which was so hot as it fell that it welded together, with the pumice slightly flattening.
The author standing at the edge of Hole in the Ground, a maar volcano. It's nearly a mile across and about 360 feet deep, and was formed about 50,000 to 100,000 years ago. Over 10 years ago, when I visited this place for the first time with Carter Kennedy, my daughter and small grandson, some folks were firing bowling balls from the rim into the crater with home-made cannons. Later that day, we had the misfortune to be hiking up the trail from the crater below at the same time that they wanted to do some target practice with rifles; so they simply fired over our heads. It was a disturbing experience.
From the GSOC Field Guide, written by Richard Bartels and Beverly Vogt, is this explanation of maar volcanoes:
"Maar volcanoes involve the interaction of rising basaltic magma and either groundwater or surface water. This mixing results in violent steam explosions. The depth of interaction and the continued availability of water control the type of feature that forms at the surface (maars, tuff cones, or tuff rings). If the mixing is at a shallow depth, the steam explosions can blast away part of the overlying bed rock to form an explosion crater known as a maar. These explosions produce a crater that extends well below original ground level and is surrounded by low rims of explosion debris. Slumping into the crater can enlarge the diameter of the crater. Hole-in-the Ground is a fine example of a maar."
From the maar, we visited Ft. Rock, a nearly one mile diameter tuff ring in the Ft. Rock basin, site of an ancient lake. From Bart and Bev's field trip guide, here is the explanation of a tuff ring:
"Tuff cones and tuff rings form by the same process but the depth of interaction is great enough to avoid blasting away of the overlying bedrock. The steam explosions drive a highly compressed column of fragmented ash, steam, and water upwards along the weakest route to the surface. The eruptive column expands at high velocity depositing layers of wet ash outward on the surface. Continued eruptions form a cone or ring of material around the vent. Tuff rings have a large basal diameter with relatively low rims (like Big Hole or Fort Rock tuff ring). Tuff cones have a smaller basal diameter with relatively high rims, like Table Rock. When the supply of water is restricted, the magma will continue to rise and will fill the crater with a lava lake."
GSOC members standing on an ancient lakeshore terrace at Ft. Rock.
Jan leads Wilven and Kurben free climbing up to the top of Ft. Rock.
View of the mound containing Ft. Rock Cave, where some of the earliest evidence of human occupation in Oregon has been found, from the top of Ft. Rock's ramparts. Sagebrush sandals found in the cave were radiocarbon dated at an estimated 9,000 years old.
From Fort Rock, where we were joined by Carol Hasenberg and her two dogs, we stopped at a quarry where obsidian cobbles were found, and then on to Crack in the Ground, the aptly if unimaginatively named feature that was so welcoming on a hot afternoon. Here, from Bart and Bev's field guide, are a few words about the geology around the Crack:
"Open cracks or fissures in the earth’s surface are not uncommon, but they usually become filled with rock rubble or lava and disappear in a very short time. However, Crack-in-the-Ground has remained open for thousands of years. It is a deep, narrow rift about 2 mi long. On air photos and topographic maps it can be traced northward toward Green Mountain summit and southward to the edge of the Green Mountain flows but then disappears into lake sediments. A somewhat parallel fault zone about 2 mi to the northeast lines up with the four craters in the Four Craters Lava Field and the vents in the East Lava Field. The origin of Crack-in-the-Ground is apparently related to a graben-like structure between these two fault trends. With the eruption of the Four Craters Lava Field, perhaps subsidence caused by a combination of emptying the magma chamber and adding the weight to the surface occurred. Indirect evidence of subsidence also occurs in the lake basin, where ephemeral ponds exist."
We found many smooth cobbles of obsidian in this gravel quarry. When cracked with a hammer, they were gleaming black inside. A few had red highlights: those were the prize specimens.
The country around Crack in the Ground, as seen through a bullet hole from inside the composting toilet at the parking area. "What did you find to take of picture of in there??" Doug asked, as I emerged with camera and a big smile. When I thought about it later, that was not a good place for a bullet hole, whether it was fired from within or without!
Clay and Rik descend into the coolness of the Crack.
Looking up at a blue sky out of the depths, it's easy to see how the two parts were once together.
The Crack runs nearly two miles, is narrow and at times difficult to navigate, but at up to 70 feet deep, it is much cooler than the ground level.
Air bubbles in the lava eroded into sculptural walls in some places.
Trish Reading and Jan Kem.
Chris takes an alternate route through the Crack, emerging near these lichen-covered boulders.
Bob and Elaine, framed in an opening between two routes through the Crack.
The first night we stayed at Green Mountain campground near this lookout tower. It was a beautiful night, and cooled off to comfortable temperatures. Kurben asked me what my favorite stop of the day was. Isn't that what adults ask kids?? I loved it!! It was hard to choose between the Crack, Ft. Rock, and the obsidian cobbles, but it had to be the Crack, because it was so nice and cool in there.
On Saturday, we packed up our tents and headed out through the town of Christmas Valley.
We had so many cars in our caravan, because we had to move from one campground to another each day. No car trouble and no one was left behind this time, thanks to shepherding by Taylor and Jean!
Bart lays out a stratigraphic section of rock specimens on the hood of his truck. He said it was the first time he ever brought rocks to a field trip!
Elaine asks a clarifying question, as Bev, Rik, and others look on.
In places it was easy to see the ancient lakeshore erosional terraces from when these dry basins were filled with enormous lakes, thousands of years ago.
The entire group, photographed at the Tucker Hill perlite quarry. Missing are Rick Meyer, Bob Lashbrook, and Elaine Glendenning, who left the trip earlier; Chris (where did he go?), and Wilven Smoody, who was exploring the area out of earshot when the photo was taken.
The group headed back to Paisley, Oregon, and from there up the canyon to Marster Springs Campground, which was just lovely, shady, and along the Chewaucan River. We stayed for an hour or so to relax and rehash the day, and then left for a motel in Lakeview. We thought we could use the shower before our backpacking trip through the Pueblo Mountains, which ultimately didn't happen. At dinner in Lakeview, we saw Bob Lashbrook and Elain Glendenning, who had left the trip earlier to visit friends in the area.
