Compared to Old Faithful, Yellowstone Lake seems fairly dull, but appearances can be deceiving. The bottom of Yellowstone Lake is hydrothermally active, and scientists are studying hydrothermal vents, spires, craters, domes, rhyolitic lava flows and other evidence of glacial, tectonic and sedimentation processes that created the Yellowstone Lake of today.
While millions of visitors view Old Faithful and the surface of Yellowstone Lake every year, only a handful of scientists and technicians know the bottom of the lake, among them Lisa Morgan, Ph.D., with the U.S. Geological Survey (USGS). Using multi-beam sonar mapping and seismic surveys, as well as a submersible craft, Morgan and her colleagues are gaining fresh insights into the geologic forces that are shaping this high-altitude lake that straddles the southeast margin of the 30- by 45-mile Yellowstone Caldera.
The caldera is the collapsed remains of a supervolcano that erupted 640,000 years ago. Below the caldera is an underlying magma body that releases tremendous heat – the source of Yellowstone’s hydrothermal features.
Generally, in Yellowstone Lake, lava flows lie within the caldera margin and have a thin veneer of glacial debris.
“This is where fire met ice,” said Morgan.
Morgan helped map Yellowstone Lake from 1999 to 2003. The first time the lake was surveyed was the Hayden Survey in 1871, a 24-day effort that gathered 300 depth readings, or data points. Morgan’s map project took four summers and generated 240 million data points, providing a much more detailed map that contains mountains of data yet to be fully analyzed.
In 1999, Morgan’s research revealed a dome – the size of seven football fields – on the bottom of the lake. That got people excited for a while, amid fears of a hydrothermal explosion that made the CBS Evening News in 2004. No explosion yet, and no changes in the chemistry of hot waters that emerge from the north basin hydrothermal dome.
“A significant change, such as CO2 [carbon dioxide concentrations], might tell us something,” said Morgan.
Her most recent research finds that the rhyolitic lava flows across the bottom of the lake help direct the flow of hydrothermal fluids underneath. The hydrothermal fluids either find their way to the edges of the lava flows or move upward through fractured, permeable zones.
About 200,000 years ago, said Morgan, post-caldera rhyolitic lava flows began moving into the lake-park-wide there were about 35 events. “We can see the record of that activity in the lake. It is a very big find. Prior to our recent mapping, no one had ever suggested lava flows in the lake.”
Morgan theorizes that hydrothermal fluids come up along deep fractures through sediments, hit the base of the overlying lava flows, divert laterally and come up along edges of the lava flows. Fractures caused by inflation and deflation of the caldera also play a role.
Another discovery emerged from observations in late September 2002, when researchers were boating above the north basin hydrothermal field. Morgan said there was a strong scent of rotten eggs in the air, as well as rising bubbles from the bottom and a plume of fine sediments. These events weren’t noticed in early summer but were noticed in late summer and early fall in subsequent years.
“We attributed this with a drop in lake level in late summer and early fall,” said Morgan. Apparently, water levels and water pressure declined enough to increase flow rates from active hydrothermal vent systems.
This actually ties in with what other scientists have observed in the Norris Geyser Basin – a “fall disturbance” that produces sudden changes in thermal springs, temperatures and water chemistry. That fall disturbance is likely due to a slight but critical drop in the water table of Norris Basin.
Morgan is also studying large hydrothermal explosion craters in and around Yellowstone Lake. Unlike volcanic explosions characterized by super-heated pyroclastic flows (hot gas, ash, rock and lava), hydrothermal explosions result from magma-heated water and steam building pressure underground. If pressure can be released by geysers or hot springs, nothing happens. But if the pressure builds up too far, the results can range from a hearty burp to a deadly explosion of steam and shattered rock.
Explosions can result in pits or craters that range in size from a few yards to hundreds of yards, tossing debris miles away.
The last really big hydrothermal explosions were 3,000 and 14,000 years ago, based on radiocarbon dating of wood fragments found in the debris. The largest hydrothermal explosion crater is Mary’s Bay, said Morgan. Since 1872, there have been 20 minor blowouts. In 1989, the Pork Chop Geyser in Norris Basin got clogged. When the pressure broke loose, rocks rained around tourists some 200 yards away.
Sites around Yellowstone Lake that were formed by hydrothermal explosions include Indian Ponds, Evil Twin over in West Thumb and Duck Lake – a crater on the edge of West Thumb Geyser Basin.
“These are relatively shallow systems,” said Morgan. “In Yellowstone, there’s no evidence for a volcanic eruption triggering a hydrothermal eruption and vice versa. Heat from the magma chamber (4 to 5 miles below) is providing the energy for heating the water in the hydrothermal systems.”
Big, powerful and deadly hydrothermal explosions are rare, said Morgan, but are more frequent than volcanic eruptions in Yellowstone.
A powerful earthquake and a big landslide could displace enough water in Yellowstone Lake to uncork a hydrothermal explosion, said Morgan. “We know we have lots of landslide deposits along the shores of the lake,” said Morgan. “We don’t know if there was just one or a series of landslides. It makes a big difference in understanding hazards associated with the lake.”
Bugs ‘n’ stuff
Morgan has also found time to work with microbiologists, taking the remotely operated submersible down to collect bacteria mats growing around hydrothermal vents.
“There’s about 200,000 years of history locked up in the sediment of Lake Yellowstone,” said Morgan. Sediment could tell Morgan what was happening locally, or even provide perspective on volcanic activity in the Cascade Range, by looking at ash deposits. Biologists could look at the pollen in the lake’s sediment, she added.
Morgan is finishing a map of the lake this summer, showing the geologic forces contributing to shaping the current lake, from fire to ice. She’s also working on a history of Yellowstone Lake, including a map of large hydrothermal explosions – most of which should be posted on a USGS web site by next fall.
Should research appropriations increase, Morgan would like to work more on the seismic data she’s accumulated, and possibly start a coring program collecting cores at selected sites in Yellowstone Lake.
Brodie Farquhar is a writer living in Lander, Wyoming. He is the contributing editor of Yellowstone Journal.
Read the study, “The Floor of Yellowstone Lake is Anything but Quiet—New Discoveries from D High-Resolution Sonar Imaging, Seismic Reflection Profiling, and Submersible Studies” at http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1076&context=usgspubs