7.7: Medicine Lake Volcano
- Page ID
- 21484
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Not Your Grandmother’s Shield Volcano
When viewed from a distance, Medicine Lake Volcano has the apparent classic shape of a shield volcano, though one covered with a pox of cinder cones (Figure \(\PageIndex{1}\)). However, as one approaches the volcano more closely, the rocks tell a somewhat different story than the overall shape might lead one to believe. The most iconic shield volcanoes such as Mauna Loa and Mauna Kea in Hawaii, are generally uniform in lava composition, with only slight variations in chemistry over their eruptive history. They are composed of only basalt. By contrast, outcrops exposed on the flanks of the Medicine Lake Volcano range from basaltic to andesitic and even rhyolitic at high elevations. Drill cores indicate that rhyolites are also found at depth. This extreme range of volcanic rock compositions indicates that the degree of crustal contamination is highly variable over the life of this volcano. At times, very little magmatic assimilation produced basaltic lavas, and at other times, extensive assimilation produced rhyolitic tuffs and obsidian flows. The range of composition might more reasonably classify Medicine Lake Volcano as a composite volcano, but it has erupted more low-viscosity basalt flows than a typical composite cone, giving it a shield, rather than cone, shape.
Medicine Lake Volcano began forming around 500,000 years ago, with the most recent eruption at Glass Mountain occurring about 950 years ago. Near the center of the shield-shaped edifice, there is an enclosed basin or caldera, 12.1 km (7.5 miles) long by 6.9 km (4.3 miles) wide, which is partially flooded by the namesake 2 km-long (1.2 mile-long) Medicine Lake (Figure \(\PageIndex{2}\)). The caldera formed when the center block began to collapse along fractured lines between 180,000 and 100,000 years ago, although no single large eruption was responsible for the caldera formation. Lava erupted after the caldera formed and lava was squeezed up the fracture lines forming rim volcanoes. The volcanoes discharged lava onto the caldera floor and down the outer edges of the original volcano edifice.
Lava Beds National Monument
Lava Beds National Monument on the north slope of Medicine Lake shield volcano contains the highest concentration of lava tube caves in North America. There are up to 500 known lava tube caves within the boundaries of Lava Beds National Monument (Figure \(\PageIndex{3}\)).
Much of the north and south flanks of the Medicine Lake shield were built from molten lava transmitted through lava tubes. These tubes formed beneath the congealing surface of basalt flows in somewhat the same way that a stream may continue to flow beneath a cover of its own winter ice. As molten lava emerges from a vent and flows down slope, congealing lava from the top and sides of the central channel often forms a bridge over the lava flow. The sticking together of bits of lava spatter and fragile lava crusts strengthens the bridge in the same way that thin crusts of floating ice raft together to cover a stream during early stages of a winter freeze. Because the viscosity of the lava depends on temperature, and the roof of the lava tube insulates it, lava can flow much further from the vent in the insulated lava tube compared to being exposed at Earth’s surface. The leading edges of basalt flows at Earth’s surface can travel as fast as 10 km/h (6 mph) on steep slopes but they typically advance less than 1 km/h (0.27 m/s or about 1 ft/s) on gentle slopes. When basalt lava flows are confined within an insulating channel or lava tube on a steep slope, the main body of the flow can reach velocities greater than 30 km/h (19 mph).
Eruption of basaltic lava is a much more violent and spasmodic process than the steady gathering of tributaries that feed a stream. If lava stops rising from its source deep within Earth, the still-molten lava moving within a lava tube will continue to drain downhill and may ultimately leave an open tube-shaped cave -- often large enough for people to walk through. Even before the top and walls of a lava flow have time to cool from one eruption, a new eruption of lava may refill the open tube, overflow its upper end, and spread a new lava flow beside or on top of the first flow. Even if the original tube is large enough to contain the renewed supply of lava, this tube can then deliver the new lava beyond the end of its original flow and thus the lava field extends farther and farther down slope. If the gradient of flow flattens, the tube may subdivide into a number of smaller distributaries, which spread laterally over the more gently sloping ground.
Within Lava Beds National Monument, most lava tubes are found within the basalt of Mammoth Crater. Complicated lava-tube systems originating from Mammoth Crater and other vents have built a broad fan of complexly interfingering lava flows that form the lower northeast flank of the Medicine Lake Volcano. Some tubes conveyed lava underground 25-30 km (about 15-20 miles) from their sources. Nevertheless, today it is hard to walk for a distance of even 6 km (4 miles) within any one lava tube. Large parts of the roofs of most lava tubes have fallen in, hiding the floor under huge piles of angular broken rock, often stacked so tightly that access to both upstream and downstream portions of the tube is closed. In some places, however, collapse of the tube's roof has provided a large entrance into the lava tube through which one can walk with ease. Multiple collapses have created the hundreds of caves within the dozen or so lava tubes that distributed lava over two thirds of the monument.
The winter of 1872-1873 was a troubled one in the Lava Beds, where a small band of Modoc Indians was besieged by a US Army force outnumbering them as much as ten to one. The conflict arose after the United States government relocated the Modoc people to the Klamath Reservation, north of present-day Klamath Falls, along with the Klamaths and members of the Yahooskin band of Paiutes. A group of several hundred Modocs left the Klamath Reservation and returned to their homeland, demanding their own separate reservation within their ancestral territory. The resulting conflict was one of the most costly wars in U.S. history. According to some estimates it cost $10,000 (about $300,000 today) per warrior to subdue the Modocs in battle. The Modoc warriors totaled between 50 and 60, while there were as many as 1000 U.S. troops at the height of the conflict. The war lasted six months, from November 29, 1872 to June 1, 1873, although tensions leading to the conflict began much earlier.
Much of the war was centered around Captain Jack’s Stronghold, a natural lava fortress characterized by lava tubes, small caves, and deep clefts within the lava flows (Figure \(\PageIndex{4}\)). The deep clefts provided excellent cover for snipers and the network of clefts and lava tube caves provided hidden, protected pathways for the Modoc to maneuver through during skirmishes with the US Army. The Stronghold was named for the Modocs’ war leader Keintpoos, or Captain Jack as he was known to the settlers. Some 150 Modoc men, women,and children lived in the Stronghold for five months of the war, including the harsh winter months. By the war’s end, the fatalities included 53 U.S. soldiers, 17 civilians, 2 Warm Springs Scouts, 5 Modoc women and children, and 15 Modoc warriors, five of which were killed in battle; many were executed for “crimes against the United States” after their surrender and the survivors were eventually transported to a reservation in Oklahoma.
Although the Modoc people did not prevail in the end, their ability to hold off the U.S. troops for as long as they did was due in large part to their knowledge and skill in using the local terrain, especially the network of lava tubes, to their advantage. The rugged landscape formed by the lava flows and collapsed lava tubes provided natural military fortifications and hidden pathways for travel for the Modoc people.
Obsidian Domes
In addition to the many cinder cones scattered throughout the Cascades and Modoc Plateau, there are also many lava domes. Two of the most iconic lava domes are Glass Mountain and Little Glass mountain, located on the Medicine Lake Volcano. These formed when silica-rich (rhyolitic) magma erupted effusively at the surface. In order for rhyolitic lava to erupt effusively, it must have a very low gas content. This usually occurs when the gasses have already escaped the magma before it erupts at the surface. Because of the high viscosity of rhyolitic lava, it does not flow long distances, as basaltic lava does, and instead builds a dome (Figure \(\PageIndex{5}\)). The relatively rapid cooling that occurs at the surface of Earth, combined with the high viscosity of rhyolitic lava, impedes the growth of mineral crystals. Instead, the lava cools into volcanic glass, or obsidian, which is solid, but with no crystalline structure.
The lack of crystalline structure in obsidian causes conchoidal fracture. Through a technique called ‘knapping’, conchoidal fracture can be controlled to produce very sharp tools, such as arrowheads and knives. This made obsidian incredibly valuable to the people of North America before Europeans introduced metal alloy technology. Not only does obsidian remain culturally important today, but it is also still used in some cutting applications, such as certain surgeries because it can produce a sharper edge than steel. Local tribes, including the Modoc and Pit River peoples collected, used, and widely traded the high-quality obsidian from sources like Glass Mountain and Lassen Creek in the Warner Mountains. Obsidian was a popular trade item to other tribes from as far away as 100 miles.
Today, collecting obsidian anywhere in the Medicine Lake Highlands is prohibited by law. The Modoc National Forest has four obsidian mines in the Warner Mountains near Davis Creek, Calif. on the east side of the forest where it is legal to collect obsidian with a free permit.
Acknowledgments
Parts of the text on this page were taken with minimal editing from sources provided by the USGS and the National Park Service, which are in the public domain. Links to the original text can be found in the reference section on this page.
References
- Alt, D., Hyndman, D. W., & Baylor, K. J. (2016). Roadside Geology of Northern and Central California (2nd edition). Mountain Press.
- Donnelly-Nolan, J. M. (1987). Medicine Lake Volcano and Lava Beds National Monument, California. https://pubs.geoscienceworld.org/gsa/books/book/786/chapter/4880695/Medicine-Lake-Volcano-and-Lava-Beds-National
- Horton, K. (2023, October 3). 150 years ago, the US military executed Modoc war leaders in Fort Klamath, Oregon. Oregon Public Broadcasting. https://www.opb.org/article/2023/10/03/modoc-war-captain-jack-execution-fort-klamath-oregon/
- U.S. Geological Survey. (n.d.). Eruption History of Medicine Lake Volcano. Retrieved March 24, 2024, from https://www.usgs.gov/volcanoes/medicine-lake/science/eruption-history-medicine-lake-volcano
- U.S. Geological Survey. (2023a, October 7). The Modoc War. https://www.usgs.gov/volcanoes/medicine-lake/science/modoc-war
- U.S. Geological Survey. (2023b, October 31). Lava tubes at Lava Beds National Monument. https://www.usgs.gov/volcanoes/medicine-lake/science/lava-tubes-lava-beds-national-monument
- U.S. Geological Survey. (2023c, November 3). Geology and History of Medicine Lake. https://www.usgs.gov/volcanoes/medicine-lake/science/geology-and-history-medicine-lake
- U.S. National Park Service. (n.d.). Geology—Lava Beds National Monument. Retrieved March 20, 2024, from https://www.nps.gov/labe/learn/nature/geology.htm
- U.S. National Park Service. (2023, December 10). Modoc War—Lava Beds National Monument. https://www.nps.gov/labe/learn/historyculture/modoc-war.htm
- U.S. National Park Service. (2024, April 4). History & Culture—Lava Beds National Monument. https://www.nps.gov/labe/learn/historyculture/index.htm