DESCRIPTION:
Medicine Lake Vicinity

Medicine Lake Caldera
Lava Beds National Monument
Glass Mountain Obsidian Flow and Domes

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Medicine Lake shield volcano (middle background).
-- USGS Photo by J. Donnelly-Nolan

Medicine Lake Volcano

Location: California

Latitude: 41.58 N

Longitude: 121.57 W

Height: 2,412 Meters (7,762 Feet)

Type: Shield Volcano Caldera

Number of eruptions in past 200 years: 0

Latest Eruptions: 1065

Remarks: Latest eruption formed Glass Mountain

The Medicine Lake shield volcano, a sleeping giant, is the largest volcano in the Cascade Range. Filling up the entire southern skyline, it has been erupting off and on for half a million years. The eruptions were gentle rather than explosive like Mount St. Helens, coating the volcano's sides with flow after flow of basaltic lava. This created a shield-shaped mountain approximately 150 miles around the base and 7900 feet high. Medicine Lake is part of the old caldera, a bowl-shaped depression in the mountain. It is believed that the Medicine Lake volcano is unique, having many small magma chambers rather than one large one.

Medicine Lake volcano is a shield volcano of basaltic through rhyolitic composition that lies east of the main axis of the Cascade Range, approximately 50 kilometers east-northeast of Mount Shasta, which is the largest of the Cascade stratovolcanoes. The small lake from which Medicine Lake volcano derives its name lies within the 7x12-kilometer central caldera. The volcano is thought to be younger than approximately 1000 years, and consists of calc-alkaline and tholeiitic lavas. The most recent eruption occurred around 1,000 years ago when rhyolite and dacite erupted at Glass Mountain and associated vents near the caldera's eastern rim. No field evidence has been found to substantiate a report of an eruption in 1910.

Primitive high-alumina tholeiitic basalt has erupted throughout the history of Medicine Lake volcano and is common around the lower flanks. The largest eruptions are basaltic, perhaps 5 times larger than the largest known rhyolitic lava flow, Glass Mountain (1 km3). Basalt and andesite also appear to be more common than thyolite, with dacite being relatively rare. Vents for both mafic and silicic flows typically form linear arrays, most trending within a few degrees of north-south, also the most common direction for faults. Medicine Lake volcano lies in a strongly east-west extensional tectonic environment behind the main Cascade arc. The most easterly alignment of vents is N55 degrees E, not very different from the east-northeast regional alignment of vents that form a highland bettween Mount Shasta and Medicine Lake volcano. Medicine Lake volcano is very similar to Newberry volcano, also located east of the main arc.

Estimates of the size of a possible silicic magma chamber beneath Medicine lake volcano differ, but geophysical studies constrain it to a very small volume. The magmatic system probably consists of numerous small bodies of differentiated magma together with a plexus of mafic dikes that represents periodic influx of the primitive basalt that provides the ultimate heat source for the volcano. Geologic mapping of the volcano's many units suggests eruption usually are relatively small and occur frequently during episodes of activity. Only one ash-flow tuff is known, an andesite tuff of late Pleistocene age, which does not appear to be related to caldera formation. Instead, the caldera probably formed by repeated subsidence over a long period as fluid lavas from summit vents flowed mostly outward down the flanks of the volcano.

Holocene lava flows range in composition from high-alumina basalt to andesite to dacite and rhyolite. Some of these are compositionally zoned, including Glass Mountain, which first erupted dacite containing mafic magmatic inclusions and last erupted rhyolite without inclusions. ... Granitic inclusions are present in many of the flows, providing evidence of crustal contamination. The inclusions may be from geophysically inferred granitic bedrock akin to that of the Sierra Nevada batholith exposed further south.

There are no hot springs and very few cold springs in this dry area in the rain shadow east of Mount Shasta. One fumarolic area is present at the Hot Spot near Glass Mountain.

Basalt flows on the north and south flanks of the volcano contain lava tubes that have collapsed in numerous places. These collapses give access to the lava-tube caves that are a prime attraction for visitors to Lava Beds National Monument, located on the north flank of Medicine Lake Volcano.

Medicine Lake volcano (MLV) is located in an East-West extensional environment on the Modoc Plateau just east of the main arc of the Cascades. It consists mainly of mafic lavas, although drillhole data indicate that a larger volume of rhyolite is present than is indicated by surface mapping. The most recent eruption was rhyolitic and occurred about 900 years ago. At least seventeen eruptions have occurred since 12,000 years ago, or between 1 and 2 eruptions per century on average, although activity appears to be strongly episodic. The calculated eruptive rate is about 0.6 cubic kilometers per thousand years during the entire history of the volcano. Drillhole data indicate that the plateau surface underlying the volcano has been downwarped by 0.5 kilometer under the center of MLV. The volcano may be even larger than the estimated 600 cubic kilometers, already the largest volcano by volume in the Cascades.

Medicine Lake volcano is a Pleistocene and Holocene shield volcano located in the Cascade Range, east of the main arc and northeast of Mount Shasta. Lavas from MLV cover about 2000 square kilometers. They range in composition from basalt through rhyolite and include both tholeiitic and calc-alkaline types. Basalt and basaltic andesite dominate the lower flanks of MLV. Higher on the volcano, basaltic lavas are mostly absent, andesite dominates, and high-silica lavas are present, including the spectacular late Holocene rhyolites of Glass Mountain and Little Glass Mountain. Volume of the volcano is estimated at 600 cubic kilometers, larger than Mount Shasta which is the largest of the Cascade stratocones. The highest point on the rim of MLV's caldera is 7,913 feet. Lava flows reach elevations as low as 3360 feet although most lavas are found above 4100 feet, the approximate elevation of the surrounding Modoc Plateau. The low shield shape, central caldera, and dominance of mafic lavas are similar to Newberry Volcano of central Oregon, also located in an extensional tectonic environment east of the main Cascade arc.

Eruptive activity during the past 11,000 years at Medicine Lake volcano has been episodic. Eight eruptions produced about 5.3 cubic kilometers of basaltic lava during an interval of a few hundred years about 10,500 years B.P. After a hiatus of about 6000 years, eruptive activity resumed with a small andesite eruption at about 4300 years B.P. Approximately 2.5 cubic kilometers of lava with compositions ranging from basalt to rhyolite vented in nine eruptions during an interval of about 3400 years in late Holocene time. The most recent eruption occurred about 900 years B.P. A compositional gap in SiO2 values of erupted lavas occurs between 58 and 63%. The gap is spanned by chilled magmatic inclusions in late Holocene silicic lavas. Late Holocene andesitic to rhyolitic lavas were probably derived by fractionation, assimilation, and mixing from high-alumina basalt parental magma, possibly from basalt intruded into the volcano during the early mafic episode. Many basaltic to andesitic lavas contain iron-rich crystals and have high FeO*/MgO -- characteristics caused by mixing of high-alumina basalt with ferrobasalt liquid produced by fractionation of parental high-alumina basalt. When ferrobasalt and high-alumina basalt are contaminated with a granitic crustal component, a calc-alkaline trend is produced. Some eruptions have produced both tholeiitic and calc-alkaline compositions. The eruptive activity is probably driven by intrusions of basalt that occur during east-west stretching of the crust in an extensional tectonic environment. Vents are typically aligned parallel or subparallel to major structural features, most commonly within 30 degrees of north. Intruded magma should provide adequate heat for commercial geothermal development if sufficient fluids can be found. The nature and timing of future volcanic activity cannot be predicted from the observed pattern, but eruptions high on the edifice could produce high-silica products that might be accompanied by explosive activity, whereas eruptions lower on the flanks are likely to vent more fluid mafic lavas.

Medicine Lake volcano is a Pleistocene-Holocene shield volcano located about 50 kilometers east-northeast of Mount Shasta, between the crest of the Cascade Range to the west and the Basin and Range tectonic province to the east. The Medicine Lake shield rises about 1200 meters above the Modoc Plateau to an elevation of 2,376 meters. Lavas from Medicine Lake volcano cover nearly 2000 square kilometers, and their volume is estimated to be at least 600 cubic kilometers, making it the largest volcano by volume in the Cascade Range. Medicine Lake volcano began to grow about one million years ago, following eruption of a large volume of tholeiitic high-alumina basalt. Similar high-alumina basalt has continued to erupt around the volcano throughout its history. Although mafic lavas predominate on the volcano's flanks, all lava compositions from basalt to rhyolite have erupted during Pleistocene time. The lower flanks consist of mostly basaltic and some andesitic lavas. Basalt is mostly absent at higher elevation, where andesite dominates and rhyolite and small volumes of dacite are present.

During the past 11,000 years, eruptive activity at Medicine Lake volcano has been episodic. Eight eruptions produced about 5.3 cubic kilometers of basaltic lava during a time interval of a few hundred years about 10,500 years ago. That eruptive episode was followed by a hiatus that ended with a small andesitic eruption about 4,300 years ago. During the most recent eruptive episode between 3000 and 900 years ago, eight eruptions produced approximately 2.5 cubic kilometers of lava ranging in composition from basalt to rhyolite. Late Holocene lava compositions include basalt and andesite, but silicic lavas dominate.

Medicine Lake volcano is a Pleistocene and Holocene shield volcano located in northeastern California about 50 kilometers east of Mount Shasta, near the western margin of the Basin and Range tectonic province.

Lava Beds National Monument is located on the northern flank of Medicine Lake volcano and encompasses mostly basaltic and some andesitic lavas. Higher on the volcano, basaltic lava is mostly absent, andesite dominates, and rhyolite and small volumes of dacite are present, the latter mainly near the 7 x 12 kilometer Medicine Lake caldera.

Medicine Lake volcano is a large Pleistocene and Holocene shield volcano in northeastern California about 50 kilometers northeast of Mount Shasta. The volcano is located in a zone of east-west crustal extension east of the main axis of the Cascade Range. The 1-kilometer-thick shield is 35 kilometers from east to west and 45-50 kilometers from north to south, and covers more than 2000 square kilometers. The volcano is composed primarily of basalt and basaltic andesite lava flows, and has a 7 x 12 kilometer caldera at the center. Eruptive activity during Holocene time has included numerous rhyolite and dacite lava flows erupted at high elevations inside and outside the caldera; cinder cones and associated lava flows of basalt and basaltic andesite have resulted from eruptions at vents on the flanks of the shield. Most vents are aligned along zones of crustal weakness that trend NNE to NNW.

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Glass Mountain from Medicine Lake caldera rim
-- USGS Photo by J. Donnelly-Nolan

Medicine Lake volcano is a shield volcano of basaltic through rhyolitic comopsition that lies east of the main axis of the Cascade Range, approximately 50 kilometers east-northeast of Mount Shasta ... The volcano is thought to be younger than approximately 1000 years, and consists of calc-alkaline and tholeiitic lavas. The most recent eruption occurred around 1,000 years ago when rhyolite and dacite erupted at Glass Mountain and associated vents near the caldera's eastern rim. ...

Primitive high-alumina tholeiitic basalt has erupted throughout the history of Medicine Lake volcano and is common around the lower flanks. The largest eruptions are basaltic, perhaps 5 times larger than the largest known rhyolitic lava flow, Glass Mountain (1 cubic kilometer). ...

(Medicine Lake) Holocene lava flows range in composition from high-alumina basalt to andesite to dacite and rhyolite. Some of these are compositionally zoned, including Glass Mountain, which first erupted dacite containing mafic magmatic inclusions and last erupted rhyolite without inclusions. Lava compositions can be explained by different petrologic processes or combinations of processes including fractional crystallization contamination with fractionation, and magma mixing. Granitic inclusions are present in many of the flows, providing evidence of crustal contamination. The inclusions may be from geophysically inferred granitic bedrock akin to that of the Sierra Nevada batholith exposed further south.

Glass Mountain consists of a spectacular, nearly treeless, steep-sided rhyolite and dacite obsidian flow that erupted just outside the eastern caldera rim and flowed down the steep eastern flank of Medicine Lake volcano. Ten additional small domes of Glass Mountain rhyolite and rhyodacite lava lie on a N25degreesW trend to the north and one to the south. The age of Glass Mountain and its preceding pumice deposits has been a matter of discussion for some time. A radiocarbon age of 885+/-40 years B.P. was obtained on a dead cedar tree without limbs or bark that is preserved in the edge of one of the distal tongues of the flow. The dated material consisted of a piece of exterior wood containing about 30 annual growth rings. This age may be too old, because some of the outside of the tree is missing. The tephra deposits that precede the flow and domes may be somewhat older but are constrained to be less than about 1050 years B.P. by the Little Glass Mountain and Lassen data.

Glass Mountain dacite-rhyolite flow ... Thirteen vents along a northwest-southeast trending fissure were active during this eruption, which occurred within the last thousand years. Nine northwestern vents and the southeastern-most vent produced domes, while lavas of the intervening three vents coalesced to for the Glass Mountain flow, one cubic kilometer in volume. The flow consists of three dacitic eastern lobes which grade westward to rhyolite and are overlain by rhyolite lobes.

Lava Beds National Monument is located on the northern flank of Medicine Lake volcano and encompasses mostly basaltic and some andesitic lavas.

Lava Beds National Monument is located in northeastern California about 50 kilometers south of Klamath Falls, Oregon. The monument, established in 1925, includes the sites of many important battles of the Modoc Indian War of 1872-73. It is also known for scores of lava-tube caves and for well preserved young volcanic features.

Volcanic eruptions on the Medicine Lake shield volcano have created an incredibly rugged landscape punctuated by cinder cones, lava flows, spatter cones, lava tube caves and pit craters. During the Modoc War of 1872-1873, the Modoc Indians used these tortuous lava flows to their advantage. Under the leadership of Captain Jack, the Modocs took refuge in "Captain Jack's Stronghold," a natural lava fortress. From this base a group of 53 fighting men and their families held off US Army forces numbering up to twenty times their strength for five months. Visitors can tour both the geologic and historic wonders of this unusual landscape.

Lava Beds National Monument was established on November 21, 1924. A paved road winds the length of Lava Beds National Monument with dirt roads leading to sites off the main road. There are several hiking trails, three of which are self-guiding interpretive trails. There are over 25 miles of wilderness and other back country hiking trails. Spend at least a day for a taste of both geologic and historic sites. Explore the geology of a shield volcano, including visiting several lava tube caves; visit Captain Jacks's Stronghold and other historic sites to learn the story of the Modoc War; visit pictograph and petroglyph sites to glimpse the prehistory of the area. Indian Well Visitor Center is located at the southern end of the Monument. Just outside, Mushpot Cave is lighted during business hours and contains exhibits describing lava tube geology.

"Visit A Volcano" - Medicine Lake and Lava Beds

Lava Beds National Monument is located in northeastern California about 50 kilometers south of Klamath Falls, Oregon. The monument, established in 1925, includes the sites of many important battles of the Modoc Indian War of 1872-73. It is also known for scores of lava-tube caves and for well preserved young volcanic features.

The monument lies at the south end of the Klamath graben, on the north flank of Medicine Lake volcano. The north edge of the monument coincides with the south edge of Tule Lake, much of which is now reclaimed for farmland. South of the lake the terrain rises from about 1,200 meters to about 1,600 meters at the southern monument boundary. Continuing south, the terrain rises gradually to the summit of the Medicine Lake shield volcano, which last erupted about a thousand years ago. Some of the most recent eruptions of the volcano were explosive, leaving a deposit of white, silica-rich pumice fragments on all units in Lava Beds National Monument. In the north, the pumice occurs as a thin dusting; near the south boundary it is locally as much as 0.3 meters thick.

Most lava flows in the monument issued from cinder and spatter cones within the monument boundaries. These numerous flank vents of Medicine Lake volcano have fed mafic lava including basalt, basaltic andesite, and some andesite. The basalt of Mammoth Crater is the dominant unit in the monument in terms of area and volume and is host to most of the nearly 300 lava-tube caves. This unit, erupted from several vents including Mammoth Crater and Modoc Crater is volumetrically one of the largest of Medicine Lake volcano, amounting to about 5 cubic kilometers. It covers an area of about 250 square kilometers, extending as far as 25 kilometers from Mammoth Crater. The basalt of Mammoth Crater was erupted in a geologically brief period of time, probably less than 100 years, as judged from the consistent paleomagnetic direction obtained from different parts of the unit.

Open ground cracks, some more than 10 meters wide and deep, are common in Lava Beds National Monument, and to the west and southeast, as well as high on Medicine Lake volcano where they formed during Holocene, mostly silicic eruptions. ... Pollard (1983) showed that east-west extension combined with dike emplacement was responsible for the ground cracks that accompanied the eruption of Little Glass Mountain and related Holocene rhyolites high on the northwest side of Medicine Lake volcano. Dominantly north-south vent alignments and major north-trending normal faults such as the Gillem fault (Donnelly-Nolan, 1983) also indicate east-west extension. In this extensional environment, it is not surprising that very primitive basalts have erupted.

In Lava Beds National Monument, elevation and rainfall increase from north to south and the vegetation changes noticeably. At the north end of the monument, vegetation consists of scattered low brush and grass with occasional juniper trees. Farther south, larger brush in the form of mountain mahogany appears, and juniper trees are more abundant. At the southern boundary of the monument, ponderosa pine is common. Continuing up the north side of Medicine Lake volcano, vegetation zones continue to change and forest cover increases. ...

The basalt of Mammoth Crater interacted with the water of ancient Tule Lake, producing pillow lava between Captain Jacks Stronghold and the northeast corner of the monument. The pillows are visible in small quarries at the edge of the flow. The lake must have been very shallow, as it is today, because the upper surface of the flow shows no effect of interaction of water. At Hospital Rock, in the northeastern part of the monument, several littoral cones, similar in appearance to small cinder cones, apparently formed where a lava tube emptied directly into water. Much older pillowed flows are exposed in Gillem Bluff and apparently entered a much older (and probably deeper) version of Tule Lake. Similarly, palagonite tuffs exposed low on the north flank of Medicine Lake volcano and around the edges of the present Tule Lake imply that larger lakes were present in the past.

Callahan Lava Flow

Slightly more andesitic lava erupted about 1100 years ago to form the blocky Callahan lava flow (sometimes known as the Black lava flow), a portion of which covers the southwestern part of Lava Beds National Monument; this flow erupted from Cinder Butte, located just outside the monument boundary. The Callahan flow is the youngest lava flow in the monument. Covering all the flows is a thin layer of white pumice fragments that formed as fallout from explosive eruptions about a thousand years ago when Glass Mountain and Little Glass Mountain erupted high on the slopes of Medicine Lake volcano.

Cinder Butte

The latest cinder cone to erupt is Cinder Butte, occurring 1110 years ago, plus or minus 60 years. Cinder Butte forms a conspicuous landmark for park visitors, especially in winter when its treeless slopes are commonly covered with a dusting of snow.

Fleener Chimneys, Black Crater, Ross Chimneys, The Devil's Homestead

Spatter cones are another formation that can occur in association with shield volcanos. Blobs of lava are thrown up through the break in the earth's crust. The lava is very thick and pasty, taking the shape of whatever it coats. A chimney may eventually be formed. An excellent example of a spatter cone is at Fleener Chimneys, 150 feet deep. Black Crater and Ross Chimneys are also spatter cones.

Picnic tables and a wheelchair accessible toilet are available at Fleener Chimneys. Fleener Chimneys and Black Crater are castle-like formations created by globs of molten lava which piled up on top of each other. Varying textures and colors will delight photographers.

Fleener Chimneys has a picnic area shaded by western juniper trees. The picnic tables were constructed by members of the CCC; the logs were obtained at Oregon Caves National Monument and the rocks were gathered locally. This spatter cone was created by globs of molten lava piling on top of each other. A fifty-foot hole is left in the center giving it a chimney-like effect. The chimneys are the source of the tremendous aa flow called The Devil's Homestead. The eruption occurred between 2,000 and 8,000 years ago.

Mammoth Crater, Modoc Crater, and Bearpaw Butte

Mammoth Crater, on the southern boundary of the monument, erupted about 30,000 years ago. There was such a tremendous outpouring of lava, that it covered the entire monument from the crater to Tule Lake, forming most of the 435 known lava tube caves in the monument.

About two-thirds of the basalt exposed in Lava Beds National Monument erupted from Mammoth Crater and related vents, including Modoc Crater and Bearpaw Butte. The basaltic lava was transported out to the northern and northeastern parts of the monument where Canby's cross, Captain Jack's Stronghold, and Hospital Rock are located, via lava tubes. Where empty, these tubes form caves, such as Balcony, Boulevard, Merrill, Skull, and Fern. The caves along Cave Loop Drive are located in lava tubes that transported basalt of Mammoth Crater to the east, to Craig Cave and beyond. This very large eruption produced at least one cubic mile of basaltic lava in less than a hundred years. The date of the eruption is unknown but almost certainly took place less than 100,000 years ago.

Petroglyph Point

You step into the past when you view the carvings in the cliff at Petroglyph Point. This formation was created when a cinder cone erupted from the floor of ancient Tule Lake to form an island. Early people paddled out in boats to carve these pictures into the soft rock. Waves undercut the base of the cliff where the petroglyphs are carved. Wind, rain, and ice have enlarged gas bubbles and faults, creating many cracks and crannies. These natural nesting sites are filled with barn owls, cliff swallows, hawks, prairie falcons, and many other birds who find an abundant supply of food nearby. A brochure is available at the site to help you better understand the significance of the area. You will long remember your visit to this special place.

Schonchin Butte

Cinder cones are easily eroded so please stay on the established trails and don't take shortcuts. Frothy lava, cooled in the air, created the large cinder cones throughout the monument. Schonchin Butte's .75 mile trail leads you to a panoramic view from the historic fire lookout. The lookout is staffed from June to September. Children of all ages can earn a Junior Fire Lookout badge. Butte is a geological word for any landform that sticks up abruptly, but cinder cone is a more descriptive geological way of describing this landmark of the monument. Erupting more than 30,000 years ago, it spewed ash and cinders into the air much like a can of soda when shaken. A lava spatter rampart is at the very top. From the lookout panoramic views of the Medicine Lake volcano, Mount Shasta, Mount McLoughlin, the Clear Lake Hills and the Warner Mountains can be viewed and photographed. On a really clear day, you can even see the south rim of Crater Lake. Below the butte, lava flows and collapses are easy to pick out in the landscape. This alone is reason enough to make the climb.

The Castles

After the basalt of Mammoth Crater, another basaltic eruption occurred that formed The Castles, located on either side of the main road near Schonchin Butte. This basalt erupted from numerous spatter vents and flowed north around Hardin Butte. Still younger are several Holocene lava flows (that is, less than 10,000 years old, and younger than the latest glaciation that formed areas of ice on top of Medicine Lake volcano). These include basalt erupted from Fleener Chimneys that forms the Devils Homestead, basalt that erupted to form Black Craters, Ross Chimneys, and their associated flows, and basalt that erupted near Tickner and Bertha's Cupboard Caves just south of the monument boundary and flowed east around Caldwell Butte to form Valentine Cave.

Valentine Cave, Tickner and Bertha's Cupboard Caves

After the basalt of Mammoth Crater, another basaltic eruption occurred that formed The Castles, located on either side of the main road near Schonchin Butte. This basalt erupted from numerous spatter vents and flowed north around Hardin Butte. Still younger are several Holocene lava flows (that is, less than 10,000 years old, and younger than the latest glaciation that formed areas of ice on top of Medicine Lake volcano). These include basalt erupted from Fleener Chimneys that forms the Devils Homestead, basalt that erupted to form Black Craters, Ross Chimneys, and their associated flows, and basalt that erupted near Tickner and Bertha's Cupboard Caves just south of the monument boundary and flowed east around Caldwell Butte to form Valentine Cave.

Lava Beds National Monument lies on the north slope of the huge Medicine Lake shield volcano, a complex volcanic edifice of greater volume than the steep-sided Mount Shasta volcanic cone, which towers as a snowclad landmark 40 miles southwest of the monument.

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 brook may continue to flow beneath a cover of its own winter ice. As molten lava emerges from a vent and flows downslope, congealing lava from the top and sides of the central channel often forms a bridge over the lava stream. The sticking together of bits of lava spatter and fragile lava crusts strengthens the bridge in the manner that thin crusts of floating ice raft together to cover a brook during early stages of a winter freeze. Eruption of basalt lava, however, is a much more violent and spasmodic process than the steady gathering of water that feeds a brook. If liquid lava stops rising from its source deep within the earth, the still-molten lava moving beneath the crusted-over top of a lava flow will coninue to drain downhill and may ultimately leave an open lava-tube cave -- often large enough for people to walk through. It is rare, however, to find such a simple scenario recorded intact among the hundreds of lava-tube caves in the monument. Even before the top and walls of a lava flow have time to cool during a pause in lava supply, a new and violent 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 must deliver the new lava beyond the end of its original flow and thus the lava field extends farther and farther downslope. 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 and intertwining lava-tube systems originating from Mammoth Crater and other vents have built a broad fan of complexly interfingering lava flows that form the northeast perimeter of the Medicine Lake shield. Most of this lava was delivered through lava tubes. Some tubes conveyed lava underground 15-20 miles from their sources. Nevertheless, today one cannot walk for a distance of even 4 miles within any one lava tube. Large parts of the roofs of most lava tubes have fallen in, hiding the floor of the tube under huge piles of breakdown or 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. In some collapse piles where access appears to be lacking, one can search the maze of tumbled blocks and perhaps find a crawlhole into a lava tube. Openings into caves may be detected by noticing the runways of small animals or testing the direction of air flow. On sparklingly clear, very cold winter days, openings into underground caverns will emit a white fog, just as one's exhaled breath does on such a day.

Lava Beds National Monument contains the highest concentration of lava tube caves in North America. The monument sits on the north face of the Medicine Lake Volcano, the largest volcanic mountain in the Cascade range. Over the last 500,000 years, the volcano has erupted periodically, sending very fluid lava over a wide area and building a large, gently sloped mountain. Formation of lava tubes was the result of very hot fluid lava issuing from cracks or fissures on moderate slopes. As the lava flowed it began to cool and solidify on the top and sides. Once the eruption ceased, the tube emptied and drained, and a new lava tube cave was born. As the rock cooled, the inner surface of the tube cracked and collapsed, producing openings to the surface.

At the last count, there are 435 known lava tube caves within the boundaries of Lava Beds National Monument. The caves vary widely in size and length. Some caves are so small that a would-be explorer must crawl the entire length, while some caves are more than 60 feet in diameter. Two dozen of the caves have been developed for public use, with ladders or steps for access and cleared paths through rubble fields. Most of the "public" caves are located along the Cave Loop Road near the Visitor Center.

Visitors are free to visit the caves on their own. The monument loans battery lanterns free of charge each day, and sells inexpensive bump hats. Cavers are also reminded to wear sturdy, long-sleeved and long-legged clothing. For some of the more challenging caves, the monument recommends knee pads and gloves. One cave, Mushpot, is a virtual extension of the visitor center, with its entrance in the middle of the parking lot. Mushpot contains exhibits and is the only lighted cave at Lava Beds. The monument also offers ranger-led tours of the caves.

REMEMBER!! Lava tube caves are irreplaceable treasures. Lava formations will not grow back once broken. Please do not break, deface, remove, or otherwise vandalize any caves or cave formations (lava, ice, etc.) in any way.

For your safety please exercise the following precautions:

• Carry a light for EACH person in your group
• Carry two EXTRA sources of light per person
• Go with a friend -- DO NOT EXPLORE ALONE
• Protect your head from low ceilings
• Wear sturdy shoes or boots and watch for uneven footing
• Carry an extra sweater or light jacket
• Tell someone WHERE you are going and WHEN you expect to return
• Do not deviate from your planned itinerary
• Explore only the caves in the IMMEDIATE VICINITY of your vehicle

Many caves have been "developed" to facilitate public use. Such development may simply consist of a trail to the entrance and a stairway or ladder into the cave. In some cases a developed trail continues inside the cave with additional interior stairways and bridges. Crawling may be required to negotiate parts of several caves.

Medicine Lake caldera is a 7 x 12 kilometer depression in the summit area of the volcano. Anderson (1941) suggested that the caldera formed by collapse after a large volume of andesite was erupted from vents along the caldera rim. However, the distribution of late Pleistocene vents, mostly concentrated along the rim, suggests that ring faults already existed when most of the andesite erupted (Donnelly-Nolan, 1988). No single large eruption has been related to caldera formation. The only eruption recognized to have produced ash flow tuff occurred in late Pleistocene time, and this eruption was too small to account for formation of the caldera. Donnelly-Nolan (1988) concluded that Medicine Lake caldera formed by collapse in response to repeated extrusions of mostly mafic lava beginning early in the history of the volcano (perhaps in a manner similar to the formation of Kilauea caldera, Hawaii). She hypothesized several small differentiated magma bodies fed by and interspersed among a plexus of dikes and sills. In her model, late Holocene andesitic to rhyolitic lavas were derived by fractionation, assimilation, and mixing from high alumina basalt parental magma.

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Aerial view, Medicine Lake
-- USGS Photo by J. Donnelly-Nolan

Medicine Lake volcano is a shield volcano of basaltic through rhyolitic comopsition that lies east of the main axis of the Cascade Range, approximately 50 kilometers east-northeast of Mount Shasta, which is the largest of the Cascade stratovolcanoes. The small lake from which Medicine Lake volcano derives its name lies within the 7x12-kilometer central caldera.

About Medicine Lake in September 1999:

• Perimeter of lake: 6,213 meters (20,380 feet)
• Area of lake: 1.65 square kilometers (408 acres)
• Volume of lake: 13,400,000 cubic meters (3.5 billion gallons or 10,860 acre-feet)
• Elevation of lake surface: 2,036 meters (6,680 feet)
• Greatest depth: 46.4 meters (152.2 feet)
• Mean depth: 7.3 meters (23.9 feet)
• Median depth: 4.9 meters (16.1 feet)

There are no hot springs and very few cold springs in this dry area in the rain shadow east of Mount Shasta. One fumarolic area is present at the Hot Spot near Glass Mountain.

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