The more you read about volcanism in North America, the more confused you become by the immense complexity of eruptive phenomenae and sequences. As will be clear from my last article about Mount Tehama (Lassen), it is never the question about a single central volcano such as Vesuvius or Etna, but about a multitude of volcanic vents and events tied to an area.
Just 50 km (30 miles) northeast of Mount Shasta lies the largest volcano of the entire Cascadia arc, the half-a-million years old Medicine Lake Volcano and on its Northeast flank, the associated Lava Beds National Monument. The Medicine Lake Volcano and the Lava Beds National monument is a great example of how complex and varied volcanism in the Cascades Arc is. That the volcano itself, despite being the largest volume-wise in the USA, does not have a proper name but is named after the lake inside its summit caldera is another example of how shield volcanoes are often overlooked.
Medicine Lake Volcano is a shield volcano that in main has erupted dry HAOT (high-alumina olivine tholeiite) and hydrous calcalkaline basalts simultaneously, but also some andesitic magma. It rises about 1,200 m (3,900 ft) above the Modoc Plateau and under the centre of the volcano, the underlying rock has been pushed down by 0.5 km (0.3 miles). On average, the shield is only about 1 km (0.6 miles) thick, but as the extent of the shield is a massive 35 km (22 miles) east to west by 45 to 50 km (28 to 31 miles) north to south, it covers more than 2,000 km2 (770 square miles). The total volume is estimated to be in excess of 600 km3 (140 cu miles), which makes Medicine Lake Volcano almost twice the size of Mount Shasta.
Unlike most other South Cascade volcanoes where a shield has first been deposited before one or more stratovolcanic cones were built on top, there has been no such development at Medicine Lake Volcano. It is believed that the Medicine Lake Volcano is “unique”, having many small magma chambers rather than one large one. Regular readers of this blog will recognise this line of reasoning as many volcanoes are believed to have a magma reservoir consisting of a system of chambers, sills and dykes rather than a single, large magma chamber and that such a magma reservoir may develop from sills and dykes into one or more larger magma chambers.
On top of the volcano lies Medicine Lake, a 7 by 12 km (4.3 by 7.5 miles) caldera structure that is thought to be the result of collapse after a large volume of andesite erupted from vents along the caldera rim. Also, there is a system of ring fractures associated with these vents, it is thought that they were there prior to the caldera collapse. As the only eruption to have produced an ash flow tuff occurred in late Pleistocene time (~200 to ~180 ka), and this eruption was too small to account for formation of the caldera, it is thought that the caldera may have formed in a similar manner to that of Kilauea.
Eruptive activity during the Holocene has been episodic and includes numerous rhyolite and dacite lava flows from vents at high elevations inside and outside the caldera. Eruptions at vents on the flanks of the shield have resulted in cinder cones and associated lava flows of basalt and basaltic andesite. Most vents are aligned along lines of crustal weakness that trend North East to North West. About 10,500 years ago, eight eruptions within a time interval of a few hundred years produced about 5.3 km3 (1.3 cu miles) of basaltic lava. After a period of dormancy of about 6,000 years, there was a small andesitic eruption about 4,300 years ago. The most recent eruptive episode lasted between 3000 and 900 years BP, when another eight eruptions produced approximately 2.5 km3 (0.6 cu miles) of lava ranging in composition from basalt to rhyolite. While the late Holocene lava compositions include basalt and andesite, silicic lavas ranging from dacite to rhyolite dominate.
The most recent eruptive episode at Medicine Lake Volcano occurred around 1,000 years ago when rhyolite and dacite magmas erupted at vents near the caldera’s eastern rim and led to the formation of Glass Mountain and ten smaller domes. Glass Mountain consists of a 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. The tephra pumice deposits that preceded the flow have been dated to less than about 1050 years before present while radiocarbon dating of a cedar tree indicate an age of about 885+/-40 years BP.
Recent exploratory geothermal drilling has identified a surprisingly silicic core mantled by mafic lavas. This result is unexpected and very different from the long-held view derived from previous mapping of exposed geology that MLV is a dominantly basaltic shield volcano. It was found that while < 6% of the ~2000 km2 of the mapped Medicine Lake Volcano lavas are rhyolitic and dacitic, drill holes on the edifice penetrated more than 30% silicic lava. It seems as if the andesite – dacite lavas that built most of the nearby Cascade stratovolcanoes never did erupt to form the usual stratovolcanic edifice on top of a basaltic shield at Medicine Lake. Since Medicine Lake Volcano is still an active volcano, even if presently dormant or in-between eruptive episodes, future large silicic and probably effusive eruptions are likely.
The Lava Beds National Monument
covers some 190 km2 (74 sq miles) to the northeast of Medicine Lake Volcano. It has one of the World’s greatest collection of different volcanic features such as lava tubes, cinder cones, spatter cones, hornitos, pit craters, maars, fumaroles, lava flows and volcanic fields. Most of the lava tube caves were formed by basaltic lava flows dated to about 30,000-40,000 years ago. 90% of the lava flows are basaltic and of those the smooth, easy-flowing and ropy Pahoehoe are the most common. The remainder of the lava flows are andesitic in nature. The park is also home to numerous cinder and spatter cones ranging in age from 114,000 to 3,025 years. The lava fields range in age from the two million years of Gillem Bluff to just under 1,000 years of the flow that created Glass Mountain.
One of the most intriguing features of the Monument is Petroglyph Point which formed about 275,000 years ago when cinder erupted through the shallow water of Tule Lake and violent explosions of ash and steam formed layers upon layers of tuff. Petroglyph Point is named for the Native American rock carvings, one of the most extensive in North America. Another interesting feature of the park is Captain Jack’s Stronghold, a natural fortress of lava tubes named after Captain Jack, a Native American who led a group of 53 fighting men and their families of the Modoc tribe that held off US Army forces for five months in spite of being outnumbered ten to one in the Modoc War of 1872-3.