Medicine Lake Volcano

Medicine Lake Volcano
Medicine Lake Volcano seen from the Lava Beds National Monument
height	2414  m
location	Siskiyou County and Modoc County California , United States
Mountains	Cascade chain
Coordinates	41 ° 36 '39 "  N , 121 ° 33' 13"  W Coordinates: 41 ° 36 '39 "  N , 121 ° 33' 13"  W
Type	Shield volcano
rock	Basalt , basaltandesite , dacite , rhyolite
Age of the rock	Middle Pleistocene to Holocene
Last eruption	1080 ± 25

The Medicine Lake Volcano is in the northeastern California befindlicher large shield volcano , the volcano arc of the Cascade Range belongs. The highest point is Mount Hoffman with 2414 m.

Geological overview

Medicine Lake Volcano (abbreviated MLV), located on the Modoc Plateau , is located around 50 kilometers behind (ie east) the southern main cascade chain and thus occupies a reararc position . In the Modoc Plateau, the geodynamic effects of the subduction of the Gorda Plate in the Cascadia subduction zone overlap with the east-west-oriented crustal expansion tectonics of the Basin and Range Province . These two factors therefore control both the structure and the magmatic development of the shield volcano.

In the regional fracture pattern, the volcano comes to lie in a kink zone. In addition, it is traversed by a crustal weak zone that connects it to the southwest with Mount Shasta and finds its geomorphological expression in a series of extraction vents. Furthermore, the northwest end of the Walker Lane Fault Zone - a tectonic fracture zone - and the south end of the Klamath Trench meet at Medicine Lake Volcano .

description

The 1000-meter-thick shield volcano measures 35 kilometers in an east-west direction and 45 to 50 kilometers in a north-south direction. It covers an area of ​​more than 2000 square kilometers. Its base has sunk by 500 meters due to its weight in the middle of the volcano. The volcano is mainly built up by lava flows of basaltic and basaltandesitic composition. At its center is a 7 × 12 kilometer caldera elongated in an east-west direction . On the eastern edge of the caldera sits the almost vegetation-free Big Glass Mountain , a geological rarity that is built up from an obsidian flow from a rhyolite to Dacite composition around 1000 years old . The Lava Beds National Monument is located on the northeast flank of the volcanic building .

The shield volcano rises 1200 meters above the Modoc plateau and in Mount Hoffman reaches a height of 2414 meters (sometimes only 2376 meters are given). The total volume of lava flows at Medicine Lake Volcano is estimated at 600 cubic kilometers, which makes it the first volcanoes of the Cascade Range, followed by Newberry Volcano in Oregon .

Medicine Lake Volcano has been active for 500,000 years. In contrast to the explosive Mount Saint Helens , its eruptions were mostly calm and covered its flanks with countless basalt currents. The 1 × 2 kilometer large Medicine Lake of the same name was only created later in the huge bowl-shaped caldera.

It is believed that Medicine Lake Volcano is unusual in its evolution as it has several small magma chambers instead of a single magma chamber.

Development history

A total of 208 volcanic units could be mapped at Medicine Lake Volcano. Donnelly-Nolan and colleagues (2008) now divide their development into 5 stages:

• Stage 1 - 500,000 to 300,000 years BP (Middle Pleistocene) - early Si-rich volcanism
• Stage 2 - 300,000 to 180,000 years BP (late Middle Pleistocene) - pre-stage of the Dazittuff of Antelope Well
• Stage 3 - 180,000 to 100,000 years BP (late Middle Pleistocene and End Pleistocene) - Calderar rim construction
• Stage 4 - 100,000 to 12,500 years BP (End Pleistocene) - large basalt rivers
• Stage 5 - 12,500 years BP to today (Holocene) - postglacial volcanism

Stage 1

With the formation of rhyolite east of Glass Mountain, Medicine Lake Volcano began to grow 475,000 ± 29,000 years ago BP . Chemically comparable rhyolite magmas with an SiO ₂ content of 75 to 77.2 percent by volume and a low δ ¹⁸ O of 6.3 ‰ were repeatedly supplied in the first stage in the development of the volcano (a total of 6 further rhyolites). There were also 2 basalts around 445,000 years BP, 2 aluminum-rich oceanic tholeiites (HAOT), 4 basalt sandesites and 3 andesites.

Stage 2

Stage 2 was dominated by 25 basaltic and 13 andesitic lava flows, most of which emerged on the northwest and west flanks. Towards the end of the stage there were 7 Dazite eruptions and one rhyolite eruption. The Dazites culminated in the Dazite ash river of Antelope Well , which ended stage 2 and could be dated 171,000 ± 43,000 years BP. This tuff is the only unit on Medicine Lake Volcano that is spread over the entire volcano and thus represents an important stratigraphic guide horizon.

Stage 3

During stage 3 around 40% of all volcanic units formed at Medicine Lake Volcano, mainly basalts and andesites (32 basalts, 9 HAOT, 27 basaltandesites and 20 andesites, but only 1 dacite and 1 rhyolite). Towards the end of the stage, the caldera emerged along arched faults, where conveyor chimneys then settled and built the caldera rim. Here, Si-rich, aphyric, low-viscosity andesites were promoted, which flowed into the caldera as well as to the outside via its north and south edges. These andesites were overprinted during the Ice Age, recognizable by the Karen . Mention should be made of the primitive basalt current of the Damons Butte lying outside the volcano , which was dated to 144,000 ± 15,000 years BP.

Stage 4

Stage 4 is clearly characterized by a predominance of basalts (16 basalts, 6 HAOT and 4 basalt andesites, but only 2 andesites, 1 dacite and 1 rhyolite). The basalts had a very high fluidity and therefore covered large areas, especially in the deeper flank area on the east side of the volcano. Examples are the Basalt Current from Yellowjacket Butte with 300 square kilometers, the Lake Basalt with around 150 square kilometers, the Basalt of Tionesta with more than 140 square kilometers and the Basalt Current from Mammoth Crater with more than 225 square kilometers. The only dacite of this stage was formed very early, but the only rhyolite was not formed until about 30,000 years BP. The latter builds the summit of Mount Hoffman on the north-eastern rim of the caldera.

Stage 5

Since the beginning of the Holocene around 12,500 years ago, funding activities at Medicine Lake Volcano have been episodic. A total of 18 volcanic units erupted, including 7 basalts, 2 HAOT, 3 basalt andesites, 1 andesite, 2 dazites and 3 rhyolites. Around 12,500 years BP, eight eruptions produced around 5.3 cubic kilometers of basaltic and basaltandesitic lava over a period of several hundred years. Particularly noteworthy is the large eruption of the Giant Crater . The basaltic magmatism of this phase is possibly explained by the melting of the 150-meter-thick ice cover at the beginning of the Holocene, which led to a pressure drop in the magma chamber.

This was followed by a longer hiatus of 7000 years, which was only ended by two dacite eruptions around 5000 years BP. Around 3000 years BP followed a basalt on the north flank and an andesite on the south flank. Between 1250 and 950 years BP, the volcano was again very active and produced 2.5 cubic kilometers of lava in basaltic and finally rhyolite composition in five eruptions. These rhyolites of the final phase are significantly poorer in SiO ₂ than the rhyolites of the first stage, but their δ ¹⁸ O content is higher (up to + 8.5 ‰). They are rich in phenocrystals and contain deterred mafic inclusions, suggesting a deeper origin.

Spatial distribution of volcanic rocks

In general, mafic magmas dominate the flanks of the Volkans, but since the middle Pleistocene the entire range of magmas , from basalt to rhyolite, has been represented. On the lower flank section, mainly basaltic and some andesitic lavas predominate. Basalts are missing in the higher altitudes of the volcano, but mainly andesites, rhyolites and rarely dazites are found.

The lava flows at higher elevations on Lake Medicine Volcano are found both inside and outside the caldera, including numerous rhyolites and dazites. Ash cones and associated lava flows of basalt and basaltic andesite, however, originate from conveyor chimneys in the flank area. Most of the vents follow crustal weakness zones that are oriented from northeast to north to northwest.

Petrology

The HAOT (46 to 50.5 percent by weight SiO ₂ ), basalts, basalt andesites (49 to 58 percent by weight SiO ₂ ), andesites (51 to 62 percent by weight SiO ₂ ), dazites (64 to 69 percent by weight SiO ₂ ) and rhyolites (72 to 76 Weight percent SiO ₂ ) of Medicine Lake Volcano represent a typical calcareous differentiation sequence. With increasing SiO ₂ content, their magmatites show an increase in the ⁸⁷ Sr / ⁸⁶ Sr ratio, which is explained by the progressive assimilation of the rock crust in the magmas originating from the earth's mantle leaves. In addition, increased values ​​of incompatible elements such as rubidium (in HAOT 0 to 5 ppm, in basalt andesite 5 to 70 ppm, in andesite 10 to 85 ppm, in dazite 105 to 125 ppm and in rhyolite 140 to 160 ppm) allow the conclusion that a fractional crystallization alone could not have been responsible for this increase, but it must have been accompanied by an assimilation process of Si-rich rocks (such as feldspar-rich sandstones and rhyolites). Fractionation alone (of olivine, plagioclase and clinopyroxene) can only produce the observed variability within the HAOT, but an additional assimilation rate of 11 to 40% can very well produce the andesites and an assimilation rate of over 40% the basaltandesites.

Basaltic and rhyolite magmas occurring together also indicate magma mixing . The dazites, which are separated in their composition, can probably only be explained by the mixing of basaltic and rhyolitic magmas. The latter assumption is confirmed by mixed dacite and rhyolite caverns as well as by imbalanced phenocrystals in basalt andesite and dacite. In a single sample, magnesium-rich olivine (Fo ₉₀ ), calcium-rich plagioclase (An ₈₅ ), reverse-zoned orthopyroxene with an iron-rich core and labradorite rim are found.

Increased values ​​of compatible elements such as B. Nickel in basalt andesite (up to 150 ppm) and andesite (up to 55 ppm) are obviously due to mixing processes of HAOT (up to 255 ppm) and rhyolite magma (up to 20 ppm).

Caldera

Medicine Lake, located in the caldera of Medicine Lake Volcano, with Mount Shasta in the background

The caldera was created by collapse, probably after a large volume of andesites had been extracted from vents along the caldera rim. It measures 7 × 12 kilometers and contains Medicine Lake inside. The arrangement of the late Pleistocene vents predominantly along the Calder Arands suggests that at the time of the outbreak of andesite already concentric distortions ( English ring faults ) were present. So far, however, none of the eruptions could definitely be correlated with the caldera collapse. The only eruption that produced ash river tufa, the dacite of Antelope Well , dates back to the late Pleistocene. However, its volume is too small to justify the formation of the huge caldera. A later attempt to explain the collapse sees the collapse as a response to repeated extrusions of mafic lava that had occurred early in the volcano's evolutionary history (ie roughly comparable to the formation of the caldera on Kilauea in Hawaii). It is probable that several smaller magma bodies were connected to one another via a knot of corridors and storage corridors and were filled by them.

Big Glass Mountain

Obsidian stream disintegrated into blocks at Big Glass Mountain

→ Main article : Big Glass Mountain

The last eruption occurred around 1000 years ago on the eastern edge of the caldera, when rhyolite and dacite magmas flowed out on Big Glass Mountain. Big Glass Mountain consists of a spectacular, almost treeless, steep-walled obsidian current that emerged just outside the eastern edge of the caldera and flowed down the steep northeast flank of Medicine Lake Volcano.

Individual evidence

1. ↑ summitpost: Mount Hoffman (Calif. Cascades)
2. ↑ ^{a b c} Donnelly-Nolan, JM, Grove, TL, Lanphere, MA and Champion, DE: Eruptive history and tectonic setting of Medicine Lake Volcano, a large rear-arc volcano in the southern Cascades . In: J. Volc. Geothermal. Res. Band 117 (2) , 2008, pp. 313-328 .
3. ↑ Hildreth, W .: Quaternary magmatism in the Cascades - geological perspectives . In: US Geological Survey Professional Paper 1744 . 2007.
4. ↑ Blakely, RJ, Christiansen, RL, Guffanti, M., Wells, RE, Donnelly-Nolan, JM, Muffler, LJP, Clynne, MA and Smith, JG: Gravity anomalies, Quaternary vents, and Quaternary faults in the southern Cascade Range , Oregon and California; implications for arc and back arc evolution . In: J. Geophys. Res. Band 102 , 1997, pp. 22 513-22 527 .
5. ^ Macdonald, Gordon A .: Geology of the Cascade Range and Modoc Plateau . In: Geology of Northern California . tape 190 . USGS, 1966, pp. 65-95 .
6. ^ Donnelly-Nolan, JM and Lanphere, MA: Argon dating at and near Medicine Lake volcano, California; results and data . In: US Geological Survey Open-File Report 20051416 . 2005, p. 37 .
7. ↑ Herrero-Bervera, E. include: Age and correlation of a paleomagnetic episode in the western United States by ⁴⁰ Ar / ³⁹ dating Ar and tephrochronology: the Jamaica, Blake, or a new polarity episode? In: J. Geophys. Res. Band 99 , 1994, pp. 24091-24103 .
8. ↑ Sigvaldason, GE: Volcanic and tectonic processes coinciding with glaciation and crustal rebound: an early Holocene rhyolite eruption in the Dyngjufjoll volcanic center and the formation of the Askja caldera, north Iceland . In: Bull. Volcanol. tape 64 , 2002, pp. 192-205 .
9. ^ Lowenstern, JB et al .: Volcanism, plutonism and hydrothermal alterations at Medicine Lake Volcano, California . In: Proceedings, Twenty-Eighth Workshop on Geothermal Reservoir Engineering . Stanford University, Stanford, California 2003, pp. 8 .
10. ↑ Grove, TL and Baker, MB: Phase equilibrium controls on the tholeitic versus calc-alkaline differentiation trends . In: Journal of Geophysical Research . tape 89 , 1984, pp. 3253-3274 .
11. ^ Grove, TL, Gerlach, DC and Sando, TW: Origin of calc-alkaline series lavas at Medicine Lake Volcano by fractionation, assimilation and mixing . In: Contr. Min. Petrol. tape 80 , 1982, pp. 160-182 .

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