HOLOCENE PLINIAN TEPHRA DEPOSITS OF THE MEDICINE LAKE HIGHLAND, CALIFORNIA
Condensed from: PLINIAN-TYPE ERUPTIONS IN THE
MEDICINE LAKE HIGHLAND, CALIFORNIA, AND THE NATURE OF THE UNDERLYING
MAGMA. Jour. Volc. and Geotherm. Res., 4, 375-402 (1978).
The Medicine Lake Highland is a young volcano located 55 km east of Mount Shasta, on the western margin of the Modoc Plateau. The volcano is east of the north-south-trending chain of High Cascade volcanoes and within a 100-km diameter depression filled with volcanic rocks of Pliocene to Holocene age. It overlies a plateau consisting of late Tertiary tuffs, lava flows and sediments, cut by north- to northwest-trending normal faults (Powers, 1932; Anderson, 1941). The volcanic center is located at the intersection of several of these fault systems.
The highland is a 1 km thick, 25 km-diameter shield, composed of andesitic and basaltic flows (Anderson. 1941). There is an 8 km by 6 km, 100-200 m deep caldera at the shield summit. Noble (1969) believes that an andesitic pyroclastic flow, interbedded with andesitic flows of the shield, may have been associated with caldera collapse. Andesitic, dacitic and rhyolitic flows were erupted along the caldera rim and partly bury the walls. Construction of the shield and caldera formation are believed to have occurred mainly during Pliocene to Pleistocene time (Anderson, 1941). During the time elapsed between the earlier eruptions of andesite, dacite and rhyolite and more recent activity, there was glaciation of the highland and substantial soil developed (Anderson, 1941).
Holocene activity was bimodal, consisting of basaltic volcanism, mainly on the shield flanks and eruption of tephra and lavas of intermediate to rhyolitic composition within and close to the rim of the caldera.
The purpose of this paper is to present data on the Holocene pumice deposits of the Medicine Lake Highland and to speculate on their origin. These deposits were erupted from Glass Mountain, a flow of rhyolite-dacite on the northeast rim of the caldera and from Little Glass Mountain. located a few kilometers west of the caldera (Fig. 1).
Age of the Holocene deposits
Underlying Holocene rhyolitic and basaltic tephra exposed in the highland is a red or yellow soil, developed on both flows and older tephra. This soil is estimated to be more than 15,000 years old (O. Miller, personal communication, 1977). The oldest tephra overlying the soil are fine-grained basaltic ash layers near Little Glass Mountain and northwest of Glass Mountain. They range from 0.5 to 14 cm thick and are reasonably fresh (sideromelane glass is well preserved). It is possible that these tephra are from Paint Pot Crater or some of the younger cones in Lava Beds National Monument (Table 1). Immediately north of Glass Mountain, the soil has developed on an older pumice bed (Chesterman, 1955). Chesterman dated trees that were growing in this soil and buried by younger pumice beds to indicate the age of the Holocene Glass Mountain tephra. Overlying the lowest Holocene basaltic tephra unit are the pumice deposits of Little Glass Mountain, with a radiocarbon date of 1065±90 years*. As will be discussed later, the tephra are similar to those of Glass Mountain. Luckily, one of the more extensive tephra beds from Little Glass Mountain has a distinctive yellow hue. By following this bed it was determined that Little Glass Mountain tephra underlie tephra units erupted from Glass Mountain. The age of the Little Glass Mountain flow is not known, but is believed to be only slightly younger than its tephra deposits; they may have been part of the same eruptive sequence.
Within the zone of overlapping Glass Mountain and Little Glass Mountain tephra, there is a thin (5 cm) bed of basaltic ash, believed to be from one of the young cinder cones on the northwest flank of the shield.
The uppermost tephra beds in the highland were erupted from Glass Mountain and have a radiocarbon age of 1360±240 years (Chesterman, 1955). The analyses were of carbon from tree trunks buried by the Glass Mountain pumice falls. The Glass Mountain flows have been dated at 130 to 390±240 years by the 14C method (Friedman, 1968); these dates were based on wood from standing trees damaged by the flow. There is an inconsistency here: field evidence indicates that little time elapsed between the pumice eruptions and extrusion of the flows at Glass Mountain. whereas radiocarbon dates indicate a hiatus of 1000 years. The older date of 1360±240 years is of tephra overlying tephra dated at 1065±90 years. Tephra from Glass Mountain also overlie the very well-preserved High Hole Crater cinder cone and Burnt Lava Flow. The youth of these basaltic vents is also indicated by the presence of tree trunks burned by the flows, resting on the flow margins.
In summary, most Holocene silicic rocks near the highland summit and the most youthful basaltic cones and flows on the highland flanks were contemporary and may have erupted during the last 1100 years.
Little Glass Mountain
Tephra from Little Glass Mountain form a highly elongate ellipse with its long axis oriented southwest-northeast (Fig. 2). Measurable tephra layers are present up to a distance of 25 km and individual rhyolitic lapilli possibly from Little Glass Mountain have been found 50 km west-southwest of the vent. (The most distant lapilli were found by D. Miller on the slopes of Mt. Shasta). The area covered by measurable tephra layers is about 200 km2; the tephra have a volume of 0.046 km3.
Within 3 km southwest of Little Glass Mountain, there are five tephra units, consisting of mainly reversely graded, pale gray pumice and lapilli-size blocks in coarse gray ash.
These units range in thickness from 20 to 50 cm and each contains between 5 and 20% lithic fragments. The only evidence for a hiatus within these units is a thin (1-5 cm) coating of orange-brown clay at the surface of the third tephra unit. Contacts between all other units exhibit no evidence of weathering or erosion. Between 3 and 8 km southwest and 6 km northeast of Little Glass Mountain, there are two tephra units; the lowest, overlying a well-developed soil, consists of a 0-24 cm thick, medium to coarse gray ash sometimes with a slight yellow hue; the upper unit consists of a 13-64 cm thick, generally reversely graded, lapilli-bearing coarse ash. It is possible that the break between units 3 and 4, near the vent corresponds to the break between the two tephra units farther from the vent. Beyond 9 km, only one tephra layer is present. This tephra bed is most readily correlated to the upper of the two tephra units, located closer to Little Glass Mountain, on the basis of its greater thickness (Fig. 3). All of the tephra units erupted from Little Glass Mountain appear to have been deposited as air-fall.
There appear to have been two periods of explosive activity at Little Glass Mountain; the only evidence for this, however, is the clay coating developed on the top of a tephra layer (Fig. 3). The tephra are overlain by the flows of Little Glass Mountain, a rhyolite flow with a volume of 0.3 km3. The flow has buried the vent area.
Pumice deposits from Glass Mountain form an irregular ellipse, with the long axis trending east-northeast (Fig. 2); the deposits with measurable thicknesses cover an area of 320 km2 and have a volume of 0.09 km3. Within an approximate range of 3 km east and northeast of the summit of Glass Mountain, there are multiple tephra beds. Within a pumice pit located 2.8 km east of the summit, eleven tephra beds, with a combined thickness of 3.7 m, overlie a red-brown soil developed on cinders. The tephra units consist of reversely graded or massive pumice block and lapilli-bearing coarse gray ash, 5-102 cm thick. Several of the uppermost beds contain 10-30% lapilli and coarse ash-size lithic fragments (mainly angular obsidian fragments and cinders). Sharp depositional contacts exist between these beds; there was no erosion or weathering at the bedding plane surfaces. Several tephra units exhibit a pale orange-pink discoloration near the top of each bed; the alteration may be due to mild vapor-phase activity that left a thin film of hematite stain on pyroclast surfaces. Between a distance from the vent of 3-11 km east, 3-11 km north and 1(?)-3 km south and west, there are two or three tephra beds with combined thicknesses of 22-131 cm. Both consist of reversely graded or massive beds of gray medium-ash to lapilli-size tephra. Due to the remarkable uniformity of the Glass Mountain tephra, it was not possible to corelate any of these beds with units described near the vent. Units 1, 7 and ll of the Pumice Pit section are only tentatively correlated with the more distant units, because they are the thickest units. Beyond the range of multiple beds there is only one bed of massive coarse ash and lapilli; this rapidly thins to zero.
All tephra beds from Glass Mountain drape the underlying terrain and exhibit no current structures; there is no evidence for deposition by flow. After deposition of rhyolite tephra, the Glass Mountain dacite-rhyolite flows were erupted along a north-northwest-trending fissure. In addition to the main body of Glass Mountain there are, along the fissure, nine small domes. Associated with the domes are small, crescent-shaped pumice cones, consisting of angular, 2-45 cm long pumice lapilli and blocks.
Other silicic vents
The Crater glass and Medicine Lake glass flows appear to be approximately the same age as Glass Mountain and Little Glass Mountain, but neither exhibited much explosive activity. There were only a few lapilli or blocks erupted from these vents; there is no evidence for extensive tephra deposits asso ciated with them.
Vents for the Holocene eruptions of silicic tephra and lavas are located along the oval caldera rim and along fissures parallel to the rim. Glass Mountain and associated small domes were erupted along a fissure trending northwest-southeast. There is no clear evidence that Little Glass Mountain was erupted from a fissure. There is, however, an arcuate trend, concentric to the caldera rim that includes the Crater Glass Flow (Fig. 1) and several en-echelon open fissures. The fissures, with a total length of several kilometers, do not appear to be associated with any of the normal faults of the region and are believed to be an expression of rhyolite dikes that are near the surface, but did not erupt. Crater Glass Flow was erupted from a segment of one of these fissures.
It was suggested by Anderson (1941) that the caldera at Medicine Lake collapsed along cone fractures that later acted as conduits for Pleistocene and Holocene silicic magma. This may indeed be the case; eruptions along the caldera rim and along fissures concentric to that rim may indicate the presence of several concentric, inward dipping cone sheets located near the summit of the Medicine Lake shield. Cone fracture systems are developed over plutons during intrusion and have been observed in many intrusive-volcanic complexes throughout the world (Anderson, 1937; Koide and Bhattacharji, 1976). Cone fractures dip inward, from the ground surface, at angles of 45° to nearly vertical. If the models for cone sheet formation developed by Anderson and Koide and Bhattacharji are correct, then cone fractures may extend downward from the surface expression of calderas and concentric fissures to a magma body that is considerably narrower than the caldera.
The remarkable similarity and contemporaneity of the eruption at Little Glass Mountain and Glass Mountain, located 15 km apart, suggest that they were erupted from the same body of magma, possibly along cone fractures. Eruptions at the two vents have the following in common: (1) the same eruption sequence, consisting of 7-8% tephra and 92-93% flow; (2) identical major-element compositions for the tephra; (3) nearly identical tephra characteristics, as outlined earlier; (4) similar ages; (5) the lavas have identical trace element compositions (Condie and Hayslip, 1975). The uniqueness of trace element data for individual magma bodies has led to the use of these data to 'fingerprint' tephra deposits for the purpose of correlation (e.g., Borchardt at al., 1971, Howorth and Rankin, 1975).
The total volume of Holocene silicic tephra and lavas is about 1.2 km3. On the basis of the small volume of eruptive rocks, it is possible to make the inference, albeit weak, that the magma body is small. The caldera has a volume of about 8 km3 (Anderson, 1941). If the caldera volume is a reflection of the approximate size of the magma chamber, this is still a relatively small chsmber. Also, the caldera volume may reflect the earlier phase of andesitic volcanism and not that of the Holocene silicic eruptions.
The ratio of 1:9 for tephra to flows, observed at Glass Mountain and Little Glass Mountain also fits general model for small silicic magma chambers developed by Smith (1976) Interpretation of the structural setting and the petrology of the silicic tephra and lavas erupted in the Medicine Lake Highland during Holocene time supports the hypothesis that there is a small silicic magma body below the highland. Concentric cone fractures may have developed over the apex of a body located at a depth of 4-8 km; this depth is consistent with the model presented by Loide and Bhattacharji (1976) and with observed intrusive-volcanic complexes and elsewhere in the Cascade Range (e.g. Erikson, 1969) and Peru (Cobbing and Pitcher, 1972). If it is true that only a small cooling body of magma is present below the highland shield, a reassessment of the area as a potential geothermal area may be necessary. This hypothesis should be kept in mind during the development of the highland which has been classified as 'known geothermal resource area' (Godwin at al., 1971).
The tephra blankets fit a general description of deposits having been formed during Plinian or sub-Plinian eruptions. A Plinian eruption is exceptionally violent and ejects copious pumice (Escher, 1933; Walker and Crosadale, 1970). Tephra were deposited as air fall with no evidence for deposition as pyroclastic flows. An eruption of this kind fits the model outlined above of vesiculated rhyolite magma erupted from the volatile-rich upper portion of a small magma chamber.
Deposition of tephra in beds may be due to individual eruptions related to time of refilling of a fissure, vent blockage, or variation in wind intensity over the volcano. In several tephra units from Glass Mountain and in all units from Little Glass Mountain, concentrations of lithic fragments at the base of each bed indicates clearing of vents or erosion of vent walls during explosive activity.
After the volatile-rich top of the magma chamber was erupted as tephra. the bulk of the lavas with low (<20-0%) vesicularities were erupted.
(1) Holocene activity in the Medicine Lake Highland consisted of basaltic eruptions on the flanks and eruptions of silicic tephra and lavas near the summit. The rhyolitic eruptions at Glass Mountain and Little Glass Mountain occurred during the last 1100 years.
(2) Air-fall tephra consists of very poorly sorted lapilli that are mostly pumice pyroclasts. The tephra falls formed elongate ovals, extending northeast of Glass Mountain and southwest of Little Glass Mountain. The tephra deposits are similar to those deposited during Plinian eruptions.
(3) Pumice pyroclasts are remarkably homogeneous and consist of blocky, angular forms. Most have elongate simple and compound vesicles and vesicularities of 45-60%. All of the pyroclasts consist of homogeneous rhyolite glass.
(4) The Medicine Lake Highland is located at the intersection of several normal faults; a 'weak spot' that allowed pooling of basaltic magmas needed for crustal melting or a 'conduit' for the rise of diapirs of silicic magma.
(5) The contemporaneity and physical and chemical similarity of tephra and lavas erupted at Glass Mountain and Little Glass Mountain, located 15 km apart, suggest that they were erupted from the same magma body. They may have erupted along cone fractures developed above the magma chamber. The relatively small volume of Holocene silicic eruptions, small caldera volume, and a ratio of 1:9 for tephra and flows suggest that the magma body had an approximate volume of between 2 and 8 km3. If this hypothesis is correct, the highland might have less potential as a geothermal resource than was previously believed.
(6) The blocky, angular pumice pyroclasts may have developed by: (a) vesiculation and elongation of vesicles by flow within 1-2 km of the surface vent; (b) comminution and disruption of the brittle, vesiculated magma by an expansion. or 'relief' wave passing down into the vent from the magma-atmosphere interface.
Samuel Gallegos, Jr. was an able field and labor story assistant throughout the summer of 1976. I wish to thank D. Mann and T. Gregory for their help as well. I benefited from discussions of the Medicine Lake tephra with J. Eichelberger, R. Crowe, and C. D. Miller. Howel Williams and an anonymous reviewer provided useful reviews of the manuscript. This research was supported by the Department of Energy, Division of Basic Energy Sciences under contract W-7405-ENG-36.
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Last Updated: 28-Mar-2006