Touring the Clear Lake Volcanics


Reported by Dan Day


Photography by Keil Albert, Mark Detterman, Phil Garbutt


Although the NCGS monthly meetings have been suspended for the summer, Field Trip Coordinator Jean Moran has filled the summer solstice with a gem. On August 2nd Sonoma State University geology professor Rolfe Erickson led about thirty geoscientists through “The Clear Lake Volcanic Field, Lake County, California.”  Rolfe is an igneous petrologist who has an obvious love of these eruptive rocks.  And he responded to our request with an elegant field trip guidebook and a full day among well-exposed volcanic deposits and bubbling geothermal springs of Lake County.


The day began with warm clear weather as the group assembled at Larkspur Ferry on the Marin peninsula and headed north on Highway 101 for Clear Lake.  The caravan jogged eastward to Calistoga and then followed Highway 29 to the first stop about 4 miles north of Middletown and 8 miles south of Kelseyville.  Here in an inconspicuous rural setting reminiscent of the Lake Tahoe area, Rolfe assembled the group at a small outcrop of Boggs Mountain andesite.



The latter was erupted from a small shield volcano located along the Collayomi fault about 6 to 8 miles southwest of the lake. The basalt-andesite-rhyolite volcanic compositional trend it belongs to traces a continuous increase in silica and alkali content that parallels the same trend in plutonic or intrusive rock of the gabbro-granodiorite-granite series.  The plutonic sequence is well documented in the intrusive rocks exposed in Yosemite National Park.  The Clear Lake volcanics are the extrusive equivalents of these deep-seated plutons.  Here, Rolfe discussed the crustal assimilation model that has been invoked to describe compositional trends and microstructural features observed in the Clear Lake volcanics.


The Clear Lake volcanic field has been active for the last 2 million years.  It covers an area of about 400 km2 in Lake County and has been intensively studied since the early 1970’s.  It contains about 100 eruptive units and its activity can be divided into four periods: 2.1 to 1.3 m.y., 1.1 to 0.8 m.y., 0.65 to 0.30 m.y., and 0.1 to 0.01 m.y.  These active periods are separated by quiescent spells of about 200,000 years.  An important geochemical indicator used to support the crustal contamination model for this volcanic field is the whole rock Sr87/Sr86 isotopic ratio.  Strontium-87 (Sr87) is generated by radioactive decay of rubidum-87.  Hence, over time, a closed rock system’s Sr87/Sr86 ratio will increase.  Siliceous volcanic rocks richer in Rb will also accumulate more strontium-87 over a fixed time span than more basic (basaltic) rocks.  The pattern observed at Clear Lake is a primitive mantle-derived basaltic magma with a whole rock Sr87/Sr86 ratio of 0.7032 associated with andesitic and more acid volcanics with Sr87/Sr86 ratios of 0.7039 and higher.  Although the difference in these two values seems very miniscule, one must remember that rubidium-87 decays very slowly, and that over a 2 million-year time span, this difference is significant.  Petrologists must therefore appeal to contamination of the original basaltic magma by a source enriched in Sr87/Sr86 to explain the higher ratios of the more acidic rocks.  As it turns out, siliceous crustal rocks are enriched in Sr87/Sr86 and have thus been proposed as the source of the higher Sr87/Sr86 values in the Clear Lake andesites, dacites, and rhyolites.  Rolfe noted that the current petrogenetic model has basaltic magmas rising to the mantle-crust boundary and residing there for a period of time while the liquid assimilates metamorphosed siliceous sedimentary material in the lower crust.  This not only boosts its Sr87/Sr86 ratio, but changes its bulk composition toward andesite and dacite.  Sampling has helped confirm this model, since many of the units contain metasedimentary rock inclusions (xenoliths) and foreign crystals (xenocrysts) that are not observed in the exposed bedrock stratigraphy.  The metamorphic grade of the xenoliths is also compatible with a deep-seated crustal source.  The model proposed by petrologist James Stimac has a the lower crustal assimilation stage followed by upper crustal fractional crystallization in a shallow magma chamber prior to eruption.  The latter helps drive the magma composition toward the more siliceous dacitic and rhyolitic end members.


Stop 2 took the group to the Loch Lomond church for a look at the basal contact between the Cretaceous Great Valley sediments and the overlying siliceous volcanics.  The contact is an erosional unconformity in the Great Valley bedrock overlain by bedded tuffs and some layers, possibly water-reworked, that contain blocks of Great Valley sediments.  Rolfe interprets this as a vent-clearing episode early in the eruption.  Subsequent vent eruption produced pumice lapilli (fragmented) tuffs and more massive units that show no evidence of reworking.  This likely represents a classic Plinian eruption, belching ash high into the atmosphere in a plume that distributes fine pyroclastic dust down-wind from the vent.

Looking up section across the contact between the Great Valley sequence (dark sandstones and shales in the left foreground) up to the section of eruptive tuff (lighter material in distance).



Eruptive tuff



The main pyroclastic unit is the Bonanza Springs rhyolite tuff (1.02 m.y.).  In outcrop exposure it is capped by the 0.92 m.y. Deiner Drive rhyodacite obsidian flow.  The Bonanza Springs is the thickest pyroclastic unit in the Clear Lake field.  Its high Sr87/Sr86 ratio suggests considerable crustal assimilation by the parent magma and prolonged fractionation of the hybrid magma before eruption.  The reworked uppermost units include an apparent lahar (volcanic mudflow) and brecciated dacite suggesting phreatic activity (hot lava in contact with water).




Details interpreted as a reworking of the tuff


Contact between the tuff and the Deiner Drive rhyodacite obsidian flow.


After a short drive from Loch Lomond church, the group assembled at an extensive roadcut through the rhyolite of Thurston Creek.  This 650,000 year-old rhyolite is the largest flow in the Clear Lake field.  It emanates from an arcuate belt 11 km. long, likely a fissure vent, and flows northward toward Clear Lake, a few kilometers to the north.  The outcrop exposes near-vertical banded obsidian interlayered with lighter pumice.  To the south the layers fan out into a series of isoclinal folds, suggesting the lava mass was flowing away from the vent area under its own weight.  At the northern end of the outcrop, the obsidian forms a sharp vertical contact with a devitrified felsite unit.  The latter may be an intrusive contact or perhaps a sudden transition to a devitrified core surrounded by a glassy rind.  To the north the rhyolite becomes more pumice-rich with blocky inclusions of obsidian glass.  This is characteristic of the Thurston Creek rhyolite as one follows it further from its source.  The Thurston Creek rhyolite is compositionally equivalent to the Bonanza Springs rhyolite, only it did not produce pyroclastics; its magmatic water was directed into the pumice interlayered with the obsidian.  The volume of magma erupted during this event (6 km3) suggests that some of the apparent tectonic dip to the flow banded units may be the result of a caldera collapse.


Isoclinal folds at the southern end of rhyolite of Thurston Creek road cut.



A view from the southern end to the more massive northern end of the road cut.




Just before lunch, the group stopped at the Maar Craters east-northeast of Kelseyville on the west-central lake shore.  The outcrop is nestled in Soda Bay on the margin of a volcanic crater now a part of the lake.  The road circumnavigating the shoreline slices through a deposit of accretionary lapilli; small, marble-sized mud and ash nodules that cling to the sides of an eruptive crater.  Maar craters are formed when rising siliceous magmas intersect near-surface groundwater, triggering a phreatic or steam-driven eruption that produces a small circular crater typically less than half as deep as its diameter.  They belong to a collective class of short-range pyroclastics known as surge deposits.  The three facies are: the sand wave facies nearest the vent, the plane bed facies at intermediate distances from the source, and the distal massive facies.  Several of these maars form the southern end of Clear Lake.  The alignment of these features suggests a fault or fissure control.  The Ferndale Marina stop exposes a wonderful sequence of lapilli layers comprised of small mud nodules ejected from the muddy lake bottom.


Accretionary lapilli - small, marble-sized mud and ash nodules clinging to the sides of the eruptive crater.  Rolfe believes it reasonable to assume this is original bedding.


Lapilli and mud details.



The marina shoreline is arcuate in shape and reflects a series of maars eruptions.  View to the north.


The Afternoon


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