DESCRIPTION:
Salton Buttes Lava Domes

The Salton Buttes comprise five small rhyolite domes extruded onto Quaternary sediments of the Colorado River delta. Rock Hill and Mullet Island are simple domes; Mullet Island is notable for its symmetrical "onion-skin" pattern of foliation, attributed to endogenous growth. Obsidian Butte consists of a central dome surrounded on all sides by a single flow. Red Island is made up of two domes, each mantled by subaqueous pyroclastic deposits. Xenoliths of basalt, partly melted granite, deltaic sediments, and their hydrothermally metamorphosed equivalents are common in the rhyolites of Obsidian Butte and Red Island. All the domes exhibit wave-cut benches carved during various stands of pre-historic Lake Cahuilla, and have been partly buried by lacustrine and aeolian deposits.

The Salton Buttes lie within the Salton Sea Geothermal Field, where temperatures at 1.5 to 2.5 kilometers reach 360 degrees C, and sediments of the Colorado River delta are being metamorphosed to greenschist facies. Both rhyolitic and basaltic intrusive rocks have been encounted by geothermal wells. A 5x8-kilometer magnetic high beneath Salton Buttes appears to represent either a batholith of a large dike swarm at depth. The dome field, the intrusive rocks, and the geothermal system are all manifestations of a spreading center beneath the sediments of the Colorado River delta, as part of the leaky transform fault that is transitional from the Gulf of California to the San Andreas fault system.

The Salton Buttes are the youngest extrusions of a bimodal basalt-rhyolite system that probably existed throughout the Quaternary. Extrusions older than latest Pleistocene, however, are now buried by sediments of the Colorado River delta.

The Salton Buttes are at the southeast end of the Salton Sea, in the Imperial Valley of southern California, approximately 165 kilometers ENE of San Diego. Obsidian Butte, Red Island, and Rock Hill are accessible by road; Mullet Island can be reached only by boat.

The Salton Buttes rhyolite center:

• Most recent eruption: Silicic pyroclastic and extrusive eruptions at four vents approximately 16,000 years ago (Potassium-Argon dating);
• Five silicic domes erupted;
• At least one silicic lava flow associated with rhyolite dome;
• Probable small-volume pyroclastic eruptions with tephra associated with dome emplacement at several vents;
• No recognized debris avalanches or debris flows;
• Most probably future potential hazard: Explosive and extrusive rhyolitic eruptions; phreatic and phreato-magmatic eruptions.

Although the youngest eruptions in this area are not known to have produced pyroclastic flows and surges, the compositions of lavas of past eruptions and the association of vents with ground water and the Salton Sea suggest that pyroclastic flows and surges and explosive eruptions could occur in the future. Such events commonly are destructive out to distances of at least 10 kilometers (6 miles) from an active vent.

The Salton Sea geothermal field lies in the Salton Trough, the landward extension of the Gulf of California, an area of active crustal spreading. Surface volcanic rocks of the field consist of five small rhyolite domes extruded onto Quaternary sediments of the Colorado River delta. Two domes are linked by subaqueous pyroclastic deposits; the others are single extrusions with or without marginal lava flows. The domes are low-calcium, alkali rhyolite with 1 to 2 percent crystals. Similar silicic rocks found in wells have been extensively altered by geothermal brines. ...

Many recent studies (Wilson, 1965; Atwater, 1970; Larson and others, 1972; Moore, 1973) have suggested that the Gulf of California is the locus of a spreading ridge along which the Pacific Plate is being rifted away from the North American Plate at a rate of perhaps as much as 6 centimeters per year (Larson, 1972). ...

The (Salton Buttes) volcanoes lie within the Salton Sea geothermal field where temperatures measured in wells drilled for geothermal brines range up to 360 degrees C at depths of 1,500 to 2,500 meters (Helgeson, 1968). The wells produce a hot brine containing up to 160,000 ppm of dissolved solids, chiefly Cl, Na, K, Ca, and Fe (White, 1968). Under the influence of this hot saline brine, the sediments of the Salton Trough are being transformed into metamorphic rocks of the greenschist facies (Muffler and White, 1969). ...

The Salton Buttes are four small volcanoes designated from southwest to northeast as Obsidian Butte, Rock Hill, Red Island, and Mullet Island. Red Island is composed of two domes linked by a deposit of subaqueous pyroclastic material; the other volcanoes are single domal extrusions with or without marginal lava flows. The domes are spaced at intervals of 2 to 3 kilometers along a northeast trend. ...

Obsidian Butte consists of a central dome of rhyolite surrounded on all sides by a single rhyolite flow. The dome is approximately 250 meters in diameter and rises some 30 meters above the adjacent alluvial surface of the Imperial Valley. The flow stands 20 to 25 meters below the crest of the central dome. The contact between the flow and the dome is exposed on the northwest side of the dome as a steeply dipping breccia zone. The central dome of Obsidian Butte is composed of medium-gray to dark-gray rhyolite that is locally pumiceous and (or) spherulitic. Flow foliation, although variable and erratic in detail, defines a crudely domal pattern. The flow around Obsidian Butte consists of rhyolitic obsidian covered by a discontinuous mantle of brown-weathering, light-gray pumice. Numerous spines of black obsidian protrude upward and through the blocky pumice mantle. Low ridges and troughs rudely concentric to the central dome indicate that the flow spread radially from a vent located where the dome now rises. At the perimeter of this flow, there is a discontinuous ring of ten obsidian hills that rise as much as 15 meters above the surrounding alluvium. ...

Overlying the eastern part of the flow and lapping onto Obsidian Butte itself is a thick deposit of gravel, much of which has been removed by quarrying (Chesterman, 1956). A bulldozer cut just east of Obsidian Butte exposed 10 meters of gravel and sand composed almost entirely or rounded clasts of pumice and ash. This material was derived from the pumice mantle of the flow and from pyroclastic eruptions that probably preceded extrusion of the flow. The pumice clasts were rounded by wave action of Lake Cahuilla and were concentrated on the eastern, lee side of the volcano. At least seven wave-cut benches can be identified on the east slope of Obisdian Butte. Thick rinds of tufa on obsidian surfaces throughout the flow also attest to covering of Obsidian Butte by lake waters in the past. ...

The morphology of Obsidian Butte and its surrounding flow indicates that they were produced by a single volcanic eruption with extrusion from a single, central vents. The obsidian domes, spines, and breccia mounds that ring the flow are interpreted to be pressure features developed at the outer margin of the flows. Obsidian Butte itself is topographically higher than the flow because of a late pulse of magma that pushed up a central, viscous mass of partly crystallized rhyolite. The Obsidian Butte volcanic rocks were extruded subaerially but were covered by Lake Cahuilla and modified by wave action soon thereafter.

Rock Hill rises approximately 25 meters above the alluviated surface of the Imperial Valley. It is composed of medium-gray rhyolite that is locally spherulitic, with minor obsidian and a discontinuous mantle of pumice. ... The foliation forms a crude dome with rather steep dips to the west-southwest and gentler dips to the north-northeast. Rock Hill is very similar to Obsidian Butte, but if a flow ever surrounded Rock Hill, it is now completely covered by the Colorado River alluvium.

Red Island is made up of two separate rhyolite domes, each mantled by subaqueous pyroclastic deposits and by later gravels and alluvium. The northern dome, which rises approximately 30 meters above the valley floor, has been extensively modified during construction of a marina and trailer park, and much of the gravel that overlies the southern dome has been removed by quarrying. ...

The part of Mullet Island presently exposed above the Salton Sea is composed of medium-gray spherulitic rhyolite. Foliation int he rhyolite clearly defines a simple dome which near the center of the island becomes a north-northeast-trending anticline sheared off on the north end. The domal foliation pattern, the crudely radial lineations, and the tension cracks nearly normal to the lineations suggest that the extrusion grew by outward expansion from a central point, resulting in an onion-skin pattern of foliation. An area of warm ground and active hydrothermal alteration coincides with the topographic and structural crest of Mullet Island. Several warm springs rich in CO2 occur on the northeast side of the island. The east side of Mullet Island is mantled by a deposit of gravel and sand composed entirely of pumice and gray rhyolite eroded from the island and deposited by wave action. The flat top of Mullet Island, although considerably modified by man's activities, pro bably represents an old wave-cut surface that now lies 12 meters above the present level of the Salton Sea.

The Salton Buttes lie within the Salton Sea geothermal field, where temperatures at 1.5 to 2.5 kilometers reach 360 degrees C, and sediments of the Colorado River delta are begin metamorphosed to greenschist facies.

The Salton Sea geothermal field lies in the Salton Trough, the landward extension of the Gulf of California, an area of active crustal spreading. The Salton Buttes volcanoes lie within the Salton Sea geothermal field where temperatures measured in wells drilled for geothermal brines range up to 360 degrees C at depths of 1,500 to 2,500 meters (Helgeson, 1968). The wells produce a hot brine containing up to 160,000 ppm of dissolved solids, chiefly Cl, Na, K, Ca, and Fe (White, 1968). Under the influence of this hot saline brine, the sediments of the Salton Trough are being transformed into metamorphic rocks of the greenschist facies (Muffler and White, 1969).

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