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Geology of California's Imperial Valley

A Monograph by Eugene Singer


VOLCANIC ACTIVITY in Southern California is not something one thinks about very much. With earthquakes carrying such a high priority in our concerns for public safety, the thought that other geologic phenomena are present hardly matters. Perhaps it is just as well, for while the valley contains a remarkable selection of volcanic wonders, they are subdued and seem to present no danger.

Why this is so goes back to the same forces that drive the earthquake question: the dynamic earth on which we live. If one accepts the fact that the Salton Trough is a part of the Gulf of California, and, therefore, the landward extension of the East Pacific Rise, and if this is the cause of our frequent earthquakes, then it does make sense that incipient volcanism may be present, as well.

This might become more clear if we describe the East Pacific Rise as a tear in earth's crust, and that sea floor spreading is opening the gap as a zipper would separate an opening in a garment, in this case from south to north. Off the west coast of Central America, deep submergence vessels, most notably the Alvin of the U.S. Navy, have extensivly observed and documented startling undersea volcanism, including smoking vents, ore formation and pillowy lava formations.

Progressing north to the Imperial Valley, geophysical evidence suggests that the crust is very thin and that a large mass of super-heated rock exists just below the surface.

All this is characteristic of a zone of sea floor spreading where new crust is being formed as molten material is brought to the surface of the earth. As the combined Gulf of California/Salton Trough structure is a transition zone between continental crust and oceanic crust, it follows that some magmatic intrusion and regional metamorphism is going on with concurrent volcanic activity. But surprisingly, it is limited. Possibly the great thickness of recent sediments in the basin act as a thermal barrier, confining the incipient volcanic activity and insulating the surface from its effects.

Even so, the limited volcanism present in the valley includes a variety of phenomena, from hot springs to recent volcanic eruptions.

The Heat Source

Volcanic activity of any type requires a heat source close to the surface. Geophysical studies of seismic activity, of heat flow in the earth, and of magnetic anomalies in the area around the south end of the Salton Sea all suggest that active igneous and metamorphic processes are now going on associated with an intrusive mass that lies below the sedimentary cover.

The intrusion under the Salton Sea is thought to be a pluton, an arm or protrusion from a deeply buried molten magma. This intrusion is parallel to the axis of the Salton Trough. It is about 20 miles long by four miles wide, and is at least one to two miles thick. It lies within the upper 10,000 feet of the crust, and possibly as close as 4,000 feet from the surface. It is centered beneath the community of Niland, at the southeast shore of the Sea.

This pluton is acting upon the sedimentary fill, altering the rocks into a low grade metamorphic series under low-temperature/low-pressure metamorphism. Associated with the metamorphism of the rocks, chemical analysis of hot brines brought to the surface by deep thermal wells in the Imperial Valley and Mexico show that active ore formation is probably taking place around the pluton. This involves the concentration of sulfides of iron, lead, zinc and copper.

Hot Springs

The Hot springs within the valley were known and used by Indians for centuries. The first commercial development in the area dates from the turn of the century, when a therapeutic spa was opened in the foothills of the Chocolate Mountains near Bombay Beach, on the east shore of the Salton Sea. This spring is still in use, and is unusual for its high water temperature, ranging from 135 to 180 degrees Fahrenheit.

With few exceptions, the hot springs are concentrated in a linear pattern along the eastern side of the valley. The line of springs extends from Desert Hot Springs into Mexico, and the arrangement strongly suggests that the warm waters are reaching the surface using fractures of the San Andreas fault system as conduits.

There are some exceptions. At a natural spring at Miracle Hill, east of Desert Hot Springs, warm water rises to the surface along the Miracle Hill fault, a north branch of the Mission Creek fault. The hot spring in the City of Palm Springs is another exception.

The warm springs in the Coachella Valley are largely confined to the city of Desert Hot Springs and its immediate vicinity. Desert Hot Springs is on the Mission Creek upland, an alluviated surface created by the coalescing alluvial fans from the east base of the San Bernardino Mountains merging with the western Little San Bernardino Mountains. The upland is a sandy plain sloping to the southeast toward the valley center.

There are more than 50 wells in a rough linear pattern from the city center southwest, following the Mission Creek fault trace. The wells deliver thermal waters at an average temperature of about 120o F. with some wells as high as 200o F. Temperature is highest in the wells located in the city, consistently decreasing southward. Desert Hot Springs waters are high in calcium and magnesium salts, primarily the sulfates, and are alkaline.

Well depths are between 20 to 340 feet into three aquifers, with the lowest being the best producer.

As there is no evidence of recent volcanic activity in the area, it is assumed that the hot waters are cool meteoric water that has traveled downward in the aquifers, there to become heated, then rising from depth along lines of fractured and faulted rocks.

Beliefs regarding the therapeutic value of natural hot springs have been popular for centuries. Since the days of the Roman Empire, mineralized hot springs have been a mecca for people afflicted with a variety of disorders. Exploiting these disabilities, zealous promoters have often used vivid imagination to lure prospective bathers. For example, early in this century the hot springs near Bombay Beach were claimed to be "veritable fountains of youth offering comfort and health-giving properties to the bathers . . .finding relief from arthritis and rheumatism." In recent years, the popularity of mineral spas has waned, as the medicinal values of the waters have been largely discounted, but the natural springs have retained some popularity as outdoor hot baths.

There are many warm springs in The San Jacinto Mountains. These are situated along the San Jacinto fault zone from Gilman Hot Springs, in Hemet Valley, south to Borrego Valley.

Agua Caliente Spring (Palm Springs)

The area that is now downtown Palm Springs has been a center of human activity for centuries. Agua Caliente Spring, located on the present Spa Hotel property, was important to the early Indians. For many years before western man came to this valley, the spring was an oasis of palm trees, saltgrass and other vegetation in profusion. It was a natural water source in an otherwise hostile environment, and the Agua Caliente Indians still attach great cultural significance to the spring.

The spring has had a long history of various mineral bath operations using the water. For more than a century, bath houses have been built on or around the spring for commercial purposes.

In its natural state, and before construction of the current hotel buildings, the spring flowed from a low mound that rose a few feet above the ground surface. In its natural development, located in the central part of the spring mound, the orifice deposits are light-gray highly permeable fine sand. This appears to extend to depth, and is the vertical conduit through which the spring water rises to the surface. Surrounding the orifice, and making up the bulk of the mound is an impermeable mass of fine-grained clay-like material.

Prevented from lateral migration by the clay, the spring water rises through the washed-sand conduit of its own making. Flowing water brings sandy material to the surface, where the fine silty material is washed to the margins of the mound while the sandy material remains in the orifice. The structure of the spring, then, is a sandy permeable flow channel surrounded by a silty, clay-like, nonpermeable confining chimney.

It is probable that the spring is very old, and has slowly extended this structure upward for centuries, with the rate of building equal to the rate of deposition of alluvium on the valley floor.

Agua Caliente Spring water is of unusually high quality. It differs markedly in chemical quality from the groundwater pumped from nearby city water wells. It is a sodium bicarbonate type, with low dissolved solids and is very soft. It has a high pH, or slightly alkaline, and is high in sodium and fluorine. Natural groundwater, pumped by the local water company for domestic water, is of the calcium bicarbonate type, has low dissolved solids and is soft to moderately hard. These differences suggest that the spring water rises in its natural conduit from a depth substantially greater than the depth of the domestic water wells.

Emission temperature of the spring water is about 107 degrees Fahrenheit, and the flow rate is about 25 gallons per minute.

The circumstance of the spring is its association with the Palm Canyon fault and the unusually thick 1,000 foot sequence of sedimentary beds at the mouth of Palm Canyon.

West of the fault are the high San Jacinto Mountains, with substantial precipitation at the upper elevations. The granitic rock mass is intricately fractured, and water readily flows in the fractures. This rainwater seepage appears to follow a parabolic path under gravity and convection. It flows vertically in the rock fractures to great depth, is heated in contact with a deeply buried heat source, then, being hindered from flowing laterally by the buried fault, rises to the surface along the fault plane. It ultimately finds its way to the valley floor by the sandy vertical conduit the spring itself has made in the coarse alluvium.

Geothermal Resources

There are several experimental geothermal developments in the Imperial Valley, extending from the south shore of the Salton Sea into Mexico. The Salton Sea geothermal field is the largest and the hottest of the several fields in the Salton Valley, and has the longest history of development.

Across the Mexican border lies the Cerro Prieto geothermal field near Cerro Prieto Volcano. It is a large field and is economically productive.

The geothermal waters are the result of a complex subsurface heat transfer system. Convection within the mantle is a continuously renewable source of heat. The heat is transferred by conduction through the thinned crust. Surface waters migrating downward are heated, then dissolve chemical compounds from the rocks undergoing metamorphism, and rise by convection through the water-saturated sediments to the surface.

The potential for the development of geothermal energy resources was first recognized in the mid-1920s. However, it was not until 1961 that the first commercial well in the Imperial Valley was drilled. It reached a depth of more than 4,700 feet.

The energy crunch of the 1970s spurred renewed interest in commercial development, and several wells were drilled to depths of 5,000 to 8,000 feet.

In the Salton Sea geothermal field, typical brines are produced at wellhead temperatures up to 6000F. During extraction, the high temperatures and reduced pressure in the drill holes cause the superheated water to flash into steam, thus bringing in a mixture of steam and hot water at the wellhead.

The steam/water solution is highly charged with chemical salts, principally sodium chloride, calcium chloride and several metallic compounds. A unique characteristic of the brine is the high concentration of dissolved rare elements, including Lithium and Potassium. The brine is slightly caustic, and severe corrosion and scaling problems complicate the production of clean turbine steam. Although the resource is large, technical problems and cost factors associated with processing the hot brines are a continuing constraint to large-scale commercial development.

Important unresolved secondary problems inhibiting commercial development are (1) a means of disposing of the spent brines without contaminating the surface or the ground water systems, and (2) a source of a large quantity of fresh water for coolant.

By contrast, the wells of the Cerro Prieto development just over the border in Mexico are producing relatively clean steam, and commercially important quantities of geothermal energy are generated.

The natural resource is extensive and valuable. The technical problems can be solved, with the principal question being the economic viability of large-scale commercial development. The opportunity is not being overlooked, as the steam resources are estimated to be adequate for the power requirements of a population of four million people indefinitely.


Volcanoes? In the Imperial Valley? At the south end of the Salton Sea, southwest of Mullet Island, are five small volcanic domes. They are oriented along a northeast trend, or perpendicular to the trace of the San Andreas fault system. The domes rise 100 to 150 feet above the valley floor, and collectively are known as Obsidian Butte. They are extruded into the Quaternary alluvium and are thought to be fewer than 20,000 years old.

Their composition is rhyolite and pumice, with subordinate obsidian.

Obsidian is volcanic glass. It is a volcanic rock that forms when the lava is cooled very rapidly. Mineral crystals do not have time to form in the molten lava, and the noncrystalline mass becomes a glass when it is subject to sudden cooling. Obsidian is often found associated with pumice, often in layers as at Obsidian Butte.

Pumice is the hardened residue of volcanic froth, or very liquid rock highly charged with gases. If the gas content is high, then the bubbles "frozen" in place upon hardening can give some buoyancy to the rock. It is commonly thought that pumice rock floats on water. Sometimes it does; usually not. Pumice boulders, because they are light and easily handled, are used extensively for landscaping.

The Cerro Prieto volcano, 15 miles south of Mexicali, is a rhyolite dome that is the product of a single eruptive cycle in the late Pleistocene.

The marked lack of erosion of the cone attests to its youthful age. Young volcanoes at Cerro Prieto are apparently part of the same suite of volcanic activity, all being associated with the East Pacific Rise.

Carbon Dioxide

From 1933 to 1954, carbon dioxide was produced from a small field near Niland. The gas was recovered from pockets 200 to 700 feet deep and was converted to dry ice for refrigeration, with much of the output supplied to the railroad for icing of refrigerator cars. The project was abandoned in 1954, a victim of refrigeration technology.

The rising water level of the lake in the early 1980s has since flooded the area, and nothing remains except a few timbers sticking out of the water.

Mud Pots

Mud pots were once a popular sight near Mullet Island. The island, originally an arm of land extending into the sea, was also submerged by the rising water.

The mud field was a dark-colored mass of lacustrine silt mud. Large bubbles of steam, some the size of a football, formed and burst, spattering wet mud into the air. Principal gasses ejected by the field were steam, carbon dioxide and hydrogen sulfide. Similar to other volcanic phenomena, the mud pots are the result of surface waters percolating downward through the sedimentary layers to the proximity of the magma body. The heated water then rises to the surface, flashing to steam as the confining pressure is reduced.

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: esinger@aloha.net