No active volcano exists within Los Angeles, and the city faces no imminent threat of a volcanic eruption. But the ground beneath LA tells a more complicated story than you might expect. The Los Angeles Basin sits on rocks that include ancient volcanic layers, and several volcanic fields lie within a few hours’ drive of the city. Understanding what those features actually are, and what they are not, clears up a surprising amount of confusion about Southern California’s geologic character.
Volcanic Rocks Beneath the LA Basin
The Los Angeles Basin does contain volcanic rock, but none of it is recent. During the Miocene epoch, roughly 12 to 18 million years ago, the region looked nothing like the sprawling coastal city it is today. Southern California’s coastline was being reshaped by the collision between the North American plate and the East Pacific Rise, a spreading ridge on the ocean floor. That interaction produced real volcanic activity across what is now the greater LA area.
The Conejo volcanic suite, exposed in the Santa Monica Mountains and the Conejo Valley west of LA, formed during this period. Geological and geochemical investigations tie its origin directly to the subducting mid-ocean ridge that was interacting with the continental margin at the time.1Geology. Petrogenesis of the Conejo volcanic suite, southern California: Evidence for mid-ocean ridge–continental margin interactions These were not minor seeps. The Conejo suite includes substantial lava flows and volcanic debris that make up a significant part of the bedrock in that area. If you hike in the Santa Monica Mountains, some of the rock under your feet is volcanic in origin.
Farther east, the Glendora Volcanics sit in the northeastern part of the LA Basin. These consist of basalt, andesite, dacite, and rhyolitic lavas, along with a roughly equal volume of tuffs and volcanic breccias, with andesitic compositions dominating.2GSA Bulletin. GLENDORA VOLCANIC ROCKS, LOS ANGELES BASIN, CALIFORNIA That variety of rock types points to a volcanic center that erupted in multiple styles over time. Today these rocks are largely buried under younger sediments, and the Duarte branch of the Sierra Madre fault zone forms a buried feature separating these volcanic and sedimentary rocks from the igneous and metamorphic rocks of the San Gabriel Mountains to the north.3GeoScienceWorld (Geology). Crustal structure and tectonics from the Los Angeles basin to the Mojave Desert, southern California
The key point is that all of this volcanism is extinct. The tectonic conditions that created it, specifically the subduction of the East Pacific Rise beneath the continent, ended millions of years ago. The plate boundary in Southern California shifted to the transform-style motion we now associate with the San Andreas fault system. No magma chamber sits beneath downtown LA, and the volcanic rocks in the basin are geological artifacts, not signs of future activity.
Why the La Brea Tar Pits Are Not Volcanic
If you have ever visited the La Brea Tar Pits in the middle of Los Angeles and watched dark, viscous material bubbling to the surface, you might reasonably wonder whether something volcanic is going on. It looks the part. But those bubbles are methane and other hydrocarbons seeping up from underground petroleum deposits, not from any magma source.
Natural hydrocarbon seeps are widespread in Los Angeles because gas migrates along faults from numerous subsurface petroleum fields.4Journal of Geophysical Research: Atmospheres. Emission of Methane and Heavier Alkanes From the La Brea Tar Pits Seepage Area, Los Angeles The oil and gas that bubble up at La Brea were formed from ancient marine organisms buried in sedimentary rock, not from molten rock. The asphalt pools at La Brea are thick crude oil that has lost its lighter components to evaporation. It is the same material that was once used to pave roads, not lava.
This confusion is understandable. Bubbling, dark, semi-liquid material emerging from the ground fits the mental image of volcanic activity. And Los Angeles sits in a geologically active region where earthquakes are routine. But seismic activity and volcanism are different phenomena, even though they can share underlying causes. The faults that allow petroleum to reach the surface in LA are the same types of cracks in the crust that make the city earthquake-prone, but they do not connect to a magma source.
The Salton Buttes, Southern California’s Nearest Active Volcanism
The closest thing to an active volcanic area near Los Angeles is the Salton Buttes, a cluster of five small rhyolite domes at the southern end of the Salton Sea, about 150 miles southeast of the city. These are not theoretical or speculative. The Salton Buttes have erupted multiple times during the Holocene, meaning within the last several thousand years.
The five domes, from oldest to youngest, are Mullet Island, Obsidian Butte, Rock Hill, North Red Hill, and South Red Hill. Research using paleomagnetic directions, uranium-thorium zircon dating, and argon dating indicates that all five domes are Holocene in age and that their volcanism spans between roughly 5,900 and 500 years ago.5Geochemistry, Geophysics, Geosystems. Episodic Holocene eruption of the Salton Buttes rhyolites, California, from paleomagnetic, U‐Th, and Ar/Ar dating More recent and detailed dating has refined the picture further, identifying the most recent eruptions at North and South Red Hill at around 800 years ago and revealing a complex history of magma storage and eruption stretching back over 12,000 years.6Chemical Geology. Warm magma storage in a continental rift: Insights from U/Th zircon dating and geochemistry of the Salton Buttes
What makes the Salton Buttes especially interesting is the evidence that their magmatic system has stayed warm for a very long time. Research suggests the presence of a long-lived, interconnected magmatic reservoir that has remained in a near-eruptible state for over 12,000 years, sustained by the thin crust, high heat flow, and active tectonic extension in the Salton Trough.6Chemical Geology. Warm magma storage in a continental rift: Insights from U/Th zircon dating and geochemistry of the Salton Buttes The Salton Sea region also hosts the second largest and hottest geothermal system in the continental United States, and thermal modeling indicates that a partially molten mush zone in the lower crust is a natural consequence of the ongoing influx of mantle-derived basalt in this area.7Earth and Planetary Science Letters. Thermal and petrologic constraints on lower crustal melt accumulation under the Salton Sea Geothermal Field
None of this means an eruption near the Salton Sea is imminent or that it would threaten Los Angeles directly. The eruptions documented there have been small-volume rhyolite events, not the massive explosive eruptions that make headlines. But the Salton Buttes are a reminder that active magmatism exists within Southern California’s borders, even if well outside the LA metro area.
The Tectonic Setting That Explains the Salton Buttes
The Salton Trough, where the Salton Buttes sit, is a place where the Earth’s crust is being pulled apart. It marks the northern end of the Gulf of California rift system, the same tectonic feature that is slowly splitting Baja California from the Mexican mainland. This active extension thins the crust and allows mantle material to rise closer to the surface.
The volcanic history of the Salton Trough stretches back tens of millions of years. Older alkaline lavas from the Oligocene and Miocene epochs were emplaced during an earlier phase of crustal extension and basin formation. Then, within the last four million years, a different style of magmatism took hold, related to the developing Gulf of California rift, exploiting crust that was already broken during the earlier stretching.8Geology. Cenozoic volcanism and two-stage extension in the Salton trough, southern California and northern Baja California The Salton Buttes are the surface expression of this younger, ongoing process.
This is fundamentally different from the situation in the LA Basin. Los Angeles sits on a transform plate boundary, where plates slide past each other horizontally. The Salton Trough, by contrast, involves plates pulling apart, which is one of the classic settings for volcanism on Earth. The San Andreas fault connects the two regions, but the styles of deformation are distinct. LA gets earthquakes from lateral sliding. The Salton Trough gets earthquakes and volcanism from extension.
Volcanic Craters in the Mojave Desert
Another set of volcanic features lies north and east of Los Angeles in the Mojave Desert. Amboy Crater and Pisgah Crater, both clearly visible from Interstate 40, are well-preserved cinder cones surrounded by dark lava flows. They look strikingly fresh, and by geologic standards, they are.
Dating using cosmogenic isotopes puts the Pisgah Crater lava flows at roughly 22,500 years old and the Amboy Crater flows at about 79,000 years old. The Cima volcanic field, farther to the north, includes flows ranging from about 11,500 to 46,000 years old.9Geomorphology. Cosmogenic 36Cl ages of Quaternary basalt flows in the Mojave Desert, California, USA These are alkali basalts, a type of magma that rises from deep in the mantle through the crust.10Journal of Geophysical Research: Solid Earth. Contamination of basaltic magma by mafic crust at Amboy and Pisgah Craters, Mojave Desert, California Pisgah Crater in particular has drawn scientific attention because its compositional variations suggest the sequential melting of different mantle sources beneath the site.11Journal of Petrology. Distinguishing Melting of Heterogeneous Mantle Sources from Crustal Contamination: Insights from Sr Isotopes at the Phenocryst Scale, Pisgah Crater, California
These craters are roughly 150 to 200 miles from downtown LA, depending on which one you are measuring from. They are not considered active in any monitoring sense, and tens of thousands of years have passed since their last eruptions. But they are young enough that geologists would not rule out future eruptions in the broader Mojave volcanic province over very long timescales. For anyone living in LA, though, the Mojave craters are more of a fascinating road-trip destination than a hazard.
Long Valley Caldera and the Broader California Volcanic Picture
If you widen the lens to include all of California’s volcanic threats, the feature that gets the most attention from hazard scientists is Long Valley Caldera, located about 250 miles north of Los Angeles near Mammoth Lakes. Long Valley is a large depression formed by a catastrophic eruption roughly 760,000 years ago, and its magma chamber remains active today.
Seismic evidence indicates that the main mass of the Long Valley magma chamber is about 10 kilometers in diameter, with its roof sitting 8 to 10 kilometers deep and smaller protrusions reaching as shallow as 4 to 5 kilometers. While a chamber that size could theoretically produce an eruption approaching 30 cubic kilometers of material, the eruptive record over the past half-million years suggests that eruptions of one cubic kilometer or less are far more likely. Ground uplift and seismicity within the caldera since the late 1970s have been attributed to the inflation of 0.1 to 0.2 cubic kilometers of additional magma, with some models suggesting the injection of thin dikes into the fracture zone beneath the caldera.12Journal of Geophysical Research: Solid Earth. Active tectonic and magmatic processes beneath Long Valley Caldera, eastern California: An overview
Long Valley is monitored continuously by the U.S. Geological Survey, and any significant changes in seismicity, ground deformation, or gas emissions would trigger public alerts. For LA residents, Long Valley represents a low-probability, high-consequence volcanic threat, the kind that scientists keep a close eye on but that is unlikely to affect the city in any given lifetime.
Geothermal Activity Is Not the Same as Volcanism
Southern California has numerous hot springs, warm wells, and areas of elevated ground temperature, particularly along active fault zones. These features can seem volcanic to a casual observer, but most of them operate without any magma involvement at all.
A comparison of thermal and mineral springs across Southern California and Baja California reveals that their distribution aligns closely with seismicity and active faulting.13Geology. Correlation between seismicity and the distribution of thermal and carbonate water in southern and Baja California, United States and Mexico In many cases, rainwater infiltrates deep into fractured bedrock along faults, gets heated by the natural increase in temperature that comes with depth, and then rises back to the surface along the same permeable fault zones. Research along the Agua Blanca fault in Baja California has documented this process directly, showing that topographic heads drive water deep into basement rocks where it is heated by the local geothermal gradient and then ascends along fault zones.14Geochemistry, Geophysics, Geosystems. Behavior of Amagmatic Orogenic Geothermal Systems: Insights From the Agua Blanca Fault, Baja California, Mexico The word “amagmatic” is the telling part: no magma is needed to explain these hot springs. The Earth simply gets hotter the deeper you go, and faults create plumbing that lets heated water circulate.
This distinction matters because people sometimes point to hot springs or warm ground near LA as evidence of hidden volcanic activity. The reality is that you can have geothermal features almost anywhere with deep faults and fractured rock, and Southern California has plenty of both. The presence of hot water at the surface does not mean magma is lurking below. It usually means groundwater has taken a long, deep trip through warm rock.
What LA Residents Actually Need to Worry About
When it comes to geologic hazards, Los Angeles has a full plate already, and volcanism is not on it. Earthquakes are the dominant geologic risk, with numerous active faults running through the metro area. Landslides, liquefaction, and tsunami risk from offshore faults round out the list. Volcanic hazards barely register in any official risk assessment for the city.
The U.S. Geological Survey maintains volcano monitoring programs for the Salton Buttes, Long Valley, and the Cascade Range volcanoes of Northern California. If any of those systems showed signs of escalating activity, Southern California would receive warnings with enough lead time to respond. An eruption at the Salton Buttes, the nearest active system, would be a regional event affecting the Imperial Valley far more than Los Angeles. Ash fall from a larger eruption at Long Valley could conceivably reach LA depending on wind patterns, but the most likely eruption scenarios there involve much smaller events than the caldera’s worst-case potential.
For all practical purposes, if you live in Los Angeles, your earthquake preparedness kit matters far more than any volcanic contingency plan. The volcanic rocks beneath the city are millions of years old, the nearest active magmatic systems are well over a hundred miles away, and none of them are currently showing signs of imminent eruption. LA is a geologically dynamic place, but its dynamism expresses itself through faults and shaking, not through fire and lava.