Mauna Loa was built by a deep-rooted column of abnormally hot rock, called a mantle plume, that rises from near the core-mantle boundary and melts as it nears the surface beneath the Pacific Plate. The Pacific Plate drifts northwest over this essentially stationary plume, so each volcano that forms is eventually carried away from the magma source, goes quiet, and a new volcano begins growing in its place. Mauna Loa is the current heavyweight product of this process, with an estimated volume of roughly 74,000 cubic kilometers, making it the largest active volcano on Earth. But getting from a deep-mantle heat source to a mountain that tall involved submarine eruptions, catastrophic landslides, and an internal plumbing system that researchers are still mapping.
A Plume From the Deep Mantle
The heat engine behind Mauna Loa sits far below the ocean floor. The Hawaiian Islands owe their existence to a mantle plume that originates at the boundary of a massive structure deep in the lower mantle known as a large low shear velocity province. These provinces are continent-sized zones where seismic waves slow down, indicating unusually hot or compositionally distinct material. The Hawaiian plume rises from this boundary and reaches the base of the Pacific Plate, where reduced pressure allows the rock to partially melt and generate magma.1Geochemistry, Geophysics, Geosystems. Insights Into the Origins and Compositions of Mantle Plumes: A Comparison of Galápagos and Hawai’i
The melting happens deep. Geochemical studies of Mauna Loa’s lavas show signatures consistent with melting within a zone where garnet is stable as a mineral phase, which corresponds to depths of around 120 kilometers or more beneath the surface.2Geochimica et Cosmochimica Acta. The aluminum conundrum in Hawaiian shield-building lavas: An argument for a deep, garnet-bearing, mantle source As the magma rises, it reacts chemically with the surrounding rock, picking up and losing certain elements along the way. By the time it reaches Mauna Loa’s internal reservoir system, it has a particular chemical fingerprint, dominated by tholeiitic basalt, the fluid, iron-rich lava that characterizes the volcano’s shield-building phase.3Earth and Planetary Science Letters. Chemical and isotopic variations in Mauna Loa tholeiites
The Conveyor Belt That Creates Island Chains
A single plume does not produce a single volcano. It produces a chain of them, because the tectonic plate overhead keeps moving. The Pacific Plate creeps northwest at roughly seven to nine centimeters per year, dragging each newly formed volcano off the hotspot and starving it of fresh magma. This motion creates age-progressive chains, where islands get progressively older the farther northwest you go.4Tectonics. Quantification of Pacific Plate Hotspot Tracks Since 80 Ma Kauaʻi, at the northwest end of the main Hawaiian Islands, is about five million years old. Mauna Loa, near the southeast end of the Big Island, is still actively erupting.
The Hawaiian-Emperor seamount chain extends thousands of kilometers to the northwest, with progressively older and more eroded volcanoes and then sunken seamounts stretching all the way to the Aleutian Trench. Mauna Loa’s position near the current center of hotspot activity means it has been receiving a generous supply of magma for hundreds of thousands of years, which is why it grew to such extraordinary size.
Growing From the Seafloor Up
Mauna Loa did not start as a visible mountain. It began as a submarine volcano on the ocean floor, likely more than 600,000 years ago, erupting pillow lavas in the cold, high-pressure environment of the deep Pacific. These early eruptions built a broad base of volcanic rock that nobody would ever see without a submersible. The transition from submarine to subaerial growth is a major turning point in a Hawaiian volcano’s life, because it changes how lava accumulates and where it goes.
Researchers dated lavas from the Kahuku landslide scarp, a 1.6-kilometer-high cliff that cuts into Mauna Loa’s submarine southwest rift zone, and found that a 1,250-meter-thick section of lava was all erupted around 470,000 years ago. That implies an accumulation rate of about 25 millimeters per year during what was likely the peak of the shield-building stage. The drop in accumulation rate recorded in the deeper submarine rift does not mean the volcano was winding down; rather, the magma delivery system migrated upward as Mauna Loa breached the ocean surface, cutting off supply to the distal underwater rift and redirecting lava to subaerial flows instead.5Journal of Geophysical Research: Solid Earth. 40Ar/39Ar geochronology of submarine Mauna Loa volcano, Hawaii
The Four Stages of a Hawaiian Volcano’s Life
Hawaiian volcanoes follow a general lifecycle with four eruptive stages: preshield, shield, postshield, and rejuvenated.6U.S. Geological Survey Professional Paper. Growth and degradation of Hawaiian volcanoes The preshield stage involves small-volume, chemically diverse eruptions as the volcano first taps the plume. The shield stage is the main event, producing enormous volumes of tholeiitic basalt that build the broad, gently sloped shape the volcano is known for. The postshield stage brings less frequent eruptions of chemically different, more alkaline lava that form a thin cap. The rejuvenated stage, which not every volcano reaches, involves a last gasp of small eruptions after a long quiet period.
Mauna Loa is firmly in its shield stage. Its recent eruptions, including the 2022 event, have all produced tholeiitic lavas, and eruption rates remain relatively high. The volcano has probably not yet entered the postshield stage, which would be marked by a shift to alkaline lavas and less frequent activity.7Journal of Volcanology and Geothermal Research. Shield-stage alkalic volcanism on Mauna Loa Volcano, Hawaii Eventually, as the Pacific Plate carries Mauna Loa away from the hotspot, the magma supply will dwindle and the transition will begin. But “eventually” in geological terms could still mean tens of thousands of years of activity to come.
The Plumbing Beneath the Summit
Mauna Loa’s eruptions are fed by a system of magma reservoirs stacked beneath its summit. For decades, geophysical monitoring suggested a single primary storage zone at about three to five kilometers below the summit caldera. Recent work using fluid inclusions trapped in crystals from erupted lavas has refined this picture considerably. The data now point to at least two reservoirs: an upper one at roughly one to two kilometers depth and an intermediate one at three to five kilometers depth.8Nature Communications. Triggering the 2022 eruption of Mauna Loa
The upper reservoir appears to contain a crystal-poor, more chemically evolved cap of magma, while the deeper zone holds hotter, more primitive material.9Bulletin of Volcanology. Fluid inclusion constraints on the geometry of the magmatic plumbing system beneath Mauna Loa — Part 1: lavas and tephras Studies of xenoliths, chunks of rock torn from reservoir walls and carried to the surface during eruptions, show storage depths consistent with this two-reservoir model. Some of these rocks formed when intruding magma reacted with piles of olivine crystal mush at around three kilometers depth, essentially the walls of the intermediate reservoir being reworked by new magma injections over time.10Bulletin of Volcanology. Fluid inclusion constraints on the geometry of the magmatic plumbing system beneath Mauna Loa — Part 2: Xenoliths
Seismic imaging supports this view from a different angle. Tomographic models using earthquake data from 1992 to 2009 show zones of high compressional-wave velocity beneath the caldera, interpreted as solidified mafic cumulates, the crystallized leftovers of older magma batches that cooled in place and now form part of the volcano’s internal skeleton.11Journal of Geophysical Research: Solid Earth. Three-dimensional seismic velocity structure of Mauna Loa and Kilauea volcanoes in Hawaii from local seismic tomography
Two Chemical Stripes in the Plume
One of the more surprising discoveries about Hawaiian volcanism is that the plume feeding the islands is not chemically uniform. The recent Hawaiian volcanoes (those younger than about five million years) fall into two parallel geographic trends: the Loa trend to the southwest and the Kea trend to the northeast. Despite being only about 30 kilometers apart, volcanoes on the two trends erupt lavas with measurably different isotopic signatures.12Geochimica et Cosmochimica Acta. The geochemical components that distinguish Loa- and Kea-trend Hawaiian shield lavas
Mauna Loa sits on the Loa trend. Its neighbor Kīlauea, just to the southeast, sits on the Kea trend. The chemical differences arise from small amounts of a distinct geochemical component in the source material feeding Loa-trend volcanoes, reflecting two geochemically distinct sides within the rising plume itself.13Geochemistry, Geophysics, Geosystems. Emergence of the Loa Mantle Component in the Hawaiian Islands Based on the Geochemistry of Kauaʻi Shield‐Stage Basalts The plume, in other words, is not a single column of identical hot rock. It carries streaks of compositionally different material, possibly including recycled oceanic crust that was subducted into the deep mantle billions of years ago and is now being brought back to the surface. This bilateral structure helps explain why Mauna Loa and Kīlauea, though close enough to interact, produce lavas with recognizably different chemistry.
Catastrophic Flank Collapses
Building a mountain as massive as Mauna Loa on the ocean floor creates stability problems. The weight of accumulated lava can exceed what the underlying rock and the volcano’s own flanks can support, leading to massive landslides. Sonar mapping of Mauna Loa’s western submarine flank revealed a 4,000-square-kilometer zone of slump and slide deposits. The most dramatic features are two debris avalanche lobes, known as the Alika slides, that traveled as far as 100 kilometers from their breakaway zone near the shoreline into the deep ocean at water depths approaching 4,800 meters.14Journal of Geophysical Research: Solid Earth. The Giant Submarine Alika Debris Slide, Mauna Loa, Hawaii
These collapses were not gentle. They moved enormous volumes of rock off the volcano’s flank in events likely triggered by seismic activity accompanying the injection of magma into rift zones. The slumping has been most intense near the large arcuate bend in Mauna Loa’s southwest rift zone, where the rift migrated westward as Kīlauea grew alongside it. Subsequent research confirmed that the western flank also shows evidence of deep volcanic spreading, a slow, gravity-driven process where the entire base of the volcano deforms outward under its own weight, further destabilizing the flanks.15Geochemistry, Geophysics, Geosystems. Mauna Loa’s submarine western flank: Landsliding, deep volcanic spreading, and hydrothermal alteration
These landslides are a normal part of how Hawaiian volcanoes grow and reshape themselves. They remove material from the flanks, which then gets replaced by new eruptions, and the process repeats. The Alika slides are geologically young enough that similar events are plausible in the future, which has implications for tsunami hazard across the Pacific.
The Weight Problem
Mauna Loa is so massive that it has literally bent the Pacific Plate beneath it. The oceanic lithosphere, the rigid outer shell of the Earth, flexes downward under the combined load of the Big Island’s volcanoes. Earthquake data reveal that the plate is not merely flexed but appears to be broken under the island, with a circular geometry and a central depression directly beneath the volcanic load.16Journal of Geophysical Research: Solid Earth. Lithospheric flexure under the Hawaiian volcanic load: Internal stresses and a broken plate revealed by earthquakes This downward bending creates a surrounding moat and a subtle arch farther out, features visible in the seafloor bathymetry around the island. The flexure also generates internal stresses in the plate that influence earthquake patterns and volcanic plumbing pathways.
The Mauna Loa and Kīlauea Relationship
Mauna Loa and Kīlauea are close neighbors, separated by only about 35 kilometers at their summits, and they share a common magma supply from the underlying hotspot. But they do not simply act as independent volcanoes. Researchers have long noticed an apparent pattern in which periods of high activity at one volcano tend to coincide with relative quiet at the other. Decades of GPS and satellite radar monitoring have provided evidence for this anticorrelation. When Mauna Loa’s summit reservoir inflates, it can alter the stress field around Kīlauea’s shallow reservoir, effectively squeezing it.17PubMed Central. Elastic interaction between Mauna Loa and Kīlauea evidenced by independent component analysis
Numerical modeling suggests the coupling goes deeper than surface stress changes. The two volcanoes may be linked through pore-pressure diffusion in a shared asthenospheric magma supply system, meaning that changes in pressure at one volcano’s roots can propagate through the partially molten zone and influence the other.18Nature Geoscience. Coupling at Mauna Loa and Kīlauea by stress transfer in an asthenospheric melt layer The relationship is not perfectly predictable; both volcanoes have erupted at the same time on occasion. But their proximity means that understanding how Mauna Loa formed and operates requires accounting for Kīlauea’s influence, and vice versa. The 2022 eruption of Mauna Loa happened shortly after Kīlauea’s long-running eruption paused, which many researchers noted fit the anticorrelation pattern, though a sample size of a few decades is too small to call it a firm rule.
Lava Types and Surface Building
The surface of Mauna Loa is built from two main types of basaltic lava flow: pahoehoe and ʻaʻa. Pahoehoe flows are smooth, ropy, and tend to advance slowly through inflating lobes. ʻAʻa flows are rough, clinkery, and typically move faster. Drill cores through the flanks of Mauna Loa and nearby Mauna Kea show that the proportion of each type varies with distance from the summit and the volcano’s growth history. Thick, inflated pahoehoe flows dominate in areas that were near sea level when the flows erupted, consistent with the broad coastal lava fields visible on Kīlauea and Mauna Loa today.19Geochemistry, Geophysics, Geosystems. Hawaiian lava flows in the third dimension: Identification and interpretation of pahoehoe and ′a′a distribution in the KP‐1 and SOH‐4 cores
This matters for understanding Mauna Loa’s shape. The gentle slopes that give shield volcanoes their name come largely from fluid pahoehoe flows that can travel long distances before solidifying. On steeper upper slopes, faster-moving ʻaʻa flows are more common. The interplay between these flow types, along with the volcano’s rift zones (linear fracture systems extending from the summit where magma preferentially intrudes and erupts), determines how the volcano grows outward and upward over time.
Ice on a Tropical Volcano
Mauna Loa stands 4,169 meters above sea level, high enough to have intersected the snowline during Pleistocene glacial periods.20Quaternary International. Pleistocene snowlines and glaciation of the Hawaiian Islands Glaciers on a tropical volcano sound paradoxical, but at that altitude, temperatures drop well below freezing during ice ages. The interaction between ice and erupting lava produces distinctive rock types, including hyaloclastite, a glassy fragmented material formed when hot lava quenches rapidly against ice or meltwater. Most of the evidence for summit glaciation on Mauna Loa itself has been buried by younger Holocene lava flows, but neighboring Mauna Kea, which is slightly taller and has been less active in recent millennia, preserves clear glacial deposits and moraines near its summit. The glacial history matters because it affected the mechanical properties of the summit rock and the way lava flows were channeled during eruptions that occurred while ice was present.
An Analog for Volcanoes on Mars
Mauna Loa’s formation process has become a reference point for understanding volcanism beyond Earth. Olympus Mons on Mars, the largest known volcano in the solar system, shares a structural kinship with Hawaiian shield volcanoes despite being far larger. Photogeologic analysis has shown that the base of Olympus Mons consists of fragmented material comparable to the hyaloclastite forming the base of Hawaiian volcanoes, the same glassy rubble produced when submarine eruptions quench lava against water.21Geochemistry. Olympus Mons volcano, Mars: A photogeologic view and new insights This suggests that Olympus Mons, like Mauna Loa, began its life erupting into a fluid medium, likely an ancient Martian ocean or ice sheet, before building above the surface. The reason Olympus Mons grew so much larger is straightforward: Mars lacks plate tectonics, so the volcano sat over its magma source indefinitely rather than being carried away. Mauna Loa’s growth, by contrast, has a built-in expiration date set by the Pacific Plate’s steady drift to the northwest.