Exploring California's Seamounts
Highlights from recent expeditions to deep-sea habitats in California waters
What Are Seamounts?
Seamounts are massive underwater volcanoes that are found in every ocean basin across the planet. Traditionally characterized as roughly cone-shaped features rising more than 3,300 feet above the seafloor, the oceans also house a large array of similar but smaller or irregularly shaped knolls, hills, mounds, and ridges. Although generally taller than terrestrial volcanoes, seamounts form through a similar process of successive volcanic eruptions. Seamounts typically follow a geologic pattern of initial growth, subsequent volcanic activity, subsidence (sinking), and eventually extinction (cessation of volcanic activity). Large enough seamounts may grow tall enough to breach the surface of the ocean and become islands, but will eventually sink back beneath the waves.
We have not mapped enough of the seafloor in great enough detail to know how many seamounts exist; however, scientists have used satellite altimetry data to estimate that more than 10,000 seamounts dot our planet. These seamounts cover an area consisting of more than 2% of the global seafloor, making seamounts a rare yet prevalent and important marine habitat. Despite this, fewer than 300 have been surveyed, limiting our ability to understand and protect this vital habitats.
Interactive Map: The estimated global distribution of seamounts based on satellite altimetry data. Data from: Harris PT, Macmillan-Lawler M, Rupp J, Baker EK. 2014. Geomorphology of the oceans. Marine Geology 352:4-24.
Why Are Seamounts Important?
Seamounts are extremely productive features in the often sparsely populated deep sea, acting as oases that support high abundances of benthic and pelagic organisms including corals, sponges, anemones, crabs, fish, sharks, seabirds, turtles, whales, dolphins. Due simply to their massive size in the water column, seamounts exert a strong influence on ocean currents, generating enhanced flows that are favorable for a wide array of benthic and pelagic organisms. Stronger currents increase food and oxygen supplies, help remove waste products, and transport larvae.
Favorable conditions on seamounts often lead to the formation of expansive benthic ecosystems. Cold-water corals and sponges are frequently the foundation, as they create complex structures that support an enormous diversity of associated invertebrates and fish. The fragility, longevity and slow growth rate of cold-water corals and sponges make them extremely slow to recover once damaged, rendering these ecosystems highly vulnerable to disturbance.
Seamounts also support highly productive pelagic systems. Altered currents above seamounts often concentrate nutrients, leading to extremely high productivity that attracts marine life ranging from tiny diatoms to enormous whales. Seamounts are often used as migratory stops, feeding grounds, and reproductive and nursery grounds for large congregations of pelagic organisms.
Pictured: Massive yellow Picasso sponges provide critical habitat for an army of squat lobsters and shrimp on Davidson Seamount. Courtesy of NOAA and MBARI.
Meet the California Seamounts
The United States is home to more seamounts than any other country. Its exclusive economic zone (EEZ) contains approximately 7.3% of the world’s seamounts (743 in total), covering an area of more than 250,000 square miles. The majority (58%) of the world's seamounts are located outside of national jurisdictions however.
Within California waters, there are an estimated 63 seamounts. The average California seamount is situated over 100 miles offshore, and has a summit depth of more than a mile. Of the well-studied seamounts, California seamounts were typically formed 10-25 million years ago on ancient spreading centers - spots where oceanic plates move apart due to plate tectonics. Many, such as San Juan Seamount and Rodriguez Seamount, were once ancient islands before sinking beneath the waves.
Californian seamounts stand poised at a critical conservation junction. With the advent of new technology and increasing commercial interest in the deep sea, a barrage of threats including fishing, oil and natural gas extraction, seafloor mining, and climate change endanger these fragile habitats. Despite the high potential to lose these seamount ecosystems in the coming decades they are shockingly bereft of protection - only Davidson Seamount is currently protected - making effective and long-term conservation planning critical for saving these habitats before they are irrevocably damaged.
In this story map, we explore some expedition highlights from recent surveys of seamounts in California waters. Scroll down to learn more!
Interactive Map: The distribution of seamounts in California waters. The better known and well-studied seamounts are named.
How Do Scientists Explore the Deep Sea?
The deep sea is challenging to explore. Surveying seamounts requires technology that can handle a highly pressurized, dark, and cold environment. Early approaches, including dredging, box coring , and towed camera arrays , are limited in their capabilities but are still widely used because they are inexpensive and easy to operate.
One of the biggest innovations in deep-sea research was the introduction of submersibles that allow for directed exploration of the seafloor. Human occupied vehicles (HOVs) are submersibles that carry small teams of scientists into the deep sea, allowing them to maneuver across the seafloor to take photographs and videos, collect biological and geological samples, and measure environmental conditions using a large variety of sensors. Remotely operated vehicles (ROVs) are robotic submersibles that are piloted from a ship. They remain attached to the ship with a long tether that supplies power to the ROV and transmits images, video, and other data back to the ship. Some expedition vessels (for example, the Okeanos Explorer and the Nautilus ) have recently begun to live telecast the video from ROV surveys over the internet, allowing anyone to follow along in real time from the shore.
Automated underwater vehicles (AUVs) are the most recent addition to the submersible fleet. Like HOVs and ROVs, these vehicles can visually survey the seafloor and carry a multitude of sensors. However, AUVs carry out their surveys without being piloted by a human, operating according to a pre-programmed mission. AUVs can generally carry out longer missions and cover much greater areas of the seafloor than ROVs and HOVs, but have reduced sampling abilities and cannot navigate highly complex environments.
The images on the right portray some of the technology that scientists use to explore our deep oceans.
Davidson Seamount - Overview
Davidson Seamount is an underwater volcano located just 75 miles off the coast of California. Its summit rises an impressive 7500 feet above the surrounding seafloor, yet still remains hidden beneath 4100 feet of water. One of the largest seamounts in U.S. waters, it is uniquely oblong in shape – extending over a massive area 26 miles long and 8 miles wide. The seamount formed an estimated 20 million years ago, with its last volcanic eruption occurring well over 9 million years ago . Today, the volcano is considered to be extinct, but is still growing at a barely perceptible rate due to the minute accumulation (1-7 mm per year) of ferromanganese oxide crusts .
Davidson Seamount was the first seamount officially recognized in U.S. waters. In 1938, it was designated by the United States Board on Geographic Names, named after George Davidson , a geographer and astronomer responsible for much of the early charting of the west coast. Accordingly, it has attracted more exploration and scientific study than other seamounts in U.S. waters, with a large number of survey expeditions over the past several decades.
In 2018 and 2019, the E/V Nautilus (Ocean Exploration Trust) explored and mapped a number of deep-sea habitats in the Monterey Bay National Marine Sanctuary , including multiple surveys on and around Davidson Seamount. These expeditions uncovered two astonishing finds: a massive colony of brooding octopuses, and a rare deep-sea whale fall.
Keep scrolling to see what they uncovered!
Davidson Seamount - Octopus Garden
On a 2018 expedition to Davidson Seamount, the E/V Nautilus made an astonishing discovery – an ‘ octopus garden ’ of more than 1,000 brooding female octopuses (Muusoctopus robustus). The aggregation occurred in a shimmering, hot vent almost 10,000 feet deep just south of the Davidson Seamount. Davidson is the best explored and well-known seamount in California waters and is already known to house more than 230 species, including 25 species of deep-water corals – some of which are bamboo corals over 200 years old .
Despite frequent surveys of Davidson Seamount, this brooding aggregation was quite a surprise, and such an astonishing discovery that the expedition team decided to return in 2019. Scientists were particularly interested in taking measurements to help understand why this site appears to be such an important nursery ground for these octopuses, leading the team to install long-term temperature and dissolved oxygen loggers which will be recovered in 2020. Subsequent surveys of the site have revealed that the octopuses are still brooding en masse, with observable development and even hatching of some embryos.
Expedition Links:
Davidson Seamount - Whale Falls and Zombie Worms
On the last dive of the 2019 Nautilus field season, the team happened upon a whale fall – an extremely rare event that was only discovered in 1977. A whale fall is simply the carcass of a cetacean that falls into deep enough waters to significantly slow down its decomposition due to the low temperature, high pressure, and different scavenger community found in the deep sea. While a dead whale carcass may not seem exciting, it supports a unique, ephemeral deep-sea community not found anywhere else.
As are commonly found on other whale falls, the team found a unique community of scavengers such as Osedax worms – mouthless ‘zombie worms’ that use acid to burrow into skeletons in search of high-energy lipids, as well as an array of fish, octopuses, and crabs. The whale fall was approximately 13-16 feet long and located in waters deeper than 10,000 feet. From the skeleton and tissue remaining after an estimated four months of decomposition, scientists have determined that the whale is a baleen whale , possibly a grey or minke whale. This surprising find shows how valuable deep-sea surveys can be. Whale falls are rare and may only exist for a few decades before completely decomposing, and previous finds have resulted in the discovery of new species.
Despite these recent expeditions to Davidson Seamount, less than 1% of its area has been surveyed. Only time will tell what other amazing discoveries may be uncovered by future expeditions to this incredible California seamount!
Expedition Link:
Guide Seamount
Guide Seamount is a volcanic seamount that lies just to the west of the Monterey Bay National Marine Sanctuary, approximately 53 miles off the coast of central California, at the base of the continental slope. Like many other oceanic seamounts off the west coast, it has a northwest-southwest orientation. It has a unique shape and shares geological characteristics with four other oceanic-island volcanoes including Gumdrop, Pioneer, Davidson and Rodriguez Seamounts. Guide Seamount is made up of four parallel volcanic ridges separated by sediment-filled troughs. Its summit is approximately one mile below sea level.
In 2016, Monterey Bay Aquarium Research Institute (MBARI) conducted surveyed more than 2 miles of Guide Seamount. The expedition found massive colonies of bamboo corals, named for their bamboo-lookalike skeleton, which alternates long stretches of white, hard calcium carbonate with black nodes comprised of softer gorgonin protein. These corals can live for hundreds of years , and even after their death their skeletons provide important paleoclimate records for past ocean conditions, giving scientists important information about prehistoric temperature, carbon cycling, and nutrients. The team also discovered extensive fields of trumpet and cloud sponges, which also form important habitat structures for a host of associated microbial, invertebrate, and fish species.
Expedition Link:
Slideshow: Scroll through the images to learn about some of the organisms that have been discovered on Guide Seamount.
Gorda Ridge - Overview
Gorda Ridge is located approximately 120 miles off the coast of northern California at the intersection of the Gorda Plate, the Juan de Fuca Plate, and the Pacific Plate. This complex ridge system contains a number of seamount features in concert with the more southern Mendocino Ridge feature. As the Pacific and Gorda plates move apart due to plate tectonics , the resulting spreading center created the ridge approximately 30 million years ago, and continues to cause volcanic and seismic activity in the region. A large eruption was observed as recently as 1996, covering an area of seafloor over 650 feet high and two miles long with lava.
As a result of this volcanic activity, the region houses the only known hydrothermal vent ecosystems in U.S. waters. Hydrothermal vents are incredible deep-sea ecosystems that form due to the venting of extremely hot and mineral rich fluids into the water column. Hydrothermal vents occur when fractures in the seafloor allow seawater and magma to meet, resulting in the venting of extremely hot ( 750°F and higher ) fluids into the water column. These fluids are hot enough to cause minerals such as silica, sulfide, and heavy metals to leach from the surrounding rocks. When vent fluids hit the cold seawater above, these minerals precipitate into a cloud of either black (iron sulfide) or white (calcium and silicon) particles that look like a large plume of smoke , and eventually solidify into large chimney-like structures. These vents support a wide array of exotic benthic organisms that rely on these venting fluids as their sole source of energy, including vestimentiferan tubeworms (Ridgeia piscesa), and a newly discovered species of limpet (Lepetodrilidae gordensis).
Gorda Ridge - Apollo Vent Field
The E/V Nautilus explored the Gorda Ridge area in collaboration with the NASA Systematic Underwater Biogeochemical Science and Exploration Analog ( SUBSEA ) program in 2019. While smaller vent systems had previously been discovered in the region, scientists onboard were astonished to find a massive new vent field at a depth of almost 9,000 feet.
The newly discovered vent system was later named the Apollo Vent Field in honor of the 50th anniversary of the moon landing. NASA hopes that studying hydrothermal vents will provide important analogs for extreme environments on planets and moons in our solar system, potentially providing key insights for upcoming life-hunting missions to Enceladus and Europa .
Unfortunately, the minerals that naturally occur in hydrothermal vents make this area a potential target for future deep-sea mining efforts, making it critical to survey and characterize these habitats before they are irrevocably damaged by commercial activities.
Expedition Link:
Gorda Ridge - Basalt Columns
While surveying an adjacent portion of Gorda Ridge, the E/V Nautilus expedition discovered towering basalt columns that delighted both the biologists and geologists onboard. Each tower contained its own mini-ecosystem teeming with glass sponges, stalked crinoids, and brisingid sea stars. As suspension feeders, these organisms rely on elevated features like these columns to protrude high in the water column where currents are faster and food is more readily available. The towers were more than a biological discovery in this case. The rock formations scattered throughout this area contained an unusual horizontal layering pattern along with some impressive cave formations.
Expedition Link:
Southern Californian Borderland
The Southern California borderland extends westward past the Channel Islands. This vast region is physically rugged, containing a wide variety of benthic habitats including seamounts, terraces, canyons, banks, knolls, islands, and ridges. The best known and studied seamounts in this region are Cortes, Tanner, and Northeast Bank. Cortes and Tanner Banks are a unique pair of twin seamounts located just 5 miles apart. The seamounts were islands as recently as 10,000 years ago, and today lie just below the surface (3-6 feet!). Due to their shallow nature, these seamounts support unusual levels of recreational activity, including SCUBA diving, lobster fishing, and big-wave surfing .
In 2016, the E/V Nautilus conducted several biological and geological surveys in the region. They visited a series of four banks and seamounts: Trask Knoll, Southwest Bank, Sictymile Bank, and Northeast Bank. Northeast Bank is a guyot, a flat-topped seamount that forms due to erosional processes during the seamount's island history. The expedition uncovered an extremely large diversity of benthic fauna, inclding octocorals, sea pigs, jellyfish, acorn worms, scallops, rockfish, catfish, eels, skates, cephalopods, and chimaeras.
The Southern California borderlands has historically been overlooked in deep-sea expeditions, despite its close proximity to Los Angelos and San Diego. Given the high levels of biodiversity and abundance discovered during the 2016 expedition, further exploration of this region would likely be extremely fruitful.
Slideshow: Scroll through the images to see some of the recent discoveries from the Southern California borderlands.
Expedition Link:
NA075, July 24-Aug. 12, 2016,
San Juan Seamount - Overview
San Juan Seamount started to form approximately 20 million years ago when seafloor eruptions spewed massive quantities of basalt and other volcanic rocks into the water column. San Juan was volcanically active for an estimated 9.7 million years , and it most recently erupted approximately 2.8 million years ago. For around a million years before it slipped back beneath the waves (10-14 million years ago), it existed as a series of eight small islands, maxing out at 460 feet above sea level with a total area of approximately one square mile (a little larger than Monaco and Vatican City – the world’s smallest countries).
Today, the summit of San Juan seamount sits 1,837 feet below the surface of the ocean. Some of the exposed surfaces of the seamount were eroded by wind and waves, as evidenced today by weathered rocks and the remnants of sandy beaches. However, the main culprit for the sinking of San Juan back beneath the waves is not erosion, but rather subsidence . Seamounts are so massive that they can cause the ocean crust beneath them to flex and compress, making the seamount sink lower and lower. In the case of San Juan Seamount, the ocean crust also compressed significantly due to thermal contraction – meaning that the crust shrank as it cooled down once volcanic activity ceased.
San Juan Seamount - An ancient island that now provides deep-sea coral habitat
Past expeditions to San Juan Seamount have uncovered an incredible array of life, including benthic communities dominated by cold-water corals, sponges, xenophyophores, and echinoderms, and fish assemblages that include grenadiers , Pacific flatnose , and halosaurs . Xenophyophores are giant amoeba-like protists comprised of only a single cell – despite their surprisingly large size of up to eight inches across! They create ‘tests’, or skeletal structures, made of sediment and shell fragments that serve as important habitat structures for worms, copepods, crustaceans, brittle stars, and snails.
The seamount is also home to rare species, including the deep-sea file clam (Acesta sphoni) that only lives on a few seamounts and canyons off the coast of California. The bamboo corals that grow on San Juan Seamount are long-living ( 150-300 years ) species that are important archives for reconstructing past ocean conditions, with significant implications for better understanding how climate change will shape our future oceans.
Most recently, in 2016, the E/V Nautilus explored a transect running from the base to the summit of the seamount. At deeper depths, the biological community was mostly dominated by soft-bottom taxa including xenophylophores and echinoderms. As the team surveyed higher up the seamount's slopes however, they found extensive cold-water coral gardens filled with dense stands of gorgonian sea fans, along with bamboo corals, mushroom corals, sponges, anemones, and fish.
Expedition Link:
Threats to Seamount Ecosystems
Despite their remoteness, California seamounts are unfortunately threatened by a myriad of human disturbances. These fragile ecosystems are extremely sensitive and may take centuries or longer to recover from significant damage.
Fishing: seamount ecosystems around the world are threatened by destructive fishing practices including bottom trawling and long-lining. Most California seamounts have limited short-term protection from bottom-contact fisheries thanks to federal and state protections. However, more permanent protections are needed to ensure that these habitats endure for generations to come.
Deep-Sea Mining: Technological improvements and rising mineral prices have resulted in rapidly increasing interest in deep-sea mining. Hydrothermal vents, like those at Gorda Ridge, often have high concentrations of rare-earth metals and will likely be targeted by mining operations, resulting in the wholesale destruction of these habitats. We need to act quickly to prohibit deep-sea mining in California before it becomes a reality.
Climate Change: Seamounts off the coast of California will be subject to future warming, deoxygenation, and acidification due to anthropogenic carbon emissions. These changes may make it difficult for cold-water corals and associated species to survive and grow.
Other threats: Seamounts and other deep-sea habitats are also affected by ship traffic, noise pollution, nutrient runoff, marine litter, pollution, and oil and natural gas extraction.
Pictured: Abandoned fishing gear frequently entangles cold-water corals, like this derelict rope on Cordell Bank, California. Courtesy of MARE and NOAA.
Conservation Status of California Seamounts
The majority of seamounts in California waters are in areas managed by the Pacific Fishery Management Council (PFMC). The council manages approximately 119 species of fish including salmon, groundfish, sardines, anchovies, mackerel, tunas, sharks, and swordfish. While the council’s stated focus is fisheries management, they are also mandated by law to identify and protect essential fish habitat (EFH). While California also contains a number of marine protected areas (MPAs), they are typically coastal and do not extend far enough offshore to cover seamount habitats. EFH designations, along with national marine sanctuaries, provide some protection for seamounts, but do not confer long-lasting protection from all threats.
Only one seamount in California waters has been granted long-term protection: Davidson Seamount. The seamount was added to the Monterey Bay National Marine Sanctuary during a 2008 expansion, making it the first seamount to gain official protection in U.S. waters (the expansion also added 775 square miles to the sanctuary). The pristine nature of Davidson Seamount, along with its incredibly high biodiversity and abundance of benthic organisms, led to its inclusion in the MBNMS.
Increasing commercial interest in deep-sea habitats highlights the importance of the protections that Davidson Seamount currently enjoys, but also contextualizes the alarming lack of permanent protection for the other 62 other seamounts in California waters.
Interactive Map: Shows the current conservation status of California seamounts. Use the buttons below to view either marine protected areas (MPAs) or essential fish habitat (EFH) closures in California waters.
Become a Seamountaineer!
Seamounts are:
- Places that shelter spectacular marine life,
- Where intrepid ocean explorers have felt the wonder of a greater power and witnessed a rare glimpse of an undersea world worthy of our stewardship,
- A means to satisfy our international ocean conservation commitments,
- A way of healing our relationship with the natural world during a pandemic caused, partly, by a break with nature
Please sign our Seamountaineer Pledge to protect seamounts.
Pictured: Brown stony corals and pink hydrocorals provide critical habitat for a large school of circling rockfish at Cortes Bank. Courtesy of NOAA.
Join Us!
Marine Conservation Institute is committed to protecting seamount and deep-sea coral ecosystems through a combination of rigorous science and advocacy. In 2017, we co-founded the California Seamount Coalition to permanently protect these deep sea treasures. In 2019, working with Mission Blue we had California's seamounts designated a Hope Spot by Dr. Sylvia Earle.
We are grateful to the Arthur Vining Davis Foundations for their generous support of the California Seamounts.
Just as we protect the special mountain environments of the Rockies and the Sierras… Just as we have created parks to protect Yosemite Valley, and Giant Redwoods, we must act to protect the great mountains underneath the surface of the ocean and the coral forests that live on them.-Marine Conservation Institute President Dr. Lance Morgan
Pictured: A small sponge garden with its resident rockfish on Mendocino Ridge. Courtesy of NOAA.
