I find submarine eruptions most interesting, as they are vigorous and poorly instrumented. We took a look at the 2012 eruption of the Havre Seamount in 2018. We took a look at a submarine eruption along the Gakkel Ridge spreading center in 2017. Granyia took a close look at the Lo’ihi Seamount in a pair of posts a week apart in June 2015. Matt took a look at an eruption from the Axial Seamount in 2015.
As we have seen with most of our other volcanoes, magma powering the system generally powers an associated hydrothermal system of varying intensity. This usually depends on both the availability of heat and the proximity of water to that heat. What better location for huge amounts of water than the bottom of the oceans? And what better source of large amounts of magma than along a mid-oceanic ridge (spreading center)? Hydrothermal vents along the mid-ocean ridge system and other spreading or rifting centers were suspected as long as 70 years ago, though not physically observed until 1977. The closer we look, the more surveys we do, the more of these we see.
Likewise, volcanic eruptions have long been suspected along the ridge systems. But continuous monitoring of these systems along the entirety of its global 80,000 km length 2,000 – 3,000 below the surface of the ocean is expensive at best, so we tend to miss most eruptions. But we do see a few after the fact from time to time. The 1999 – 2001 Gakkel Ridge eruption is one example. Like eruptions associated with seamounts, eruptions along the ridge system tend to be periodic events, with a start and a stop.
The Northern Section of the East Pacific Rise (EPR) is a spreading center that extends from Baja California south to the Pacific-Antarctic Ridge, which in turn merges with the Southeast Indian Ridge at the Macquarie Triple Junction south of New Zealand. It is one of the world’s great tectonic structures.
The East Pacific Rise (EPR) is where the first oceanic hydrothermal vents (black smokers) along an oceanic ridge were discovered by the RISE project in 1979. At the time of this discovery, these sorts of vents were thought to be an anomaly rather than a common geologic feature.
This post will take a closer look at three suspected volcanic eruptions along the rise between 9.8° – 17° N, a distance of some 800 km along the rise. But first, a small detour to look at subsea manifestations of volcanic activity, hydrothermal vents.
Deep sea hydrothermal vent systems are distributed along oceanic plate boundaries, generally spreading centers or mid-oceanic ridge systems. There are around 500 active vent systems known today. These vents were first suspected when a deep-water survey reported hot brines in the central portion of the Red Sea. Later work in the 1960s confirmed the presence of saline brines and metal-rich muds. But it wasn’t until 1977 when anyone actually looked at one.
Vigorous vents eject superheated water with large amounts of dissolved rock and metals. These immediately precipitate out of the solution as the stream cools upon mixing with surrounding water, leading to the name, black smokers. Black smokers typically spew iron sulfides and built chimneys around the flow. The structures are unstable and collapse after growing too tall. Temperatures of the water is in the 350 – 400 C range. Less vigorous vents do not eject sulfides and operate at lower temperatures. These carry silicas and barties dissolved, making the flow white as are the chimneys. They are called white smokers. Snowblowers are even cooler vents, spewing white fluffy particles that are a product of a microbiological bloom. Final type of hydrothermal vent is a seep, which does not eject anything noticeable other than shimmering warm water. All of these vents support a biological community, generally larger and more complex based on how vigorous the vent is.
The discovery of communities of life surrounding oceanic hydrothermal vents was relatively recent, with the first being found in the vicinity of the Galapagos Islands on the Galapagos Fracture Zone, the spreading center between the Cocos and Nazca Plates east of the East Pacific Rise in 1977. This was the discovery of the world’s first active hydrothermal vent. The most shocking thing about the discovery was that the heat energy of the hydrothermal vent powered an extensive population of worms, crabs, shrimp, algae, bacteria, and other animals. Invertebrates dominate these ecosystems. This discovery demonstrated that complex communities of life on earth do not need sunlight to survive and thrive.
Ocean temperatures at the 2,550 m depth are near freezing while the temperatures from the vents are over 400 C. The vents build chimneys as tall as 55 m and eject dissolved minerals and metals. They were named black smokers because of the black cloudy water ejected from the vents. There are cooler, less vigorous vents called white smokers which eject cooler plumes and build smaller chimneys. Even cooler, weaker vents are called seeps. The more vigorous the vent, generally the larger the surrounding community. Since the initial discovery, over 500 other vent sites have been discovered.
Two years after the Galapagos discovery, a second expedition exploring the East Pacific Rise south of the tip of Baja California discovered a second population of hydrothermal vents in the vicinity of 21° N, 2,900 km north of the Galapagos rift zone. This one was a joint US – French expedition a year after initial French scouting found some interesting rock samples. This one used a piloted deep sea submersible, Alvin.
Initial encounter with a vent melted the plastic temperature probe on the submersible. Temperatures were eventually measured at 350 C, the first proof of the temperature of water from these systems. The dissolved metals in the vent waters precipitate out as soon as the hot water cools, building deposits of metals around the vents. The precipitation of dissolved metals and rock also colors the water stream. This means that subsurface hydrothermal vents may be responsible for many of the great sulfide ore deposits known today. Sulfide ore deposits connected to black smokers have been dated as far back as 2.4 billion years on an ancient sea floor in Canada.
The active vents were also surrounded by a robust, complex community of invertebrates. This established that what was found near the Galapagos was not unique. Better yet, it demonstrated that a community of life based on chemicals and hot water emitted by volcanism, not dependent on sunlight, not only existed, but was a possible model for how life might have begun on earth. These undersea communities are ephemeral, existing as long as the vents that power them are active.
The first black smoker along the Mid-Atlantic Ridge (MAR) was discovered in 1979. Average depth of these tend to be around 2,100 m. They are generally found in clusters. The first chimney was in a cluster of 6 chimneys 12 – 14 m high. A 1994 expedition along a 240 km stretch of the MAR discovered seven additional vent sites, at the time doubling the number of known sites in less than three weeks. They were spaced roughly 30 km apart on the ridge. Seabed in this region is 3,000 m.
The Trans-Atlantic Geotraverse hydrothermal field was discovered in 1985. This discovery demonstrated that geothermal fields at global spreading center ridge systems also occur at systems spreading at low rates. At spreading rates less than 2 cm/yr, volcanic activity decreases, the crust becomes very thin, and the upper mantle is exposed at the seafloor. Magmatic heat flux in ultraslow spreading ridges is an order of magnitude less than rapidly spreading ridges. This led to no small amount of speculation that hydrothermal vents could even exist at the slowly spreading centers. Yet they do. Surveys of the Southwest Indian and Gakkel Ridges have shown that these vents are more common than expected.
The Arctic Mid-Ocean Ridge is one of the slowest spreading ridge systems on the planet. A black smoker vent field was discovered in the ridge at the crest of the Axial Volcanic Ridge in 2008. Axial tops out at 2,000 m below the surface in this location. It is associated with an unusually large hydrothermal sulfide deposit indicating the system was long-lived, something that requires a stable heat source and conduit. The wildlife around this field is also different from that found along the Mid-Atlantic Ridge.
The sulfide deposits from these vents are thought to contain high grade copper and gold deposits near the vents which are now being targeted by deep sea miners. This is setting up an environmental standoff between the miners and those who want to protect the life around the active vents.
Northern EPR at 17° N
This segment of the EPR (Segment RO2 and RO3) is between the Orozco ad Rivera transform faults, south of the Rivera transform. It generally has a small rifted axial crest 2 – 3 km wide and 20 – 50 m deep, though the southern portion is wider. The trough almost disappears in some places, indicating high rates of volcanic activity. There is a seamount just east of the axial crest. Date measurements on basaltic glasses dated a lava flow around 2,000 years ago and another of a contemporary (decades to hundreds of years) date.
A detailed geochemical sampling effort during a 1997 expedition targeted the widest portion of this segment, ranging from 3 – 4 km to 10 km wide at 15° 40’. They sampled the ridge axis and off-axis seamounts. The R03 segment near 15° 40’ has a spreading asymmetry and a parallel ridge 10 km east of the current EPR. This suggests a western jump of the axis of the rise some 100k years ago. This location has a marked central bulge where the axial summit is flat and nearly 10 km wide. Average depth of the inflated segment is around 350 m above that of the average for neighboring segments of the rise, making it the most inflated of the entire northern EPR. This bulge and erupted lavas may indicate a small hotspot located beneath the ridge at this location.
The broad axial plateau has a hemisphere on each side, suggesting a seamount was bisected by the ridge. Chemistry of lavas from this portion of the segment suggest three different magma sources feeding it with the newest lavas closest to the ridge axis. Off axis magmas have a greater variation in chemistry. Most samples are dated some 2,000 – 3,000 years ago. New oceanic crust on either side of the rise is 50 – 75 ka. The broad distribution of the 2,000 – 3,000-year-old lavas are thought to be due to intense volcanic activity that flooded the axial rift and flowed down the ridge flanks. The most recent eruptions were small volume eruptions.
In contrast, volcanism along the typical R02 Segment to the south is somewhat magma-starved, though still driven by intermediate spreading rates. The summit has a trough 20 – 50 m deep and is 2 – 3 km wide. There is an unusually high level of hydrothermal activity along faulting across the summit trough. There are two locations where the trough almost disappears. This is thought to be evidence of recent volcanism. There are a few off-axis seamounts on either side of the rise. Volcanic activity along this segment has been smaller and dispersed along the length of the ridge and across time.
Northern EPR at 10.7° N
Submersible exploration discovered a very fresh lava flow at 10° 44’ N in Nov 2003 at a depth of 2,900 m. The flow was covered with bacterial mats, large amounts of bacterial floc from diffuse vents and a small population of small animals. Sample dating of the lavas put the eruption some weeks to months before the visit. This segment is located just north of the Clipperton Transform Fault, an area with an estimated low magma production rate, a magmatically starved fast spreading ridge.
The expedition expected to visit an established hydrothermal vent field. Instead, the submersible Alvin discovered fresh rock, bacterial mats, diffuse snow blower vents from lava collapses, leading to the conclusion that they were seeing a recent eruption.
The area has been monitored by a hydrophone array since 1996, though not monitored in real time. No data was recorded 2002 – 2004.
Dating very recently erupted lavas requires a dating method developed to deal with observations of an EPR 1991 eruption at 9° 50’ N. It has been successfully applied to multiple eruptions on the EPR and other seamounts. Results have been variable, with estimates of erupted lavas 2 – 3 months old largely related to the date of analysis following the eruption. This technique works better the closer to the eruption date it is as it is based on very short half life of some erupted isotopes. It is capable of dating samples with weeks to years resolution.
Northern EPR at 9.8° N
A series of submersible Alvin dives in Nov – Dec 1991 along the EPR at 9° 50’ N exploring hot vent communities discovered some that had been buried by fresh basaltic lava flows, including scorched soft tissues of partially buried animals. This site had not yet attracted scavengers, so was thought to be very recent. Fresh black smoker chimneys were draped by fresh lava flows. The location south of the Clipperton Fracture Zone at 2,500 m coincided with fresh lava flows previously estimated at less than 50 years old. Dating of dredged samples by a new radioactive technique confirmed the recent date and identified another event late 1991 – early 1992. A third eruption 2005 – 2006 produced lava flows that entrapped seismometers located to monitor the system. The location is 10 km SE of the southern end of the Lamont Seamount Chain.
The initial exploration of the site in Mar – Apr 1991 found evidence for recent volcanic activity. There were 25 Alvin doves during the expedition. They found extremely murky bottom waters, suspended particular matter plus white biological particles swept from the floor by a strong hydrothermal system. Hydrothermal fluid was very hot at 403 C. Animal communities documented in Nov – Dec 1989 had been buried by fresh lavas and not yet attracted scavengers. Some collapsed chimneys were draped by fresh lavas. A number of sites had high temperature fluids venting directly from the basalt. Apparently, chimneys had not yet had time to form. Vent animal communities were absent. These observations echo what was learned following post-eruption observations around Kick-‘em-Jenny in Dec 1988.
By May 1991, ocean bottom seismographs detected frequent micro seismicity. Many of the events were at distances of 0.5 – 2 km below the surface, centered on the ridge axis, and along at least 10 km of the ridge axis.
A new expedition in Apr – May 2006 discovered recent volcanism. Only 4 of 12 deployed ocean bottom seismometers failed to return to the surface when instructed, leading to the conclusion they had been buried by lava. Dredging along the axial trough recovered new samples showing fresh and older lavas along the 13 km dredge track. Comparisons between these samples and photos and those taken by Alvin in 1992 led to the conclusion that there had been a recent eruption 1 – 6 months earlier.
A possible eruption was predicted based on observed changes in hydrothermal vent fluid chemistry and temperatures in 2004 and increasing microseismicity in 2006. An increase in seismicity along this portion of the EPR was reported in late Jan 2006. This included a pair of M 5.4 quakes at a depth of 10 km. Both were within several kilometers of the eruption site.
Following this eruption, an expedition found extensive particulates in the water column. Towed camera images suggest the lavas erupted from fissures within the axial summit trough. Highest effusion rates were near 9° 50’N. The flow extended some 18 km along the ridge axis, up to a kilometer off axis, similar in length to the 1991 – 1992 eruption. Data from the seismometers did capture precursory activity, the eruption itself, and a rapid decrease afterwards. The event has been interpreted as an intense diking event for 6 hours on Jan 22, 2006 with a rapid taper off afterwards.
The EPR is a mid-oceanic ridge, a spreading center along the floor of the Pacific Ocean. For most of its length, it separates the Pacific Ocean to the west from the Cocos and Nazca Plates to the east. North of the Cocos Plate, it separates Baja California on the Pacific Plate from the North American Plate in the Gulf of California Rift Zone under the Sea of Cortez, converting from a spreading center to a strike-slip boundary north of Baja California and into California and North America via the San Andreas Fault and Walker Lane systems.
For most of its length, the EPR is a quickly spreading center. The segment near Easter Island is among the fastest in the world, at 15 cm/yr. In its northern reaches, the rate drops to 6 cm/yr.
The structure of the rise generally tops out around 2,500 m, and as much as 2,000 above the neighboring abysmal floor. The line of the rise is generally a parallel line of peaks 3 – 4 km wide with a trough a several to many tens of meters deep between the axial boundaries. Hydrothermal vents tend to occur along faults within the axial trough or on the line of axial peaks. Spreading is not continuous along the length of the EPR, and the rift tends to jump from time to time as chunks of new crust are carved off as underlying mantle fluids find more favorable locations to reach the surface. The entire system is bounded by transform faults where the spreading center offsets along cold lines on the ocean floor.
Narrower portions tend to be somewhat magma starved. Wider sections tend to have more magma available. Off axis magma supplies will either erupt along the axis of the rift or off axis creating seamounts. Sometimes the seamounts are bisected by the spreading rift.
The EPR has asymmetric spreading over the last 51 Ma with 55 – 60% of new lithosphere added to the Nazca Plate while 40- 45% added to the Pacific Plate. A symmetric spread would put the EPR some 500 km east of its current position. The existence of the EPR is tied to a long-lived mantle upwelling in the Pacific.
The East Pacific Rise is a massive, active tectonic feature, that has triggered intense volcanic activity in three known spots along its length. We can see evidence of past activity with the presence of seamounts on either side of the EPR. We have seen evidence of recent volcanic eruptions in at least three known locations along the northern segment. Those eruptions have been periodic, exhausting available magma for at least a time. The ongoing tectonic activity has weakened the new crust, so ocean water interacts with magma, powering a vigorous hydrothermal system along the EPR. This hydrothermal system is treated as individual sites, also with a finite lifetime. Sometimes the cluster of vents is disrupted by a volcanic eruption. Sometimes the emplaced monitoring seismographs are covered with fresh basalt. And with all of this, we’ve only closely studied small segments of the entire system. I believe we will find more activity the closer we look at the entire system.