Tag Archives: oceanic ridge

Tamu Massif is an intriguing new type of hybrid volcano

Tamu, the largest volcano on Earth, shares characteristics of both a mid-ocean ridge and a shield volcano. Its unique characteristics may force us to rethink a classic volcano formation theory.

A 3D map of Tamu Massif, the largest known volcano on Earth. It is around 4 miles high from its base and around 120,000 square miles across — approximately the size of New Mexico. Image credits: IODP.

When the Tamu Massif was discovered in September 2013, researchers suspected that it might be a single volcano. If this were, in fact, the case, it would make Tamu the single largest shield volcano on the globe. A new study, however, casts doubt on that idea — but shows that Tamu might be even more interesting than we thought.

Tamu is an extinct volcano, dating from the Mesozoic, some 145 million years ago. It is located 1,600 km (990 mi) east of Japan at a spreading ridge triple junction, where three tectonic plates are diverging from each other. However, Tamu was considered to be a shield volcano, comprising almost entirely of fluid lava flows from an emerging mantle plume.

This might not be the case.

Spreading ocean ridges typically form large volcanoes themselves. They also have a very distinct magnetic signature, which researchers can analyze. Essentially, whenever lava comes up the surface it solidifies, and the magnetic minerals inside it tend to align to the Earth’s magnetic poles — like compass needles frozen in time.

The magnetic poles are in constant movement, so when the next generation of lava bubbles up, those minerals will have a slightly different magnetic alignment, and so on. This magnetic analysis can also highlight polarity changes in the magnetic poles, which researchers can then detect.

Depiction of polarity around an ocean ridge. Image credits: WHOI.

Linear magnetic anomalies formed by the three ridges had previously been found around Tamu Massif, but it was unclear whether they continued within the volcano itself. Existing information seemed to suggest that this was not the case, hence the argument for Tamu being a shield volcano.

Now, a team of researchers from Texas, China, and Japan analyzed data from 4.6 million magnetic field readings carried over 54 years by ship tracks carrying magnetic measurement equipment. They also had new surveys over the area, finding that linear magnetic anomalies around Tamu Massif blend into linear anomalies over the mountain itself, indicating that the ridge is directly connected to the volcano formation.

“For Tamu Massif, we find dominantly linear magnetic field anomalies caused by crustal blocks of opposite magnetic polarity. This pattern suggests that Tamu Massif is not a shield volcano, but was emplaced by voluminous, focused ridge volcanism,” the study reads.

This is important because it suggests that the Tamu Massif (and potentially other similar areas) were formed through entirely different processes than we thought. A commonly accepted model in volcanology suggests that a hotter (and therefore, lighter) blob of magma, called a mantle plume, slowly rises through the mountain. This plume creates a volcano when it reaches the surface through a vertical succession of lava flows.

But in the case of Tamu, this succession is lateral, not vertical, which the mantle plume theory struggles to incorporate.

Depiction of a mantle plume. This explains many of the earth’s volcanic systems, but not Tamu. Image via Wikipedia.

William Sager, a geophysicist at the University of Houston and senior author for the paper, was one of the authors of the study which concluded that Tamu is likely a shield volcano, but he says questioning old ideas and putting them to the test is an essential part of science.

“Science is a process and is always changing. There were aspects of that explanation that bugged me, so I proposed a new cruise and went back to collect the new magnetic data set that led to this new result.”

“In science, we always have to question what we think we know and to check and double check our assumptions. In the end, it is about getting as close to the truth as possible—no matter where that leads.”

Also, in light of these findings, Tamu also can’t be considered the world’s largest shield volcano, since it’s not a shield volcano. That honor flows back to Mauna Loa, on the island of Hawaii. As for the largest overall volcanic system in the world, that is dominated by the mid-ocean ridges.

“The largest volcano in the world is really the mid-ocean ridge system, which stretches about 65,000 kilometers around the world, like stitches on a baseball,” Sager said. “This is really a large volcanic system, not a single volcano.”

The study ‘Oceanic plateau formation by seafloor spreading implied by Tamu Massif magnetic anomalies’ has been published in Nature Geosciences

Seafloor sensors provide unprecedented view into underwater eruption

A seafloor observatory has gathered a trove of data from the 2015 eruption of Axial Volcano, some 480 km off the coast of Oregon, allowing researchers to “see” the eruption in unprecedented detail.

Image credits: University of Washington.

Whenever a volcano erupts, it rumbles and shakes, similar to an earthquake in some ways. It sends ripples (seismic waves) in all directions, which can be picked up by specialized equipment. By then studying these seismic waves, we can tell a lot about the geologic setting of the area, something particularly interesting for the Axial Volcano, where two tectonic plates are moving apart. Basically, geologists believe that by studying this volcano they can get a better understanding of the volcanic activity around mid-oceanic ridges.

“The new network allowed us to see in incredible detail where the faults are, and which were active during the eruption,” said lead author William Wilcock, a UW professor of oceanography. The new paper in Science is one of three studies published together that provide the first formal analyses of the seismic vibrations, seafloor movements and rock created during an April 2015 eruption off the Oregon coast. “We have a new understanding of the behavior of caldera dynamics that can be applied to other volcanoes all over the world.”

The Axial Volcano is a particularly complex case. It is part of the Axial Seamount in the Juan de Fuca Ridge, located both at the center of both a geological hotspot and a mid-ocean ridge. This means that it is “fed” by underlying mantle that is anomalously hot compared with the surrounding mantle and also by magma exposed by the tectonic spreading – two unrelated phenomena which make everything more difficult to understand.

A seismic instrument (long black cylinder, right) installed in 2013 on a level triangular metal plate on the seafloor atop Axial Volcano. The green plate holds electronics that communicate between the instrument and the orange cable sending data back to shore as part of the National Science Foundation’s Ocean Observatories Initiative.
Credit: University of Washington/OOI-NSF/CSSF-ROPOS

The Axial Seamount is the most active volcanic site in the North Pacific. Study of magnetic delineations along the seamount have modeled the ridge’s history up to 30 million years ago. The place is marked by an unusually rectangular caldera, with several dome-like structures. An advantage of studying this particular volcano is that the location of the magma chamber is well known, and Axial is quite active.

“Axial volcano has had at least three eruptions, that we know of, over the past 20 years,” said Rick Murray, director of the NSF’s Division of Ocean Sciences, which also funded the research. “Instruments used by Ocean Observatories Initiative scientists are giving us new opportunities to understand the inner workings of this volcano, and of the mechanisms that trigger volcanic eruptions in many environments.

“The information will help us predict the behavior of active volcanoes around the globe,” Murray said.

Most people don’t know, but the vast majority of volcanic activity takes place underwater – which makes sense when you consider that most of the Earth’s surface is covered in water, and the vast majority of volcanoes take shape at the edge of oceanic tectonic plates.

Journal Reference: W. S. D. Wilcock, M. Tolstoy, F. Waldhauser, C. Garcia, Y. J. Tan, D. R. Bohnenstiehl, J. Caplan-Auerbach, R. P. Dziak, A. F. Arnulf, M. E. Mann. Seismic constraints on caldera dynamics from the 2015 Axial Seamount eruption. Science, 2016; 354 (6318): 1395 DOI: 10.1126/science.aah5563