Scientists have discovered that one of the most extreme volcanic events in Earth’s history caused more than just the construction of a giant underwater plateau. It also changed the structure and chemistry of the underlying oceanic plate.
A research team led by Lecturer Azusa Shido of Okayama University of Science, in collaboration with Associate Professor Akira Ishikawa of Tokyo University of Science and Professor Masako Yoshikawa of Hiroshima University, used seismic waves to investigate the deep underground structure of the Ontong Java Plateau. Their results suggest that vast amounts of magma penetrated the pre-existing plate, forming a network of vertical intrusions and chemically altering the surrounding rocks.
The survey results are Geophysical Research Letters.
Hidden structures beneath the Ontong Java Plateau
The oceanic plate beneath the Ontong Java Plateau (OJP) does not appear to have the relatively simple structure expected of a typical oceanic plate. Instead, the researchers found evidence of a complex interior made up of horizontal layers traversed by swarms of vertical magma channels.
These vertical features are known as embankments. Dykes form when molten rock is forced through cracks and cools within them. Large groups of these intrusions, called dykes, can preserve records of intense volcanic activity long after the magma has solidified.
The research team also detected unusually low seismic wave velocities within the plates. This suggests that magma rising from deep within the Earth did not simply pass through the plates. The chemical composition of the plates may also have changed.
Largest oceanic plateau on earth
The Ontong Java Plateau (OJP) lies beneath the western Pacific Ocean and is the world’s largest oceanic plateau. A marine plateau is a huge raised area on the ocean floor formed by the eruption of large amounts of lava.
OJP was formed during a period of unusual submarine volcanic activity approximately 110 to 120 million years ago. This event is considered the largest known volcanic eruption in Earth’s history.
Scientists claim the eruption released enough heat, gas, and volcanic material to seriously damage the Earth’s environment. It also changed ocean chemistry, climate, and the amount of oxygen available in seawater, which may have contributed to the mass extinction.
Recent studies have shown that this volcanic phenomenon may have been caused by thermochemical plumes rising from deep within the mantle. A mantle plume is a column of unusually hot material that moves upward through the Earth’s interior. Thermochemical plumes are chemically different from the surrounding mantle and may carry materials recycled from ancient oceanic crust.
Such plumes can generate huge amounts of magma, but scientists don’t fully understand how that magma affects the oceanic plate that already sits above it.
Seismic waves reveal the interior of plates
To study the plate beneath OJP, the researchers studied high-frequency seismic signals called Po waves and So waves. The waves were recorded by submarine seismometers placed around the plateau and instruments on nearby oceanic islands.
Po and So waves travel through the oceanic plate rather than just through the surrounding mantle. Because its speed and strength depend on the rocks it passes through, it can reveal hidden layers, cracks, and other structures deep under the ocean floor.
Under typical conditions, Po and So waves are formed when P and S waves are repeatedly scattered through layered structures within oceanic plates. This repeated scattering allows signals to travel thousands of kilometers.
Waves recorded near OJP showed unusual behavior. Po waves moved efficiently through the region, but So waves weakened dramatically.
This difference provided an important clue that the plate beneath the plateau has a more complex internal structure than most oceanic plates.
Ancient magma channels cut across plates
Scientists used seismic waveform modeling to determine what types of structures could produce the observed waveform patterns.
Their results show that the plate contains dyke complexes (vertical intrusions) and intersecting layered structures (horizontal stacking). Horizontal layering allows some seismic waves to travel long distances, while vertical intrusions appear to disrupt and weaken other seismic waves.
This discovery provides evidence that magma once rose through a pre-existing oceanic plate along many separate paths. Together these channels formed an extensive underground network beneath the growing plateau.
Magma may have changed the chemistry of the plates.
The research team found another major difference between OJP and regular oceanic plates. Both Po and So waves traveled quite slowly beneath the plateau.
Seismic waves tend to slow down when passing through rocks that are hotter, less rigid, more fractured, and chemically different compared to typical mantle material. The researchers concluded that structure alone cannot fully explain the unusually low speed.
They propose that magma from a thermochemical plume rose through the plate, forming dykes and then reacting with surrounding mantle rocks. This process led to chemical modification (i.e. re-fertilization) of the plates.
Reduction occurs when magma returns chemical components lost during partial melting to mantle rocks. Most of the mantle is made up of a rock called peridotite. When a portion of that rock melts, some elements are removed along with the melted material. The magma can then bring those components back and change the mineral content and physical properties of the rock.
Giant eruptions could change oceanic plates
The results suggest that large-scale volcanic activity can cause more than just coating the ocean floor with a thick layer of lava. Magma rising from deep within the Earth can disrupt oceanic plates, form extensive dyke networks, and change the chemistry of the plates themselves.
This physicochemical modification model could improve scientists’ understanding of how oceanic plates develop and how large volcanic regions reshape Earth’s interior.
The study was published in Geophysical Research Letters.
Main findings
- The oceanic plate beneath the Ontong Java Plateau (OJP) has a complex structure consisting of horizontal layers intersected by dykes.
- The unusually slow seismic waves suggest that magma from a thermochemical plume chemically altered the plates.
- The results of this study show that large-scale volcanic activity can significantly alter both the physical structure and chemical composition of oceanic plates.
Precautions
Thermochemical plume:
A mantle plume rising from deep within the mantle. Its composition is chemically different from the surrounding mantle and may contain components derived from ancient oceanic crust.
reference:
Peridotite in the mantle is depleted of melt components when the melt produced by partial melting is extracted. The process by which molten components are reintroduced into such depleted peridotite is called illumination.
