New research conducted by an international team of experts has managed to reach the first conclusive evidence that links deep Earth’s water cycle and its behaviors with magmatic productivity and earthquake activity.
Water (H2O) and other volatiles (e.g., CO2 and sulfur) that are pushed through the deep Earth have played a crucial role in the evolution of our planet, more so in the formation of the continents, the onset of life, the accumulation of mineral resources, and the dispersion of volcanoes and earthquakes.
Lead author of the study, Dr. George Cooper, Honorary Research Fellow at the University of Bristol’s School of Earth Sciences, explained: “As plates journey from where they are first made at mid-ocean ridges to subduction zones, seawater enters the rocks through cracks, faults and by binding to minerals. Upon reaching a subduction zone, the sinking plate heats up and gets squeezed, resulting in the gradual release of some or all of its water. As water is released, it lowers the melting point of the surrounding rocks and generates magma. This magma is buoyant and moves upwards, ultimately leading to eruptions in the overlying volcanic arc. These eruptions are potentially explosive because of the volatiles contained in the melt. The same process can trigger earthquakes and may affect key properties such as their magnitude and whether they trigger tsunamis or not.”
Where exactly and how volatiles are released and how they change the host rock is still a mystery, but researchers are determined to find out.
The Connection is Now Clear
Most studies have concentrated on subduction along the Pacific Ring of Fire. This research, however, focused on the Atlantic plate, namely on the Lesser Antilles volcanic arc, located at the eastern border of the Caribbean Sea.
“This is one of only two zones that currently subduct plates formed by slow spreading. We expect this to be hydrated more pervasively and heterogeneously than the fast spreading Pacific plate, and for expressions of water release to be more pronounced,” said Prof. Saskia Goes, Imperial College London.
The Volatile Recycling in the Lesser Antilles (VoiLA) project has collaborated with a large multidisciplinary team of scientists for this study.
“We collected data over two marine scientific cruises on the RRS James Cook, temporary deployments of seismic stations that recorded earthquakes beneath the islands, geological fieldwork, chemical and mineral analyses of rock samples, and numerical modeling,” said Dr. Cooper.
To monitor the effect of water along the length of the subduction area, the researchers analyzed boron compositions and isotopes of melt inclusions, which are small pockets of trapped magma within volcanic crystals. Boron prints unveiled that the mineral serpentine filled with water, contained in the sinking plate, is an inactive supplier of water to the core zone of the Lesser Antilles arc.
“By studying these micron-scale measurements, it is possible to better understand large-scale processes. Our combined geochemical and geophysical data provide the clearest indication to date that the structure and amount of water of the sinking plate are directly connected to the volcanic evolution of the arc and its associated hazards,” said Prof. Colin Macpherson from Durham University.
“The wettest parts of the downgoing plate are where there are major cracks (or fracture zones). By making a numerical model of the history of fracture zone subduction below the islands, we found a direct link to the locations of the highest rates of small earthquakes and low shear wave velocities (which indicate fluids) in the subsurface,” further explained Prof. Saskia Goes.
The cycle of subduction of water-rich fracture areas can also explain why central islands of the arc are the biggest and the reason behind the large production of magma over geologic history.
“In this research, we found that variations in water correlate with the distribution of smaller earthquakes, but we would really like to know how this pattern of water release may affect the potential — and act as a warning system — for larger earthquakes and possible tsunami,” said Prof. Colin Macpherson.
The study is titled ‘Variable water input controls evolution of the Lesser Antilles volcanic arc,’ and has been published on June 24th, 2020, in the journal Nature.