Scientists have for the first time linked the deep Earth’s water cycle to earthquakes and volcanic activity.
Water, sulphur and carbon dioxide, which are cycled through the deep Earth, play a key role in the evolution of our planet – including in the formation of continents, the emergence of life, the concentration of mineral resources, and the distribution of volcanoes and earthquakes.
The Lesser Antilles volcanic arc is one of only two zones where we can see these slow-moving plates. Professor Jenny Collier Department of Earth Science and Engineering
Subduction zones, where tectonic plates meet and one plate sinks beneath another, are a key part of the cycle – with large volumes of water going into and coming out from the Earth, mainly through volcanic eruptions. Yet, just how (and how much) water is transported via subduction, and its effect on natural hazards and the formation of natural resources, has been poorly understood.
Now, a new paper from a project led by researchers from Bristol, Durham, and Imperial has shown that water in the deep Earth triggers earthquakes and volcanic activity by releasing fluids along fault lines and lowering the melting point of rocks.
The researchers say this is the first conclusive evidence that directly links the water-in and water-out parts of the cycle with magma (melted rock) production and earthquake activity.
The paper is published in Nature.
As tectonic plates journey from where they are first made at mid-ocean ridges to subduction zones – where they meet other plates – 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 chain of volcanic islands, called a volcanic arc.
Lead author Dr George Cooper, of the University of Bristol, said: “These eruptions are potentially explosive because of the volatiles (water, carbon dioxide, and sulphur) 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.”
The wettest parts of the 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 the presence of fluids in the subsurface. Professor Saskia Goes Department of Earth Science and Engineering
While most studies look at the Pacific Ring of Fire, the subducting plates that surround the Pacific Ocean, this research focused on the Atlantic plate, in particular on the Lesser Antilles volcanic arc at the eastern edge of the Caribbean Sea.
Study co-author Professor Jenny Collier, of Imperial’s Department of Earth Science and Engineering, said: “The Lesser Antilles volcanic arc is one of only two zones where we can see these slow-moving plates. We expect this one to be hydrated more pervasively than the fast spreading Pacific plate, and for expressions of water release, like earthquakes and tsunamis, to be more pronounced.”
To conduct the study, the research team, known as the Volatile Recycling in the Lesser Antilles (VoiLA) project, collected data over two marine scientific cruises. They deployed seismic stations that recorded earthquakes beneath the seafloor and the islands and undertook geological fieldwork, chemical and mineral analyses of rock samples, and numerical modelling.
To trace the influence of water along the length of the subduction zone, the scientists studied compositions of the element boron and isotopes of melt inclusions (tiny pockets of trapped magma within volcanic crystals). Boron fingerprints revealed that the water-rich mineral serpentine, contained in the sinking plate, is a dominant supplier of water to the central region of the Lesser Antilles arc.
The researchers say that by studying these microscopic measurements it is possible to better understand large-scale processes. The 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.
Co-author Professor Saskia Goes, also of Imperial’s Department of Earth Science and Engineering, said: “The wettest parts of the 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 the presence of fluids in the subsurface.”
The history of subduction of water-rich fracture zones can also explain why the central islands of the arc are the largest and why, over geologic history, they have produced the most magma.
Dr Cooper said: “Our study provides conclusive evidence that directly links the water-in and water-out parts of the cycle and its expressions in terms of magmatic productivity and earthquake activity. This may encourage studies at other subduction zones to find such water-bearing fault structures on the subducting plate to help understand patterns in volcanic and earthquake hazards.”
Next, the researchers will look into how this pattern of water release may affect the potential for larger earthquakes and possible tsunamis.
This story was adapted from a press release by the University of Bristol.
“Variable water input controls evolution of the Lesser Antilles volcanic arc” by George F. Cooper, Colin G. Macpherson, Jon D. Blundy, Benjamin Maunder, Robert W. Allen, Saskia Goes, Jenny S Collier, Lidong Bie, Nicholas Harmon, Stephen P. Hicks, Alexander A. Iveson, Julie Prytulak, Andreas Rietbrock, Catherine A. Rychert, Jon P. Davidson & the VoiLA team, published 24 June 2020 in Nature.