How water in the deep Earth triggers earthquakes and tsunamis

Date:
June 24, 2020
Source:
University of Bristol
Summary:
Scientists provide the first conclusive evidence directly linking
deep Earth's water cycle and its expressions with magmatic
productivity and earthquake activity.



FULL STORY ==========================================================================
In a new study, published in the journal Nature, an international team
of scientists provide the first conclusive evidence directly linking
deep Earth's water cycle and its expressions with magmatic productivity
and earthquake activity.


========================================================================== Water (H2O) and other volatiles (e.g. CO2 and sulphur) that are cycled
through the deep Earth have played a key role in the evolution of our
planet, including in the formation of continents, the onset of life,
the concentration of mineral resources, and the distribution of volcanoes
and earthquakes.

Subduction zones, where tectonic plates converge and one plate
sinks beneath another, are the most important parts of the cycle --
with large volumes of water going in and coming out, 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 historically been poorly understood.

Lead author of the study, Dr George Cooper, Honorary Research Fellow at
the University of Bristol's School of Earth Sciences, said: "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." Exactly where and how volatiles are released and how
they modify the host rock remains an area of intense research.

Most studies have focused on subduction along the Pacific Ring of Fire.

However, this research focused on the Atlantic plate, and more
specifically, the Lesser Antilles volcanic arc, located at the eastern
edge 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 brings
together a large multidisciplinary team of researchers including
geophysicists, geochemists and geodynamicists from Durham University,
Imperial College London, University of Southampton, University of Bristol, Liverpool University, Karlsruhe Institute of Technology, the University
of Leeds, The Natural History Museum, The Institute de Physique du Globe
in Paris, and the University of the West Indies.

"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 modelling," said Dr Cooper.

To trace the influence of water along the length of the subduction
zone, the scientists studied boron compositions 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.

"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, 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," said Prof. Saskia Goes.

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.

"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," said Dr Cooper.

"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.


========================================================================== Story Source: Materials provided by University_of_Bristol. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Cooper, G. F., Macpherson, C. G., Blundy, J. D., Maunder, B.,
Allen, R.

W., Goes, S., Collier, J. S, Bie, L., Harmon, N., Hicks,
S. P., Iveson, A. A., Prytulak, P., Rietbrock, A., Rychert, C.,
Davidson J. P. & the VoiLA team. Variable water input controls
evolution of the Lesser Antilles volcanic arc. Nature, 2020 DOI:
10.1038/s41586-020-2407-5 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200624120450.htm

--- up 22 weeks, 1 day, 2 hours, 34 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1337:3/111)