What do earthquakes and volcanoes have in common? Water!

What do the biggest earthquakes, most destructive tsunamis and most explosive volcanoes have in common? They are all caused by water. How? It is all because of subduction. Earth's crust is made up of interlocking plates, like puzzle pieces that move around. As these plates move, they collide, pull apart and grind next to each other. Subduction occurs when two plates come together, but instead of colliding, one plate dives into the Earth beneath the other. As the plate goes deeper, the minerals within it are heated up and put under pressure, and the water they absorbed at Earth’s surface is forced out. This adds water deep inside of the Earth (depths of 40-120 kilometers) where it normally is not found. As one plate dives underneath the other, they slide past each other creating earthquakes. Adding water makes it easier for the plates to slide creating more earthquakes. Tsunamis are generated where these earthquakes occur under the ocean. Adding water to rocks also makes it easier for them to melt, like adding salt to an icy road melts the ice. When the water is added deep in the Earth, it causes melting of rocks making magma that becomes explosive volcanoes like Mount Vesuvius in Italy, or Mount St. Helens in Washington.

Fig. 1: Cross-section of subduction showing one plate diving underneath another.

Fig. 2: Map showing the outlines of Earth’s plates and the location of subduction zones.

We know that water is released during subduction because of experiments that simulate the temperatures and pressures. But it is much harder to observe the release of water in nature since it happens at 40-120 kilometers beneath our feet, so we don't really know how it behaves. One way to study the behavior of water during subduction is to find places where rocks that were subducted have made it back to Earth's surface. In these rocks, we can find veins which are the solid remains of where water used to flow. When the water was flowing through these veins it left behind chemical traces that were preserved when the vein was filled with minerals. By measuring these traces we can learn about how water behaves during subduction. Water from different places carries different chemical traces, so by measuring what is left in the vein we can determine where the water came from. If we compare the chemical traces in rocks from different places in the plate we can make a map of how water moved.

Fig. 3: Photo of a subducted rock cut by a vein. The diagram on the right shows how water might have moved through the vein leaving behind chemical traces.

We can also calculate the amount of time that water flowed through the rock by studying a process known as diffusion. As water moves through a vein, some of it leaks into the surrounding rock. Over time, more water leaks into the rock and it spreads farther from the vein. We call this process of leaking diffusion. But when the water stops flowing through the rock, there is no more water to leak, so diffusion stops. If we know what chemical traces the water was carrying, we can measure how far these traces are from the vein, and calculate how long water was flowing through it.

By using the information from the chemical traces left behind, we can better understand how water behaves during subduction. So far, we have found that water generally flows through veins for a few hundred years, the same amount of time between large earthquakes. We have also found rock trapper in veins that was shattered into pieces by an earthquake. And if we look at the chemistry of lava from volcanoes like Mount St Helens, we can see the chemical traces of water from subduction. By continuing to study water and its relationship to earthquakes, tsunamis and volcanoes, we can better understand how they happen and help keep humans safe.