Subduction Zones: making natural disasters commonplace

Subduction zones cause volcanoes, earthquakes, and tsunamis, but can we understand something we can't directly observe?

If you threw a rock into an active subduction zone, perhaps the Japan Trench just east of Honshu Island, it is going to be transported tens to hundreds of kilometers into the Earth. Why? Because the thin veneer on the surface of our planet, known as the crust, is moving. In some areas, known as ridges, the crust is separating and magma is moving in to fill the gap (check out The Interior of the Earth is Balloons! for more on this). In other areas, such as the Japan Trench, the plates are moving towards and into each other. Where oceanic crust converges with continental crust, a subduction zone forms, and the oceanic crust is driven beneath the continental crust.

Active subduction zones create a lot of problems for the societies that live above them. If you live in Japan, or Chile, or even Northern California, I’m sure you’re no stranger to earthquake drills, tsunami sirens, or volcanic “red zones”. But what is it about living above a subduction zone that makes catastrophic natural disasters commonplace? It all comes down to what happens when cold and wet material (ie. the oceanic crust) interacts with the hot and dry material in the Earth’s interior tens of kilometers beneath our feet –too deep for us to directly observe. So, how do we learn about these interactions if we can’t directly observe them? To answer that question, let’s go back to that rock you threw into the Japan Trench.

The active subduction zone beneath Japan causes quite a few natural disasters. Here, the red and green circles represent the origin (epicenter) of an earthquake, with the size representing the magnitude of the earthquake. Red circles are "shallow" earthquakes and green are "deep". You can also see that the volcanoes' magma originates on the subducting slab.

Over the course of the next couple million years, your rock is going to be transported deeper and deeper into the Earth at a rate of a few centimeters per year. The deeper it gets, the more heat and pressure it is exposed to, which causes the minerals (like garnet and quartz) in it to slowly change –similar to how cookie dough changes when it is baked. As these minerals change, they act like transcribers for the rock, recording everything that happens to it.

The minerals in rocks change similarly to how cookie dough changes when it is baked!

Although most of the material in the subduction zone is moving down, it doesn’t always stay there. There is a chance that your rock will be brought back to the surface by a process that geologist call exhumation. There are several exhumed terrains all over the world that we study in order to understand how subduction zones work –Santa Catalina Island off the coast of California, the Cyclades in Greece, New Caledonia, the Western Alps in Europe, and Tian Shan in China are just a few (the rest are shown in the map below). By studying the minerals in these rocks, we can get a glimpse at how deep into the Earth they went, how hot they got, how long they were subducted for, and the amount of time it took to get them back to the surface (among other things). All of this knowledge adds up to understanding the mechanics of the subduction zone, which in turn leads to knowing how the resultant earthquakes and volcanoes are generated.

Exhumed subduction zones around the world. The colors represent geographical "groups" of exhumed terrains. From Penniston-Dorland et al (2015).

So, what can these exhumed rocks tell us about the 2014 Mont Ontake eruption that killed 57 people or the 2011 Fukushima Daiichi nucleardisaster? Well, just like when cookies are over-baked, when the cold wet rocks are subducted into the dry and hot interior of the Earth, they dehydrate. Since the released water can’t just evaporate like it does on the surface of the Earth, it moves into the surrounding dry and hot rocks –this is known to cause both earthquakes (which generate tsunamis) and volcanoes. The exhumed rocks tell us where (and when) in the subduction zone this dehydration happens, allowing us to predict where earthquakes and volcanoes may occur. Of course, no prediction is perfect, but the more we know about subduction zones the less catastrophic the resultant natural disasters are!

Image sources:

Earthquakes, Volcanoes, and Japan | Cookies Baking | Exhumed Terrain Map

Do the chemicals in our rivers tell us who we are?

Leonardo da Vinci provided us with profound hydrologic insight, albeit unintentionally, when he stated, “In rivers, the water that you touch is the last of what has passed and the first of that which comes; so present with time.” Rivers continuously drain water from our landscapes and carry it to the oceans. After it rains, the water moves both over the land surface and underneath the ground towards the river.  When you look at a river you’re seeing the same water that once blanketed the land for a brief moment. As the water makes its way into the river, we often see it take small items along for the ride, like soils, stones, woodchips, newspapers, and empty soda cans. However, there is something else being carried that we don’t see; chemicals. Unlike heavy items like trash, the chemicals cannot settle out of the water downstream because they go through a transformation and instantly become a part of the water. Hydrologists call these chemicals the dissolved load of a river. As we apply nutrient fertilizers to our crops to grow food for our families, the chemicals (nitrate, phosphate) become a part of the dissolved load.  As we build our houses and roads, the chemicals (salts, carbonates) follow the pipes into our rivers. As we improve our quality of life and innovate new technologies, the chemicals of raw materials (sulfur, lead, arsenic, PCBs, PFOAs) enter the dissolved load.  As we drink more coffee, cure disease, and fight our wars, the chemicals involved (caffeine, hormones, amphetamines, antibacterials, radioactive uranium and tritium) show up in our rivers. 

Water discharge from a pipe (iStockphoto/Viktor Balabanov from ScienceDaily, 2008)

Although these chemicals in our rivers might be fascinating to anthropologists, they are catastrophic for our environment and our health. Nitrates and phosphates cause huge algal blooms in stagnant water (like beaches) that suck up all of the oxygen and kill fish, crabs, and other marine life. Salts, carbonates, and metals can create toxicity for the delicate plant life that relies on the water. These chemicals also attract invasive plants that are tougher, and that can outcompete the native species and eventually lead to a loss of biodiversity.  Organic chemicals like antibacterials, hormones, and plastics are carcinogenic for the animals that rely on the water and can cause reproductive issues, physical and mental abnormalities, and death. If these chemicals continue to show up in our rivers, we can ultimately lose the ecosystem services that we are dependent on for our quality of life, like seafood, clean drinking water, sewage disposal, and recreational spaces. If aliens ever landed on Earth and wanted to know more about us, I’d tell them to look in our water.

People have significantly altered the landscape, and the fingerprints of human activities show up in the water (Kaushal & Belt, 2012)

Cosmic Dust Bunnies

     

One of many new micrometeorites found by Project Stardust. Photo credit: Jan Braly Kihle/Jon Larsen. © Project Stardust

 Jon Larsen is a modern day renaissance man. He’s an accomplished guitarist, composer, and painter, but lately he’s been spending his time sweeping up and collecting dust on Norwegian rooftops. Why would an artist spend his or her free time this way? Normally the answer would probably be drugs, but instead of angel dust, Larsen is getting high on a much more far-out kind of star dust, “urban micrometeorites”. 

A dusty night sky during the Perseid meteor shower in 2015.
Photo credit: Scott MacNeill/Frosty Dew Observatory.

Larsen is the principal investigator for a research organization called Project Stardust in Oslo. His goal over the past few years has been to prove that micrometeorites can be found in urban settings (usually on large, flat roofs).

As the name implies, micrometeorites are microscopic cousins of meteorites. You may be more familiar with the visual spectacle produced by the entry of micrometeorites into our atmosphere—shooting stars. Micrometeorites started their life off as dust particles floating between planets in our Solar System. That dust is what remains of planet-forming processes that occurred as long as 4.5 billion years ago. Some of those dust particles wind up drifting into Earth’s orbit, streaking through the atmosphere, and coming to rest somewhere on the surface.

We’ve known for years that micrometeorites exist.

Researchers have historically found them in seemingly unlikely places, such as ocean floors and antarctic ice sheets. These particles often contain a significant amount of iron, and can be separated from other dust particles with the aid of a simple magnet. Until recently, many in the scientific community thought searching for micrometeorites in more developed cities would be a fruitless endeavor. Cities tend have a higher concentration of artificial dust particles that are also magnetic and usually sourced from industrial processes. With a very small amount of micrometeorites falling to Earth each year, artificial dust particles far outnumber anything that fell from space.

Larsen was not deterred by the challenge, and developed a method of discriminating between artificial and cosmic dust particles. His search consisted of over one thousand field searches, which usually involved sweeping up dust on rooftops, separating out magnetic material, thorough washing, and using sieves to separate out different dust sizes. The remaining particles were observed under the microscope for features that are characteristic of micrometeorites— aerodynamic shapes, beads of iron-nickel alloys, unusual surface textures, and chemical compositions that are similar to meteorites.

Micrometeorites come in many forms that distinguish them from artificial dust particles.
Photo credit: Project Stardust - Jon Larsen

The identification of urban micrometeorites provides new insight into the variety of interplanetary dust in the Solar System. Most of these particles were deposited within the last century, while micrometeorites collected in Antarctica and on ocean floors may have been deposited over a much greater span of time. An ongoing program examining changes in micrometeorite compositions over time could lend some insight into the distribution of interplanetary dust in the Solar System. This information could be used to understand processes that may have occurred during the formation of planets over 4.5 billion years ago.

Jon Larsen can't get his mind out of the gutter, as he uses a powerful Neodymium magnet
 to pick up magnetic dust particles in Oslo, Norway.  Photo credit: Project Stardust.

A basic guide to collecting urban micrometeorites can be found on Project Stardust’s website, although you’ll need a very high resolution camera to photograph them yourself. You can also buy his book, In Search of Stardust online. Even if you don't find any micrometeorites on your first search, at least it’s one more reason for you to finally get around to cleaning out your rain gutters!

Old Faithful Plumes on Europa?

Europa, one of Jupiter's four largest moons, has an ocean lurking underneath its ice shell. The ocean's presence has been speculated based on Europa's surface features including rift zones, ice ridges, and chaos regions (Fig. 1).  Plumes on Europa are significant because of Europa's potential to harbor life in its ocean. Europa's ice shell is potentially tens of kilometers thick, making drilling to its ocean nearly impossible. These eruptions may deposit material from Europa's interior onto its surface where it can be more readily studied by a fly-by mission or lander mission. Thanks to the Hubble Telescope, Europan plumes have now been imaged.

Figure 1. Cross section of Europa's ice shell and subsurface ocean. In the background are Jupiter and Io, one of the most active bodies in the solar system. The plumes eject material from the surface ocean, but how the plumes are activated remains a mystery. From left to right, surface features include plumes, rifts and chaotic terrain. Image Credit: NASA JPL/ Caltech

            In 2012, scientists at the Southwest Research Institute discovered water vapor signatures coming from Europa. The signatures are likely plumes due to cyrovolcanism (ice volcanoes). More recently, Hubble was used again to directly image plumes erupting from Europa (Fig. 2). Out of ten observations, the plumes were spotted three times. Since Enceladus experiences eruptions during certain points in it orbit, one hypothesis is that Europa would experience a similar eruption cycle. Initial investigations suggest this may not be the case.

Figure 2. Image of Plumes with high-res image of Europa superimposed. Image Credit: NASA/ESA

            The locations of Enceladus' plumes tend to cluster in the southern hemisphere, near the tiger-striped patterns. Europa's plumes originally clustered near the southern pole, but the recent observations showed the plumes may originate closer to the equator. The timing of the eruptions was compared to Europa's orbital position relative to Jupiter, and indicated that the eruptions do not always occur at the predicted times. This has lead several studies to suggest that plumes may not be controlled by tides, or Europa's orbit, but may be caused by internal convection in the subsurface ocean. Until more observations can be made, the eruptions on Europa remain unpredictable. While the plumes may reach great heights (100km), they won't be as faithful as Yellowstone's geysers.

   

Figure 3. Artists illustration of plumes on Europa's surface. Jupiter can be seen rising in the background. Image Credit: NASA JPL/ CalTech

            

Tracing Water(falls): Understanding the role of water in subduction processes

Earth's crust is made up of a number of plates that move around and interact with each other by smashing together, pulling apart or sliding next to each other. Subduction occurs when instead of colliding, one plate slides underneath another. Subduction is important because some of the biggest earthquakes, most explosive volcanoes and richest mines are the result of processes that occur during subduction. One of these processes is the transportation of large amounts of water deep into the earth where they don't belong. When a plate is moving across the bottom of the ocean, water is absorbed into the crust and stored in minerals. When the plate is subducted, these minerals are heated up and put under pressure and they release the water stored inside them. The addition of water makes it easier for rocks to slide and cause earthquakes. It also makes it easier for rocks to melt (like salt added to an icy road).

Fig. 1: Schematic diagram of a subduction zone showing one plate diving underneath another and water being released from the down-going plate. (altered from Bebout, 2007)

We know that all of this water is released because of experiments that simulate the temperatures and pressures of subduction, but it is much harder to observe the water in nature (since it happens at 30 to 80km beneath our feet) so we don't really know how it behaves. One way to study the behavior of water in a subduction zone is to find places where rocks that were subducted have made it back to Earth's surface (something else that we don't really understand). In these rocks, we can find veins where water used to flow and left behind chemical traces. Depending on where the water came from, it will leave behind different traces, so we can track where the fluids came from and, by comparing different rocks, map out their pathway as they leave the subducting plate.

Fig. 2: Photo of a rock that was subducted and returned to the surface. The white and brown vein can be seen cutting across the rock with a green zone around it showing where water, carrying chemical traces such as lithium, has leaked outward. Just as water at Earth's surface flows in rivers, water that moves through Earth's crust can be concentrated in channels, such as this vein, with only limited interaction with the surrounding rock. (from Taetz et al., 2016)

We can also calculate the amount of time that water flowed through the rock by using the element lithium. Water moves through the rocks along channels, like rivers across Earth's surface, but as the water passes, some of it leaks into the surrounding rock. Over time, these leaks penetrate further and further into the rock. We call this diffusion. But when the water stops flowing through the rock, diffusion stops. Lithium diffuses faster than any other element so even if the water is only flowing for tens to a few hundred years (this seems like a long time, but to a geologist this is like the blink of an eye), it still diffuses a measurable distance. So by measuring the distance that lithium has diffused, we can calculate roughly how long fluid was moving through the rock. We then combine this with other data to understand things like how far water can move in a subduction zone, which direction it moves, how much of it there is and what kinds of elements it can dissolve and transport. This helps us to better understand volcanoes, earthquakes and ore deposits.