Tectonics on Earth's "Twin" Planet

Venus had often been called Earth’s twin planet.  While it has many similarities to Earth- size, density, bulk composition, radiogenic heat production- it turns out to be a very different world.  There are surface temperatures of 740 K (hot enough to melt lead) and an atmosphere of almost entirely carbon dioxide (a runaway greenhouse effect).  Another important difference is its absence of plate tectonics.  Earth appears to be the only planet with plate tectonics, which is the theory that Earth’s lithosphere (the crust and upper mantle) is made of individual mobile plates that can collide, spread apart, or slide past one another.  Although Venus currently lacks plate tectonics, it is impossible to know if it existed before 500 Mya.  It was around this time that Venus underwent a global resurfacing event; this age has been constrained with crater count data.  The physics behind this catastrophic process is still mysterious, and some suggest it could be cyclic.  Instead of plate tectonics, Venus is currently in stagnant lid convection regime, which means that tectonics is driven by basal shear from mantle convection (Hot mantle rises and cools, then descends. The same principal as boiling water on the stove- see Fig. 1).

Figure 1: A simulation showing convection in a one-plate planet, such as Venus. Red indicates warm, rising mantle, and blue is cold, descending mantle. (credit: Walter Kiefer and Louise Kellogg, lpi.usra.edu)     

On Earth, plate tectonics require a global network of shear zones (areas of low strength and concentrated deformation) throughout the entire lithosphere.   The localization processes are dominated by water content and minerals with layered sheet-like fabrics, such as micas.  Venus’ lithosphere is dry and lacks layered fabrics, and yet the rift zones of Venus are remarkably Earth-like.  The area of Beta Regio (Fig. 2) has often been compared to the East Africa Rift System due to their similar geologic morphology.  Beta Regio has a rift called Devana Chasma, which is part of a triple junction system similar to the one seen in East Africa.  Beta Regio is a volcanic highland region; the elevation and other geophysical data suggest that it is the result of a young underlying mantle plume (a thermal perturbation which rises through the mantle and impinges on the base of the lithosphere…think of a lava lamp!).  For Devana Chasma to be as narrow as it is, we expect some form of localization to be active to concentrate the deformation into a localized region.  It is likely that the presence of melt will have a significant impact on localization.  The basal shear force induces a small stress on the lithosphere, and may not be enough to cause rifting.  The addition of gravitational potential energy due to the area’s high elevation might be enough to induce rifting.  

Figure 2: Image of Beta Regio. The black lines are regions that were not imaged. (credit: Rathbun et al. Formation of Beta Regio, Venus

It is thought that the addition of the appropriate localization mechanism(s) and including the fact that tectonics are not driven by the same forces on Venus as they are on Earth, that an agreeable model for rifts on Venus can be developed.  By understanding the tectonic regime of Venus we are able to better constrain its evolutionary history and interior processes.  Venus can also represent an appropriate analogue for early Earth, prior to the initiation of plate tectonics, which will allow us to better understand how it developed here.

See the Earth's interior through noise

What does a local earthquake sound like

For many of us, the first impression of earthquakes is hazard, disaster or catastrophe. However, on the other side, seismologists can make use of the record from seismic wave because of the information it carries from the deep interior of the Earth. This even leads to the fact that for tens of years, seismologists struggles in studying most part of the Earth’s interior due to the lack to seismicity in those areas. Now there might be a new way of exploring the Earth that meets the interests of both the public and scientists -- seismic noise tomography.

Detecting seismic signals from noise(Weaver, 2005). Among the diffusive wave field of noise, the detectors record random signals. Infrequently a wave may travel through both detectors. As a result, the signals are weakly correlated.
Like the definition of most noises, seismic noise is a non-interpretable or unwanted component of the signal recorded by seismometers. The recent study is challenging this assumption. The main idea of seismic noise tomography is to detect and amplify the common feature between two noisy waveforms from different detectors. Specifically, cross-correlation function is used here as a measure of similarity between two seismic records. The physical meaning of this coincidence is the propagation of the weak but repeated signals through the Earth between these seismometers. By looking through a monthly long noise record and adding up the detected signal, seismologists are able to provide a strong, clear cross-correlation function between a pair of seismic stations. This information is then translated into the time needed to travel through this path. Combined with the distance; the velocity of the Earth’s interior is obtained. While the analysis of one pair of stations only provides the velocity structure in a limited area, same work can be done between various pairs of seismometers within a dense seismic array. Then seismologists can use tomographic techniques to generate a map of seismic wave velocity.

Group-speed maps constructed by cross-correlating 30 days of ambient noise between USArray stations. (A) 7.5-s-period Rayleigh waves. (B) 15-s-period Rayleigh waves. Black solid lines show known active faults. White triangles show locations of USArray stations used in this map. (Shapiro et al., 2005)

The noise tomography result from Shaprio et al. has been suggested to be consistent with geological structures in the studied area. As more and more dense seismometer arrays become available, such work can be carried out throughout the Earth with numerous possible applications.

Reference

Shapiro, N. M., Campillo, M., Stehly, L., & Ritzwoller, M. H. (2005). High-resolution surface-wave tomography from ambient seismic noise. Science,307(5715), 1615-1618.

Weaver, R. L. (2005). Information from seismic noise. Science, 307(5715), 1568-1569.

How do Geoscientists “Hear” the Melt Underground?

Earthquakes generate “earthquake sounds” (terminologically called seismic waves) that travel though rocks and fluids. Seismic waves have different velocities in different materials. By listening to the sound from the inside Earth, seismologists tell where the “sound” is from and what kind of materials it might have traveled through. And that gives us the information about the Earth interior. Figure 1 shows us how fast the seismic wave travels at different depth of the Earth.

                                    Figure 1: Seismic velocities in the Earth

If we zoom in the velocity profile, we can see a low velocity zone (LVZ) around depth 100 km. Seismic-wave velocities rely on the property of the material in which the wave propagates. Generally, the harder and denser the material is, the faster the wave travels in it. As depth increases, the rock density gets higher. So the question now is why there is a velocity decrease at the LVZ. Many studies were done on this topic, and currently the most convincing interpretation is partial melt. As depth increases, the temperature also increases. At LVZ, part of the rocks starts melting, which generates fluids that cause the seismic velocity anomaly.

                                                Figure 2: At about depth 100 km underground, there is a low velocity zone.

However, the details of how the melt shape and amount affect the velocity are not totally clear. Geoscientists want to find a quantitative relationship between the melt and the seismic velocity change. First, they measured the shape of the melt using X-ray scanning in the lab. Figure 3 shows the melt shapes in partially molten rock cubes at different degrees of molten. The next step, which hasn’t been done yet, is to simulate the seismic wave propagation on the computer and then compare the calculated velocities with our observations. Since we are not able to dig into any depth of our Earth, a combination of the seismic methods and rock physics analysis would be particularly helpful with exploring the Earth interior.

                                                Figure 3: Melt distribution in partially molten rock samples

Figure1 and figure 2 are from wikipedia, figure 3 is from Miller, Kevin J., et al. "Experimental quantification of permeability of partially molten mantle rock." Earth and Planetary Science Letters 388 (2014): 273-282.

Call a Plumber!!! I Think Our Watershed is Broken.

A general definition of a watershed is the region of land within which every drop of rain and surface water flows into a common drainage. Think of it as a big basin.  The Chesapeake Bay Watershed is a large and well-studied area that covers 64,000 km² and includes parts of Delaware, Maryland, New York, Pennsylvania, Virginia, and West Virginia.  It is also home to about 17 million people and 185 million livestock animals.  In addition to the typical pollutants we tend to associate with populated areas—industrial chemicals, heavy metals, vehicle exhaust, road salt—we generate a lot of nitrogen (N) and phosphorus (P) that eventually makes its way to the Bay.  But wait, those are good, important nutrients, right?  Nitrogen and phosphorus are natural parts of aquatic ecosystems. Nitrogen is also the most abundant element in the air we breathe. Nitrogen and phosphorus support the growth of algae and aquatic plants, which provide food and habitat for fish, shellfish and smaller organisms in water.  However, too much nitrogen and phosphorus in the water cause algae to grow faster than ecosystems can handle. Significant increases in algae harm water quality, food resources and habitats, and decrease the oxygen that aquatic life needs to survive. 

Large growths of algae, called algal blooms, can severely reduce or eliminate oxygen in the water, leading to illnesses or even death in fish populations. Some algal blooms are also harmful to people because they produce elevated toxins and bacterial growth that can cause water contamination. 

So what are we doing about this in the Chesapeake Bay Watershed?  A LOT!  Policies are in place to reduce nutrient loads entering our rivers from farms and cities.  Education efforts from elementary schools to town hall meetings teach people about the problems and some of the solutions.  Throughout the region, parts of many streams have been “restored” to stabilize their banks to reduce erosion, add engineered structures to slow down the water, or reconnect streams with their floodplains through the creation of wetlands.  The goal is to reduce the volume of N and P being transported, and allow more time for nutrients to be transformed to inert states before they reach the Bay.   But a number of the questions remain regarding how well some of these restoration practices work:

·         Are they successfully changing the chemistry of the water?

·         How much improvement in N and P removal can we expect ?

·         What are the best practices?

·         Where are they most effective?

We begin to answer these questions through long-term monitoring and frequent sampling of the water upstream and downstream of where a restoration strategy has been put it in place.  We measure the water chemistry to look for seasonal differences, changes between normal flow conditions and during storm events, and any entry of nutrients from sources we can’t see.  Until now you might have thought that a watershed was two-dimensional (2-D), only at the surface.  We have to learn to think in FOUR dimensions; in addition to the area above ground, there is a complicated, dynamic system underground (3-D), and things change through time (4-D).  This is especially true when we think about urban areas.  All of those leaky pipes and sewer systems are part of the problem!

Credit:  Rich Puoyat

Rapid land development that brought rise to large cities and agricultural areas has altered our watershed permanently.  We have a long way to go to identify how much damage has been done and how we can make improvements.  People aren’t moving away from our nation’s waterways anytime soon, so we have to learn how to coexist and create sustainable cities.  For now, save your call to the plumber (but put a scientist on speed dial!) and surf your watershed to find out what’s going on in your own area, and how you might help work toward solutions.  

Mid-ocean Ridge Melting - The Ballon Analogy

     The mid-ocean ridges are mountain chains that curve around the Earth like baseball seams. Sadly, these exist almost entirely at the bottom of the oceans (but you may have guessed that from the name!). From these mountains, lava (liquid rock) comes slowly pouring out, from somewhere within the Earth. Exactly where, is an important, and not well understood question, but we'll get to that later. The lava becomes solid rock and creates new "crust." The "crust" is a specific portion of the Earth's anatomy. It is a region of the Earth that is more rigid (like a pie crust), than the inside (which is still rock, but since it is hotter, can 'flow' in a similar manner to a bending spoon). The inside of the Earth is actually split into a few different parts, but for the sake of simplicity, I will leave that to your interests to figure out elsewhere (try wikipedia). Anyway, the part of the inside of the Earth that I study is called the "mantle." It moves around slowly, similar to a boiling pot of water, heat from the bottom causes the mantle/water to move up, as it moves up (away from the heat source) it cools down, and when it reaches the top of the Earth/pot, it returns downward. This is called a convection cell, and there are many of them inside of the Earth's mantle. 

     The mid-ocean ridges are where two of these convection cells are moving upwards together, and then move away (see this image for a better understanding of all this stuff).

Artists conception of the interior of the Earth: The motion of the Mantle (the middle, orange region), can really be seen nicely here. The Core (the red part, at the center) is the source of heat that causes the mantle to move up in this image. The Mid-ocean ridge is the thin yellow line on the surface of the Earth. The melting region is the triangle formed just below the ridge.              Image Source: National Geographic

     Now, there is a lot of discussion about what causes these movements, but the basics are that the heat inside of the Earth is escaping, and that the ridges are an area of the Earth where things are moving away from each other. Just below the surface, there is a region of the mantle which is just hot enough to cause some of the material to melt. This melted material is lighter than the rock, and so it moves upwards. Now as you can imagine, the region containing melt is quite large, because a lot of the mantle is hot here. One of the main problems when this is realized, is that if everything is moving up in this large region, why do we only see a thin ridge, where all the lava comes out. One idea that explains this, that I use, is that there is a layer in the mantle that funnels the lava to the ridges. Think of it as a roof, and picture yourself in an attic. In this attic you have a bunch of balloons, if you let go of them, you would see them travel up, hit the bottom-side of the roof, and then get funneled to the center. This is the way that we think melt from a large area, arrives at the much smaller mid-ocean ridge. In this analogy, the center of the roof would be just below the mid-ocean ridge. 

The 'Balloon Analogy": The crust is represented by the green panels, and the arrows show that the crust is moving away from the mid-ocean ridge. The Grey is the roof that focuses the balloons (melt) to the ridge. The red balloons turn into green crust. The red arrows show the direction of the convection cells in the mantle. 

     This "roof" is a somewhat complicated structure, and the way we understand and explain it actually allows us to predict the way and the locations that it will cause the lava to come out. Now you'll have to imagine a roof with more than two sides, maybe a house with a corner in the middle. This corner, will get more of those balloons than other parts of the ridge. And extending the analogy to the Earth, a larger amount of lava, and hence a larger amount of rock will appear here. 

     The beauty of this relatively simple idea, is that we can now start to add more complicated things to different parts of the attic, and see where, and how the roof structure will create changes that we can then see at the surface of the Earth (except this part of the surface is at the bottom of the ocean, but that just means we get to go down in submarines and explore!). And when we find the things that our complications predict, we then add this small piece of knowledge to the growing body of knowledge about the Universe. Which is really what it's all about.

The 'Balloon Analogy 2: Son of Balloon Analogy": The crust is represented by the green panels, and the arrows show that the crust is moving away from the mid-ocean ridge. The Grey is the roof that focuses the balloons (melt) to the ridge. Only in this cartoon we have a corner. There is more crust (the blippy lines at the top of the image) at this corner. And we've also added a different kind of rock (yellow balloons) to the idea. The red balloons turn into green crust, the yellow balloons turn into yellow-green crust. You can begin to imagine what adding more colors, in different regions can do!

Clues in the Crust Help Resolve an Atmospheric Conundrum

             Despite taking over 23,000 breaths per day, you probably don’t often wonder where the oxygen necessary for your survival comes from. In fact, this question is still being asked by scientists and many are currently working to better understand how and when oxygen arose in our atmosphere. The term Great Oxidation Event, or GOE, is used to describe the point (or period) when the Earth’s atmosphere began to accumulate oxygen and is thought to have occurred about halfway through Earth’s lifetime, around 2.4 Ga (Ga = billion years ago).

             One of the methods used to constrain the timing of the GOE is to track where the element molybdenum (Mo) is found throughout Earth history. While uncommon, Mo is produced in the igneous rocks in the Earth’s crust as magmas rise towards the surface and is often associated with hydrothermal alteration of igneous rocks. It is thought to concentrate in sulfides, as it has been shown to have an affinity to the element sulfur. One of the most common sulfides in the crust is pyrite, FeS₂, as pictured below. The uniqueness of Mo lies in its sensitivity to oxygen – when exposed to oxygen in the atmosphere, the soluble Mo6+ ion will weather out of pyrite in the crust and will be carried to rivers and oceans through erosional processes. 

             Geologists measure Mo concentrations in shales throughout time to determine when Mo began to appear in ocean sediments, an analog to when free oxygen accumulated in the atmosphere.  While this method has timed the GOE in concordance with other approaches, the use of Mo as an oxygen proxy is a method that is still being refined.

 Figure 1 | Mo enrichment in shale through time. The dashed box represents the GOE at 2,400 Mya and the small Mo peak at around 2,500 Mya may represent a "whiff" of oxygen before the GOE. See Scott et al., 2008 (Nature) for further information.

Figure 2 | A photomicrograph of a pyrite crystal (light yellow) with exsolved chalcopyrite (gold) and a magnetite rim (silver). Sulfides like pyrite have long been considered to be the primary host of Mo in the crust. Field of view is 3 mm.  Scott, C., Lyons, T., Bekker, A., Shen Y., Poulton, S.W., Chu, X., Anbar, A., Tracing the stepwise oxygenation of the Proterozoic ocean, Nature 452, 456-459, 2008

From Rainwater to Mineral Water: a Chemical Journey

            You may think of rainwater as an ultra pure form of water, but this is not actually true.  While high in the atmosphere, the water droplets interact with numerous gases, which dissolve in the water droplets before they fall to the ground.  Carbon dioxide (CO₂) is the most important of these gases, as it combines with water to form carbonic acid (H₂CO₃), which is a small component of rainwater.  This small amount of carbonic acid causes natural rain to be slightly acidic.  But most other kinds of water such as seawater, river and lake water, and water in the ground is slightly basic, the opposite of acidic.  How does this transformation occur? 

The answer lies in the fact that as rainwater flows down through the soil and rocks on its way to the ocean, it interacts with rocks in a process called “chemical weathering”.  A major weathering reaction involves the breakdown of the mineral calcite (CaCO₃) by the acidic rainwater, releasing calcium (Ca⁺²) and bicarbonate (HCO₃^-) ions.  The bicarbonate ion is slightly basic, changing the water into a slight base.  This is a necessary process, as most aquatic and marine life could not survive if the water were acidic like rainwater.  

A lot of weathering reactions are poorly understood however, and while I was an undergraduate, I undertook research with a graduate student named Hyojin Kim to better understand how water changes as it flows from cloud to stream to ocean.  The site we studied was in a nature preserve in Northern California, in the watershed of the Eel River. 

The Location of Rivendell, the site we studied, within California the wells drilled are shown

The Elder Creek, which sits at the bottom of our hill slope and has basic water

            We drilled wells into the hill slope and devised a suction system to sample the groundwater from these wells everyday for several years.  We then analyzed these water samples for many kinds of ions. 

One of our auto sampling machines on the steep hill slope

            The data showed us that chemical weathering is a complex and dynamic process.  Weathering reactions of all sorts of different minerals made the groundwater chemistry change throughout the year and up and down the hill slope.  We monitored the rainwater and creek water at the site to show how much the water had changed during its time flowing through the soil and rock.  Our data showed that different ion concentrations vary differently throughout the year and across multiple years, demonstrating that groundwater chemistry is a complex system waiting further exploration.