Bayes' Theorem

Idealized Plume Ascension Rate (and Lava Lamps)

Particle Reynolds Number for Sediment Transport

General Form

Specific Form

Empirical Geoxybarometer Using V paritioning

Olivine Control Line to determine V content of the melt:

Batch Melting and Fractional Crystallization

S wave velocity from the apparent incidence angle

Molten Rocks Might Have Changed the Earth's Voice

Rocks are NOT that hard stuff

As is known to all, the ice becomes water at a certain temperature. We call that procedure “melting”! Although it’s not usually seen in everyday life, melting also happens in rocks. A pretty good example of the molten rock is the lava coming from the volcano (see figure 1).

                                                       Figure 1: Floating melt rock!

In fact, the temperature in the Earth increases with the depth. When the temperature gets high enough (at a depth about 100 km underground), some part of the rock starts to melt. Scientists use an advanced technique called X-ray imaging to take a close look at the partially molten rock sample, see figure 2. By intuition, we can imagine that the melting procedure would make the rock not so hard as before.

Figure 2: Images of molten rock samples with different amount of melt between the rock grains. There are four rock samples shown here, A, B, C, and D. They are tiny rock cubes with edge length of 0.14 millimeters. The opaque stuff in gray is the melt; the transparent part is the solid rock that hasn’t been molten; and the red area refers to the melt boundaries at each cube face.

Listen to sound inside the Earth

However, properties of the molten rock are still not totally clear. A better understanding of that would help us find answers to other problems in Earth science, for example, the “anomalous sound” geoscientists hear from the Earth. When people talk to each other, we hear the voices, and we get information from that. The Earth also talks. Earthquakes generate “earthquake sounds” (terminologically called seismic waves) that travel though rocks and fluids. Earthquake sounds, i.e. seismic waves, have different traveling speeds in different materials. By listening to the sound from inside of the Earth, seismologists tell where the “sound” is from and what kind of materials it might have traveled through. And that is how we get the information about the Earth interior. Figure 3 shows how our Earth looks like in view of the seismic wave speed at different depths.

Seismic wave speeds rely on the property of the material which the wave travels in. 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 velocities, normally, should also increase. But a strange thing happens at depth around 100 km which we call a Low Velocity Zone (LVZ). As we can see in figure 3, there is a velocity decrease at the LVZ. Why do we have this? What changed the Earth's normal voice? There is much debate on this topic, and some scientists are trying to relate this with partial melt. As depth increases, the temperature also increases. At LVZ, the temperature gets high enough, and the rocks start melting. The melt softens the rock and causes seismic wave velocity decrease.

Figure 3: The two colored curves respectively represent the velocities of two different types of seismic waves along the depth. The dashed lines show the depth of some interesting zones. Here in this article, our interests focus on the depth about 100 km, where the seismic velocities have an unexpected decrease. That area is called the Low Velocity Zone (LVZ).

The melt shape and amount are supposed to have a direct effect on the velocity. But scientists are still looking for the detailed relationship between the melt property and the seismic velocity change. One way to do this is to simulate how the wave travels through the molten rock, and then compare the calculated velocities with existing observations. The simulation work goes on computers, but there are still a lot of issues waiting for clever solutions.