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Thursday, July 18, 2019

Objective #1: What is actually going on in there?!?

By Annie Kell

Our 3D survey has the first objective of imaging Axial volcano is to see the details of the main and satellite magma chambers. Though we have this idea that volcanoes hold magma in a single and simple reservoir of magma and where there is no variation in the types of magma melt, the reality is that volcanoes have a more complicated system! There are many arms and pathways inside the system and there are variations of the actual chemical composition of magma.

The magma body:
When we think volcano and magma chamber, we think of it like a single tank underground with an opening on top, like what is illustrated in the first figure below. In this mental picture, pressure builds and the magma either blows or seeps out of the top of the “tank.”

In an idealized setting, the volcano is very simple with one magma chamber and one vent.

In observations of volcanic systems, the tank that holds the magma is complicated. It can be a combination of small chambers that are connected through fractures or conduits. The chambers can be mostly vertical, called dikes, or can be more horizontal, called sills (see the next image). Because there are few examples of the “text book” magma chamber, a lot can be learned by seeing an actual system and by seeing that system in 3D. The AXIAL3D Expedition will answer the fundamental questions that cannot be answered with 2D data and that are still lingering about this system. By seeing the large and small paths that make up the magma chamber, we can see how melt is stored in reality. The magma is moving from beneath the crust, within the crust and then to the surface of the earth. We will get to see that path! And in 3D! 

Observations actually show that volcanoes are complicated structures with many chambers and many vents. This survey will image the internal workings in 3D.

Some of these magma are not like the others:
Not all magma is the same. Magma is a variable material! Some magma is very melted, meaning it has a larger proportion of liquid. In other systems and (as we are learning) sometimes within the same system, the magma is more crystalized. More crystalized means the material is not as melted but rather has some “solidus” in the melt.  Within the Axial magma chambers, there are pockets where magma is more melt rich and where there is more crystallization. How do we tell this?!? The magma is deep within the earth so we cannot sample it! This is where the seismic data comes in.

As mentioned earlier, we record reflections of seismic waves off of all the material deep underground. Depending on the material, the amplitude of the reflected wave changes. Seismic data shows variations in the recorded amplitudes and we can plot those variations in many different ways. In order to distinguish areas that are more crystalline from areas that are more melted we conduct a type of seismic analysis called Amplitude Versus Offset or AVO. Offset is simply the distance between the seismic source to the seismic receiver. We look at the dependencies between the amplitude of the returned wave and the offset from the source. The cartoon image of a 2D profile below gives a bit of a picture of how offset and amplitude change along the length of the receiver array. AVO analysis can give a lot of details about a materials density, fluid percentage (thus melt variation) and material porosity.

This cartoon illustrated the changes in offset along the length of a receiver array.


So depending on HOW we analyze the data, we can tell the internal structure AND we can get indicators of material composition. All from the same data collection! These 2 direct uses of the seismic data are the first learning objective for this study. We will cover more about further objectives later.

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