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Wednesday, July 17, 2019

The Expedition

By Annie Kell

For this seismic survey we are studying the Axial Volcano! While the word “volcano” alone triggers interest and imaginative curiosity, the Axial Volcano is a unique and amazing system that is worthy of the hype (or at least we think so). Read on to learn some details.

Geology 101 Review:
To understand the specifics of Axial Volcano, let’s have a quick review of a few geologic terms. Volcanoes can occur in 3 tectonic settings (outlined in the figure below). We have Volcanoes like Mt. St. Helens that are associated with subducting oceanic plates. Then we have underwater volcanoes are associated with a mid-ocean ridge or spreading between plates. Many of these systems are poorly observed because they are in the middle and at the bottom of the ocean! Lastly, we have volcanoes like we see in Hawaii, which are called hot-spot volcanoes. These types of volcanoes occur in the crust above where a plume of hot magma is rising from deep within the earth. These hot spot volcanoes leave what is called a “seamount” on the seafloor. Tectonic plates move, and the actual hot-spot remains stationary. For this reason, the volcanoes become inactive because the mantel magma plume no longer feeds them. As time passes, the volcano erodes but the mark of a volcano remains for millions of years. Think of the chain of Hawaiian Islands -- they are a system of former volcanoes and you can track more seamounts across the pacific ocean that are part of the same hot-spot.  Remnant seamounts show the history of tectonic plate movement over the course of millions of years and create specific and diverse ecosystems for ocean life.

This cartoon illustrates the various tectonic locations for volcanic activity. Axial Volcano is located at an intersection of a hot-spot volcano and a rift-volcano. 

Why Axial:

OK back to this specific study. Axial is located ~300 miles off the west coast of the US, where the Pacific plate is moving northwest and the Juan de Fuca plate is moving east (thus subducting beneath North America). So Axial is located along a mid-ocean ridge volcanic system. But wait! Axial is also located at the very southeastern end of the Cobb-Eickelberg Seamount chain, a hot spot volcano! So, while Axial volcano is located within a mid-ocean ridge, it is the youngest and most active seamount of the Cobb-Eickelberg system. Axial volcano is the paramount location to understand the complex dynamics of seamounts. It is continuously monitored for volcanic activity and earthquakes. It is also observed for magma chemistry, specialized volcanic vent ecology and geologic changes associated with eruptions. Axial Volcano is the whole package.

The volcano itself is a u-shaped caldera that is ~1000 meters taller than the seafloor on either side. The image below shows a perspective view of the seafloor and some of the geophysical studies that hint at underground structure. You can clearly see the signature caldera that marks Axial.

Perspective view of the velocity structure across Axial volcano magma reservoirs (MMR & SMR), along seismic lines JF54 and JF44. The red mesh shows the 3D contour of the MMR. The
dashed black line marks the contour of the SMR. Two possible fluid pathways (low-velocity regions)
connect the SMR to a dense fracture network (magenta line) on the southeastern flank of the volcano
where a new hydrothermal field has recently been observed (New Dymond ). Magenta cylinders mark
the locations of hydrothermal vents. Green contours outline the 2015 eruption flows [Chadwick et al.,
2016a]. Gray dots are relocated earthquakes from the 2015 eruption [Wilcock et al., 2016], with a
possible fault mechanism for caldera inflation/deflation dynamic (black lines; Kennedy et al., [2004]).

Back to Axial:
In the past, there have been a lot of studies of Axial because it is so active and unusual. For most of the world, continuously monitored volcanoes are on land, but Axial actually has been continuously monitored for 2 eruption cycles (2011 and 2015). Axial has had numerous 2D seismic surveys collected across it, but this is the only 3D data volume.

About 3D Seismic Imagery:
This particular study focuses on 3D seismic imagery. What that means is that the end product will be a volume of data that reflects the intricate structure and pathways that carry the magma flows to the surface of the seafloor.

Now to the nitty-gritty of seismic imaging! Seismic imaging uses seismic waves to image the structure beneath the surface of the earth. Seismic waves can be created by an earthquake, or as is the case for our imaging efforts, we use something to make a seismic source. For this marine 3D survey, the seismic source is an array of air guns, which use compressed air to generate an impulse wave. That wave moves through the water column and then into the layers of rock beneath the seafloor. From there, the waves refract and reflect --  these are recorded by an array of receivers. For this survey, we are towing 4 streamers, each of which are 5.8 km long. The streamers are spread out away from each other by a type of water kite (called a paravane) that pulls the streamer out to the required distance so that a very large area is covered by the receiver array.

For 3D seismic, the reflections off of the submarine structures are recorded at all different angles. This is important! When the subsurface geology is complex, when there are many faults or fractures, many changes in material or fluid pathways, the returned seismic waves are scattered in different directions. Complex geology makes it very hard to process the waves and determine from where the recorded wave reflected. In 3D imaging, each reflected wave is recorded on thousands of receivers at all different orientations. Through a workflow of processing, we can place those reflections in space and get a full 3D picture of what is happening deep within the earth! Yay for applied math! 

This simplified schematic of a 3D seismic survey shows the receiver array, the seismic source near the boat and the reflected seismic waves that are recorded and processed to achieve a 3D volume. http://www.pge.com/includes/images/shared/edusafety/dcpp/seismic/seismic_survey_2_web.png

So in this amazing and complex expedition we will image Axial Volcano in 3D. We will have an unprecedented picture of a volcano that is located at a collision point between a mid-ocean ridge and a hot-spot. Axial volcano is an ideal location for this study because it complements so much past and ongoing research. The images will extend the surface observations of erupting magma into the earth and allow us to trace how fractures carry hydrothermal fluids and how magma migrates.
The objectives of this study are to determine details of how volcanoes form and then deform, where the composition of magma changes within the magma chamber, what role fractures play in magma movement, and then to connect surface vents an lava flows to their internal sources.  These objectives will be detailed more in tomorrows post.

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