THE AXIAL 3D SEISMIC EXPEDITION
With an ocean-bottom cabled observatory, recent well-documented eruptions of lava on the seafloor, and the detection of a series of magma storage regions within the edifice, Axial Seamount is one of the best monitored and studied deep-sea volcanoes in the world. So what do we still not know?
One of the most informative aspects of volcanology is an understanding of the location, volume and pathways of magma in the subsurface beneath a volcano across all depths. A large part of our knowledge of volcanic systems comes from surface observations (e.g., geology, outcrops sampling, geochemistry, geodesy, earthquake seismology) and numerous computational models. A combination of systematic monitoring and detailed imaging of magma reservoir structure has not been done in the same location for a deep-sea volcano.
INTERIOR STRUCTURE OF AXIAL VOLCANO
Previous studies of Axial Volcano have gained useful insight into the location of melt and magma within the volcano using 2D seismic reflection surveys and information from ocean-bottom seismometers (OBSs) during periods of high seismic activity.
This most recent work identified two main reservoirs of magma beneath the surface of Axial: 1) the main magma reservoir (MMR), which is approximately 14 km long, 3 km wide, 1-2 km thick and centered 2 km below the summit caldera; and 2) a secondary magma reservoir (SMR) at a similar depth centered beneath the SE limb of the seamount.
In this expedition, we will be using 3D seismic imaging to capture the full 3D structure of these magma reservoirs, large scale rift-zone and caldera fractures, and, possibly, new locations of melt beneath the surface of Axial Volcano since the most recent summit and North Rift Zone (NRZ) eruption in 2015.
|A 2D seismic stack through the Axial caldera identifying a 14km long shallow reservoir of magma beneath the volcano (collected in 2002). The southern, shallower portion of the reservoir is interpreted have a greater magmatic melt proportion.|
WHAT ARE THE GOALS OF THIS EXPEDITION?How do volcanoes form and deform? This is a rather simple question for which the answer still has significant gaps. There are several main objectives of this expedition to Axial Volcano to help build one of the most comprehensive studies of an active submarine volcano:
- Imaging the detailed 3D geometry of the main and satellite magma reservoirs, including their internal structure (imbricated sills, melt/mush zonation) and interconnection. This will tackle questions such as: What are the magma transport mechanisms beneath, within and above the crustal reservoir? How is melt stored in the upper crust?
- Imaging the 3D fracture network exploring its relationship to magma bodies, caldera formation and volcano deformation. These observations help us formulate the following questions. Is hydrothermal venting observed around the caldera walls responsible for the physical change of the caldera floor? How are those processes connected? Are the south and north rift zones structurally different? How does the NRZ relate/connect to the summit caldera and underlying MMR?
- Connecting internal structures to surface observables such as hydrothermal vents, faults & fissures, lava flows and biological communities. What drives hydrothermal venting?
As well as the main 3D seismic survey, we will also be conducting several 2D seismic slices through the volcano. These transects will target the imaging of deeper regions within the crust that the 3D tracks cannot fully resolve. But what exactly IS a seismic survey?
WHAT IS A 3D SEISMIC REFLECTION SURVEY?IN SHORT: A seismic reflection survey uses sound waves generated from a ship to identify regions in the subsurface where sound travels at different speeds e.g. melt lenses, lava flow series, sediment layers, or fluids in the crust.
IN DETAIL: A series of air guns are towed off the back of a moving vessel. These air guns generate periodic acoustic pulses (shots) every 15 seconds that produce sounds waves which travel down through the ocean. These sound waves penetrate the seafloor and then reflect off of various structures in the subsurface. The reflections bounce back through the ground and ocean to a series of acoustic receivers along "streamers" also being towed behind the ship (see below).
|Configuration of the 3D seismic array being used on the Axial 3D survey on board the R/V Marcus G. Langseth. Four ~6km long streamers and an airgun array are deployed off the back of the vessel.|
The time it takes for the waves to reflect off a feature and travel a certain distance can tell us about the "velocity structure" beneath the volcano (see figure below). Features such as porous lava, dense lava, fresh melt, magma crystal mushes, and sediments all have different sound wave velocities. We can use a seismic survey to identify these features many miles within the oceanic crust and even to the boundary of the upper mantle!
|Schematic of a marine seismic survey with sound waves reflecting off of interfaces between layers with different density. Source: Wikipedia.|
In a 3D survey, the reflected sound waves are recorded by several parallel streamers towed behind the ship to acquire a series of 3D slices, which can then be stacked together to generate one large 3D section of the entire volcano! In this cruise, we will conduct both 3D and 2D seismic reflection surveying.
We will be out at sea for 33 days from July 11 - August 14, 2019. We depart from Seattle, WA and will be on board the R/V Marcus G. Langseth (Lamont-Doherty Earth Observatory).