By Annie Kell
|Lines on the seafloor highlight seafloor features in the vicinity of a seismic line through Axial caldera, such as: fractures, ridges, caldera walls, lava units. Black lines on seismic section are possible uppercrustal faults.|
The second objective of AXIAL3D is to understand how the fractures connect to the magma bodies. The presence of fractures in a volcanic setting is not unusual. Many other volcanic systems have similar fractures that form paths between magma chambers. The fractures are thought to be caused by several different processes, and we want to know more about the cause.
Expansion and deflation of the magma body or spreading of the magma body can cause fractures. When magma enters a storage chamber, the pressure and stresses on the nearby rocks changes. Subsequently, when the magma evacuates that chamber, the weakened rock will break into fractures. As this process is repeated the surrounding rocks are weakened and result in cracks that extend from the magma bodies to the surface of the ocean floor.
Think about a cake that rises too much while in the oven. The over inflation on the top of the cake weakens the cake, and then when it cools and collapses the surface is covered in cracks.
Similarly, when the location of the magma chamber moves (so the location and amount of the inflation moves around), stresses are placed on the surrounding rocks, expanding the area and then forming fractures after collapse.
The various types of fractures seen from existing 2D imagery on Axial volcano are comparable to those seen in systems globally. Axial, however, is more heavily monitored and, with this 3D survey, will be more accurately imaged than most other analogues. By collecting these data, we gain information to test a series of hypothesis for what causes the fractures and what role they play in the volcano dynamics.
The two specific hypotheses that will be tested using the processed data are 1) is the fracturing caused by magma intrusion/motion into the main magma reservoir or 2) is the fracture network attributed to the motion within the more secondary reservoir?
Because the fractures change the physical properties (things like porosity and how easily fluids flow) within the caldera and the rocks surrounding the system, the physical changes play a role in how exactly the magma migrates. It’s feedback system! The magma creates the fractures, the fractures change the physical properties of the volcano, that dictates where the magma flows which leads to more fractures and so on!
We want to know if the changing fracture network plays a part of the intrusion events that are far from the main magma reservoir. Also, could hydrothermal venting cause the fractures? Lastly, what types of differences are there in the structure across the region? The findings of this study will help to answer these questions.
This image comes from Stewart and Davies, 2006. A concentric ring of faults surrounding a mid subvolcanic caldera. (a) This map of the concentric ring faults is based on the disruption of shallow seismic reflection amplitudes from earlier seismic studies. The second objective of AXIAL3D is to understand how these fractures correlate with the various magma chambers within the entire system.
The second main objective of AXIAL3D is to address the fundamental questions about diking and the correlation of fractures to the secondary magma reservoirs within the Axial system.