Science Background

This post will describe the scientific (and in this case economic) purpose of this cruise, for those who are interested in that sort of thing. This cruise is somewhat unique because it represents collaboration between academia and industry. The cruise is entirely funded by Nautilus Minerals, an Australian seafloor mining company. Typically, research cruises are funded by grants from the National Science Foundation (the US government body that funds nearly all scientific research in the country). My advisor at the University of Hawaii at Manoa, Fernando Martinez, is the Chief Scientist, but the exploration program and the instruments that we are using were all decided by the Nautilus people. This company has used data collected in this area by my advisor previously and they are using a University of Hawaii ship. We will use the data collected to learn more about the study area, and at least a few of the scientists on board will publish papers based on some of the data we collect, so it is a mutually beneficial operation. The first paper for my PhD will be on the same general area, but most of the data we collect here is at too small of a scale to be particularly useful for my purposes.

Location map showing Lau basin bathymetry (seafloor topography). My Master's study area is outlined by the blue box, the red lines mark the spreading centers. We are looking at the northeast corner of the map, near the northernmost spreading center.

The Study Area:
We are going to be sailing around the ocean a few hundred km south of Samoa, and ~500 km east of Fiji. We are in the Lau basin, which is the same geologic feature that we studied during the last cruise in 2009, but we are ~500 km N-NE of the previous study area. The Lau basin is a backarc basin associated with the Tonga subduction zone. Very briefly, a subduction zone occurs where two tectonic plates converge, in this case two oceanic plates. One plate, in this case the Pacific plate, sinks down under the other and penetrates deep into the mantle. This is occuring around nearly the entire rim of the Pacific plate, creating what is known as the "Ring of Fire," as well as various other places around the world. Subduction zones are the locations of the largest earthquakes and volcanic eruptions around the world, e.g. the recent Japan quake, the large quake in Chile a few years ago, and the infamous 2004 tsunami in Sumatra, so they are a very important geologic setting to learn about. In order to understand what we will be looking at, I'll give a brief introduction to some of the major processes along subduction zones before I get into what we'll be doing:\

A simplified cross-section showing the main features of a subduction zone.

1) Earthquakes (large and small, deep and shallow) occur along the interface where the two plates come together and grind past each other. Large ones occur when the plates "stick" and lock together for a long time, building pressure until it is all released in one large event. These quakes often cause tsunamis as well. Smaller quakes occur when the plates move a little bit at a time and don't get the chance to build up a lot of energy. Earthquakes also occur due to the plate bending and cracking as it sinks down into the mantle, but these are typically smaller than those that occur where the two plates directly rub against each other.

2) Volcanoes: the subducting plate (called the "slab") is saturated with water and as it descends into the mantle this water is slowly squeezed out of the sediments and rock and released into the overlying mantle. Also, at ~100-200 km depth, minerals that have water bound in their crystal structure undergo a chemical reaction that releases this water, creating a sudden influx of water at these depths. When water enters the mantle, it lowers the mantle melting temperature and decreases it's viscosity by a huge factor, causing melt to buoyantly rise and penetrate into the overlying plate above. This creates a linear chain of volcanoes on the seafloor, called the arc volcanic front, or the arc for short. These volcanoes can grow to become islands that break the sea surface or they may just continue erupting under water. When the overlying plate is a continental plate, you get volcanic chains such as the Andes Mountains in South America or the Cascade range along the west coast of North America, where Mt. St. Helens, Mt. Rainier, Shasta, Lassen, etc. are located. There are other materials and chemicals released from the slab as it subducts, but water is by far the most important in creating this massive amount of volcanic activity.

3) Backarc extension: This part is a little more complex and not fully understood, but this is the part of the system that we are currently looking at and that both my Master's and PhD work focus on. While convergence is the dominant overall plate motion in a subduction zone, the region behind the arc, away from the trench where the two plates come together, can actually undergo extension. The first thing to help understand how extension can occur in a convergent setting is to realize that the slab is not being shoved down into the mantle, but rather sinking and falling away from the overlying plate. The trench where the two plates come together actually migrates away from the arc in a process called trench rollback, although it should be noted that this does not occur in every subduction zone. Because the two plates are coupled together at their interface, the slab actually pulls the overlying plate with it, causing the overlying plate to be stretched. This stretching causes rifting near the volcanic arc, where the crust is thick and weak, which over time matures and concentrates along a single spreading center, similar to “normal” spreading centers along the mid-ocean ridge system, where extension rather than convergence is the dominant tectonic process. As the plate is pulled apart, the mantle beneath rises to fill the space and actually melts because of the decrease in pressure, causing volcanism along the spreading centers and new oceanic crust to form. So in a global sense, crust is continually being destroyed at subduction zones and created at spreading centers. These two processes are balanced, otherwise the earth would be growing or shrinking, which we know is not the case. One amazing fact is that the entire process of plate tectonics is driven by the fact that the solid plate is ~0.5% denser than the mantle below. Without this density difference, the plates would not sink into the mantle, subduction would not occur, new crust would not be created, and all of the land above sea level would simply slowly erode and shrink over time until we would all be living in a world entirely covered by water.

4) Hydrothermal Activity: The process that we will be focusing on during this cruise is hydrothermal activity. Hydrothermal vents can occur in any location with active volcanic activity where there is magma residing within the crust, in this case individual volcanoes and spreading centers. Water penetrates the crust through faults and cracks, and even through pore spaces in the sediments and rocks. When the water comes into contact with magma or hot rock, it is quickly heated, often to 4-5x the boiling point, and it is ejected through hydrothermal vents on the seafloor. On its way up through the crust, the water dissolves metals and various minerals out of the crust, which are then ejected at the seafloor vents. These metals and minerals precipitate out of the superheated water when it hits the cold seawater and are deposited on the seafloor around the vents. This is actually the original source of nearly all precious metals, such as gold, silver, platinum, copper, and zinc. These metals end up on land by being scraped off of the slab onto the overlying plate during subduction under a continent. All of the gold in the Sierras in California was initially deposited near hydrothermal vents on seafloor that was long ago subducted under North America.