Plate Tectonics: The theory of plate tectonics was advanced in the 1960s and 1970s and represented a revolution in our understanding of how the earth works. With this new concept, earth scientists could explain, for example, why large earthquakes occur in some places on earth but not others; why fossils of the same relatively rare animal species are found on widely separated continents; why there is evidence of cold glacial deposits in areas now located near the equator.
In the theory of plate tectonics, the outer shell of the earth (called the lithosphere) consists of rigid, 100 km-thick plates, which slowly move relative to one another, and ride on the more ductile and partially molten aesthenosphere beneath.
Dramatic earth events often occur at the boundaries between the lithospheric plates – of which there are three types: convergent, strike-slip or divergent plate boundaries. At convergent plate boundaries, two plates move toward one another; for example, the Indian plate converges northward toward the Eurasian plate, raising the Himalaya Mountains. At strike-slip plate boundaries, two plates slide past one another horizontally; along the San Andreas fault in California, for example, a portion of the Pacific plate slides northward relative to the North American plate. The third type of plate boundary – and the type we will focus on during this expedition – is where two plates move apart (diverge) from one another. The most prominent example of divergent plate boundaries is the global mid-ocean ridge system. Here, divergence of the two plates leads to decompression and melting of the mantle below the ridge; the melt then rises buoyantly and erupts onto the ocean floor as lava and solidifies to form new ocean crust. The physical and chemical properties of oceanic crust help us to better understand how it forms.
Our Expedition: An underlying question in the study of mid-ocean ridges (also called spreading centers because the two plates spread apart) is how they initiate and evolve.initiate and evolve in different settings around the globe. Previous studies have explored divergence and initial rifting in continental environments such as the East African Rift. But there are few places along the mid-ocean ridge system where one can explore various stages of divergence from initial rifting through full seafloor spreading. The Cocos-Nazca spreading center offers an opportunity to examine these and related questions.
Located at ~ 2°20’N and ~ 35 km east of the East Pacific Rise , the Cocos-Nazca Rift provides a unique opportunity to study progressive stages in the establishment of tectonic and magmatic seafloor spreading.
This expedition will use bathymetric, gravity and magnetic data to reconstruct how each segment (see figure below, S1 through S9) has evolved to its current configuration. Rock samples dredged from the ocean floor will be used to explore geochemical changes that may accompany the evolution of the spreading segments through time, and possible influences of proximity to the EPR.
Bathymetric data indicates that the western most portion of the Cocos-Nazca spreading center (S1,) is amagmatic, and the transition into magmatic seafloor spreading occurs between ridge segments S2 and S3 while full magmatic seafloor spreading is occurring along S3. Further sampling along this portion of the Cocos-Nazca spreading center will help determine whether this observation is correct. Another interesting observation is that ridge segments increase in length from west to east (red lines). One possibility is that initial magma supply established beneath newly formed rifts tends to become enhanced with time, and manifests this enhancement by increasing segment length. These and other questions and hypotheses will be tested with the data we collect on this research cruise and subsequent analyses on shore.