Background I – Introduction

Part I – Overview of my research

mountains and volcanoes

My research focus is volcanism in central Andean plateau of South America. The plateau is rivaled in elevation and extent only by the Tibetan plateau of Eurasia. Although these plateaus (some say plateaux) have different origins, their broad similarities in elevation, crustal thickness, lithospheric process, and the presence of potassic volcanism suggest that a common link between mountain building and volcanism.

The relationship between volcanism and the non-collisional mountain building in the Central Andes

The rise of both the Tibetan and the central Andean plateaus is mostly due to compressional forces that squeeze the crust until rocks begin to thrust over one another in large faults and folds. In the central Andes, research is still being conducted to understand the basic reasons for this intense compression (in Tibet, the collision of India with the continent is an obvious culprit). In Brian Isacks’ seminal paper, he suggests that the central Andean lithosphere (composed of lithospheric mantle and continental crust) was initially weakened during low-angle subduction, allowing otherwise typical convergence of the subduction zone boundary to construct the inland plateau. Other researchers expanded on this idea and suggested that subduction of aseismic ridges (oceanic hot spot trails), increased plate velocities, and plate motion changes all played an important role in causing the low-angle subduction and subsequent plateau uplift, although this idea (and pretty much everything else related to the central Andes) is disputed). Lamb and Davis (2003), suggested that the single most important factor causing plateau growth was cooling of the global climate during the Cenozoic. These changes led to decreases in rainfall in the region, which limited sediment supply to the streams, thereby starving the trench interface of lubrication, increased the shear stress at the interface, and ultimately resulted in plateau uplift. This process was self-reinforcinging; as uplift further decreased rainfall and erosion, further starving the trench of sediments, leading to more uplift. Other researchers argue that the arid climate conditions only began some 19 million year ago and post-date uplift, which began at least 40 million years ago. Whatever the initial cause, most researchers agree that crustal shortening (squeezing) played the dominant role in construction of the both the central Andean and Tibetan plateaus. The causes of volcanism on these plateau are controversial, but they are very likely linked to the growth of the plateaux.

What is causing volcanism in in our planet‘s plateux?

Structural growth by compression does not directly lead to volcanism. In fact, the majority of volcanoes on the planet occur where extensional forces pull the lithospheric plates apart. This causes partial melting within in the asthenosphere as it rises to lower pressures beneath the thinning lithosphere. An often under appreciated concept about our planet is that very few of its volcanoes are the direct result of temperature increases. Mantle rocks are warm, but generally not hot enough to melt on their own. Instead, reducing the overlying pressure of the asthenosphere is by far the most common way volcanoes form on Earth. A secondary process that causes the asthenosphere to melt is by changing its chemistry – rocks that contain water and other volatiles melt at lower temperatures than rocks that are dry. If water can be introduced into the warm asthenosphere (or possibly even the cooler mantle lithosphere or continental crust), it will cause melting, without any increases in temperature. This occurs most often at subduction zones, where water-rich sediments and oceanic crust are dragged down with the dense oceanic lithosphere as it subducts into the mantle. High-pressure metamorphic reactions then cause the water-rich material (minerals) to become unstable, releasing their volatiles into the asthenosphere and inducing melting. Volcanoes that result from subduction are present along much of the western coastline of the Americas, but haven’t been seen in Tibet since subduction gave way to continental collision some 50 million years ago. Since subduction zone volcanoes (known as arc volcanoes) are limited to a thin band parallel to the plate interface , this process cannot readily explain the abundant volcanism across the central Andean plateau either. As orogenic plateaus such as Tibet and the central Andes are also characterized by volcanism, a causal effect may be occurring.

Lithospheric removal

As described above, the mantle beneath plateaux is likely not hot enough to melt on its own. Subduction and the addition of volatiles into the asthenospheric mantle can explain the arc volcanoes in the western portion of the central Andean plateau, but the numerous volcanoes to the east of the arc likely need a distinct trigger.  One proposed process is lithospheric removal (also described as lithospheric degradation, thinning, delamination, foundering, dripping,or convective instability). Seismic evidence that the lithospheric mantle and lower crust are thinner than expected given the large amount of shortening thought to occur in the region suggests that the removal of much of the lithospheric mantle and lower crust has likely occurred. This material must have been dragged into the depths of the asthenosphere, and may be an inevitable process in regions of crustal shortening. If such a process triggers a volcanic response, the timing and location of this removal might be identified in the temporal and spatial patterns of volcanism on the plateau surface.

Lithospheric removal and lava lamp volcanism

As mentioned above, there are two common ways to melt the rock of the mantle: decrease the pressure or introduce water. Both of these processes may be occurring beneath orogenic plateaus, where vertical movement caused by lithospheric removal transports dense material to regions of higher pressures and allows the less dense asthenosphere to rise upward to lower pressures. R.W Kay and Suzanne Mahlburg Kay (1991) expanded on earlier research of lithospheric removal by adding the idea that as the lithosphere gets squeezed together, the lower crust is pushed to greater depths and higher pressures – causing the mafic (basaltic) lower crust to transform (metamorphose) into dense, garnet-bearing eclogite. This transformation causes a density inversion where the eclogite is denser than the underlying mantle lithosphere and asthenosphere resulting the dense material to sink. The mantle lithosphere might already be denser than the underlying asthenosphere because of lower temperatures further facilitating the sinking. As this material moves downward, warm asthenosphere must rise to fill its place, thereby reducing its pressure and invoking melting. As the sinking material is brought to greater depths, hydrous minerals such as amphibole and biotite will breakdown, releasing their volatiles into the surrounding mantle (lithosphere or asthenosphere) creating a second mechanism to trigger melting and volcanism. Adding further complexity, is the strong possibility that magmas generated by either of these processes, will provide enough heat to melt small, but significant, portions of the lithospheric mantle, lower crust, and/or upper crust during ascent creating hybrid magmas prior to eruption.

Unraveling all of these complexities in the central Andes will be a main focus of my research.

Part II – Geochemistry of the central Andes