Crustal Mantle Evolution

The entire surface of the Earth is made up of a number of tectonic plates (lithosphere) that "float" around on the surface of the Earth as the interior convects. The continents are located on some of these plates (40-70 km thick crust) while other plates consist entirely of oceanic crust (mostly 7km thick but in some places up to 30 km thick). These plates bump and grind against each other at their boundaries; the plate boundaries can be divided up into three general types. At most of the long undersea mountain ranges (Mid-Atlantic Ridge, East Pacific Rise) the plates are spreading apart from each other (divergent plate boundaries); these mid-ocean ridges consist of volcanoes that form where magma rises to fill in the gap. Strike-slip plate boundaries are areas where two plates are sliding against each other (southern California).

The Mariana Islands are located on a convergent plate boundary, where two plates are moving toward each other; this motion can be accommodated by a process called subduction, where one plate is bent downward and slides beneath the over-riding plate. The down-going plate has been chemically altered by interaction with seawater and is normally covered by a layer of seafloor sediment. As a result, the subduction of the down-going plate introduces a large amount of water into the Earth's upper mantle. As the plate plunges deeper into the mantle and heats up, water is released into the mantle. Water has a very strong effect on the viscosity and melting point of the mantle; as a result, the mantle above the down-going plate melts extensively. As this water-rich magma rises to the surface, volcanoes are formed on the top of the over-riding plate. The Mariana Islands are themselves these same volcanoes, which are often referred to as "island arc" or "arc" volcanoes (due to the geographic arc shape of the island chain). The magma delivered to these volcanoes during eruptions can be very explosive, as the large amount of water in the magma turns to steam and violently degasses during eruption.

There are many convergent plate boundaries around the world with arc volcanoes (Aleutian Islands, Andes, Cascades, Indonesia, Antilles just to name a few), but the Mariana arc is ideally suited to the study of the deep Earth's water cycle. Water is clearly being introduced into the mantle source regions of Mariana volcanoes, but we simply do not have enough data to know whether all of this water is coming back out in the volcanic eruptions, or whether some of the water is retained by the subducting plate to be carried into the deepest regions of the Earth's interior. Similar or larger uncertainties exist for all convergent plate boundaries, and as a result we do not even know whether the Earth's interior is getting wetter or drier with time. Future studies in the Mariana Islands will be aimed at determining how much water is contained in Mariana arc magmas before eruption and how much water may be emitted to the atmosphere during the gentle degassing that occurs between eruptions.