|Source: adapted from National Park Service and R. J. Lillie. 2005, Parks and Plates|
Where in the world do we find the worst earthquakes, and many of the worst volcanic eruptions? Looking at maps of earthquake epicenters and volcanic eruptions, it doesn't take long to realize that there are specific zones where disasters and human misery occur. They follow oceanic trenches and their associated volcanic arcs (curving series of active volcanoes). Horrific events like the Sumatra earthquake of 2004, the 2011 Tohoku earthquake near Japan, and the 1991 eruption of Mt. Pinatubo in the Philippine Islands were the result of oceanic crust and upper mantle (the lithosphere) sliding beneath the adjacent continental or island landmasses. It is the sliding and grinding of the oceanic plate in these subduction zones that produces the huge earthquakes, and it is the complicated interaction of extreme heat and volatile materials in the mantle that leads to the formation of magmas and resulting volcanic eruptions.
|Mt. Shasta and Little Glass Mountain, in the active part of California's subduction zone|
Subduction zones are complicated places, and it can be difficult to study active systems. We may get highly accurate maps of the seafloor, and geophysical data may reveal the broad outlines of what lies beneath the bottom of the sea, but direct sampling and observation of the deep crust is mostly beyond our technology. So how to study and understand the dynamics of these zones of terror?
Maybe you have noticed that I sometimes say nice things about the geology of California. Once in a while, anyway. The state has such a rich variety of geologic and tectonic landscapes that one could spend a lifetime exploring them all (which for the record is what I am currently doing, although I am known to explore other places as well). As such, the state provides a nice outdoor laboratory for looking in the active parts of a convergent boundary, as well as a marvelous place to observe the deep interior of a fossil subduction zone complex.
|California's Great Valley may seem to be a monotonous flat valley (clarification: it is a monotonous flat valley), but it hides a violent and complicated past.|
Besides the trench, there are three structures within many subduction zone complexes: an accretionary wedge, a forearc basin, and a magmatic arc (see the diagram at the top of the post).
The accretionary wedge is a gathering place for the flotsam and jetsam of the seafloor and oceanic crust, as well as sediments from the continent. These trench deposits include a clay- rich sandstone called graywacke, dark colored shale, pillow basalt, deep-ocean chert, and the occasional volcano or coral reef. These rocks are carried deeper and deeper into the subduction zone, and are put under tremendous pressure. The rocks are churned up, faulted, and deformed into a chaotic mass called a mélange (from the French word for mixing).
A forearc basin forms in a relatively shallow sea between the crest of the accretionary wedge and the volcanic arc inland. Sediments, primarily sandstone, siltstone, and shale derived from the continent, accumulate to depths of tens of thousands of feet. This can happen in a shallow basin because the weight of the sediment pushes the crust downward, making room for more sediments. The foundation of the forearc basin is ocean crust rocks collectively called an ophiolite sequence.
A magmatic arc is a chain of volcanoes fed by magma generated when the subducting slab of oceanic crust reaches the semi-molten layer within the mantle called the asthenosphere (from the Latin "weak shell"). Water in the subducted slab serves as a catalyst to lower the melting points of the silica-rich minerals, causing the rock to melt and form plutons of magma that rise through the continental crust. If the magma reaches the surface and erupts, it may form andesite, dacite, or rhyolite lava. If it cools slowly deep in the crust, it will form a variety of granitic rock, such as actual granite, granodiorite, tonalite, quartz monzonite, or diorite.
|Part of the Diablo Range, a subdivision of the Coast Ranges, from the summit of Mt. Hamilton, which houses the Lick Observatory complex.|
I hope you'll join me on this coming blog journey across California and through the guts of an ancient subduction zone. I got the seed of an idea for this series when I finally drove the winding road from San Jose to the Great Valley past Lick Observatory and down Del Puerto Canyon (I've been in Del Puerto many times, but never drove beyond the head of the canyon). I can't believe it took me this long to get around to it, but that's what happens sometimes.
This series is also meant to coincide with the long-awaited opening of our Great Valley Museum of Natural History, which opens to the public on April 4. Information is available on Facebook at https://www.facebook.com/GreatValleyMuseum?pnref=story, and at http://www.mjc.edu/instruction/sme/gvm/. I hope to see you there!