It can be very exciting to watch an erupting volcano or look at super fresh deposits. You get to see rock that was inside the earth days to moments before. The inaccessible becomes accessible, and in a dramatic way.
|The author touching fresh rock from Kīlauea Hawaii 2009.|
While I love watching active processes and fresh rock, I also love looking at rocks that have been sitting on earth's surface for millions of years waiting to share their story. The exhumed insides of volcanoes provide a great opportunity to see a more complete history of how a volcano grew. One of the cornerstones of volcanology is understanding what volcanoes have done in the past, that way we can better understand what might happen in the future. While a regularly erupting volcano like Stromboli or Etna gives us an idea of the most frequent processes, we need to look at older volcanoes to get an idea of the range of eruption types and sizes and to understand their long term growth.
Earlier this summer I took a field trip to Colorado and Arizona to look at the exposed guts of two very different types of volcanoes. The aim was to investigate how magma gets through the crust and to the surface. At an active volcano we cannot see the plumbing system directly, but if we could it might help us know where the melt would reach the surface and how much was left. Geophysics is the remote study of the earth’s interior using different rock properties, such as seismic wave speed, magnetics, gravity and electrical conductivity/resistivity. These techniques have given us a way to image what is happening under a volcano, but in order to make the models precise and accurate we first need some knowledge of what is reasonable. If we’ve never seen inside a volcano, how do we know what is possible, let alone happening at a specific volcano? This is where eroded volcanoes come in.
So what about those drawings in our introductory geology text books where there is a magma chamber and with some sort of straw like conduit taking magma to the surface? Well, the honest answer is we had to start somewhere.
|Classic drawing of a volcano and its basic parts. 1) Ash plume, 2) conduit, 3) falling ash, 4) layers of older volcanic deposits, 5) older rock, 6) magma chamber. © Sémhur / Wikimedia Commons, via Wikimedia Commons|
It is always a good idea to start out with the simplest model, however, it is now recognized that volcanoes have much more complicated plumbing systems, but the systems are so complex that no one drawing would do to cover all volcanoes. That’s ok, it means we have more to learn, and there is work for future volcanologists!
But there is a misconception that I wish to correct. It is usually right next to the drawing of the simple conduit drawing. The text books bring up examples of ‘plugs’ or ‘frozen conduits’ of a stratovolcano, and most of them show a picture of Ship Rock, in New Mexico, USA.
The sharp craggy rock pokes up out of the desert, rising a little over 1,500 feet (480 m) from the surrounding rock. Ship Rock also has several dikes, or intrusions of magma that do not follow local bedding, that radiate away from it in multiple directions. It is pretty impressive to behold and is a great example of the exhumed insides of a volcano. The lie the intro book is telling is that it represents a plug of solid material that cooled inside a classic stratovolcano.
|Ship Rock as viewed from space, courtesy of Google Earth. You can see the central structure that stands up like a ship's sail in the center and the radiating dikes. The black line indicates the approximate size of the central structure|
So what is it then? It is actually a pipe full of debris, bits of cooled magma, broken sedimentary rock, and more intrusions. Ship Rock is a diatreme, which means it would have been below a maar volcano. The ground surface was likely several hundred feet higher than it is today, and when rising magma interacted with water on its way to the surface caused a series (10’s to 100’s) of explosions underground. These explosions form a crater at the surface (the maar crater) and a downward facing cone of debris underground (the diatreme). This debris has enough hot sticky magma in it, and hot water moving around, that it ends up much stronger than the surrounding rock. As the landscape erodes the diatreme and the solid intrusions of magma that fed it (the radiating dikes) are left behind, standing proudly above the horizon. Other examples of exhumed diatremes are present in Arizona (the Hopi Buttes Volcanic Field in the Navajo nation), Germany (parts of the Eifel volcanic region), Montana (Missouri River Breaks) and a few more. Maar diatreme volcanoes typically form as a result of one or only a few eruptions and have a narrow range of compositions within a given volcano. Most of them are basaltic (or a similar low silica melts like basanite or monchiquite).
So what does the inside of a stratovolcano look like then? Summer Coon in Eastern Colorado is an excellent example of an eroded stratovolcano. The structure stands out really well even in Google Maps.
So what is the big deal? Why does it matter that Ship Rock and Summer Coon are different? While we could talk about a few issues, the one I am interested here is the plumbing system. How does the magma move through the volcano to the surface? In the diatreme the magma explodes before it ever gets to the surface and creates a crater by breaking up the surrounding and overlying rock. When the explosions are shallow they throw debris into the atmosphere and have a plume and some other hazards similar to other explosive eruptions. The stratovolcano builds upwards, piling up its deposits to form a cone. In the diatreme the magma arrives and mixes into the debris-filled mess that is the diatreme. In a stratovolcano the magma can intrude into the conical pile of deposits and erupt at the top, like we expect, or anywhere along the side where it finds an easy way to the surface. We have a lot to learn about where an eruption is likely to occur next, and investigating these systems that were once underground is a great way to study some of the possibilities. Eroded volcanoes also give us a chance to see some of these underground structures in 3D, a unique opportunity to remember that magma doesn’t just move in a straight line from inside the earth to the surface, but can take more complicated paths to the surface. This is a good reminder for all of us who spend time looking at two dimensional sketches of these typically hidden structures.
Both of these locations are gorgeous to visit for geology, weather, and the general landscape. But there are many more examples around the world!
* If you want some really neat technical descriptions of diatremes from the Hopi Buttes area I refer you to some scientific papers.
Hooten JA, Ort MH (2002) Peperite as a record of early-stage phreatomagmatic fragmentation processes: an example from the Hopi Buttes volcanic field, Navajo Nation, Arizona, USA. . Journal of Volcanology and Geothermal Research 114:95-106
Lefebvre NS, White JDL, Kjarsgaard BA (2012) Spatter-dike reveals subterranean magma diversions: Consequences for small multivent basaltic eruptions. Geology 40:423-426
Lefebvre NS, White JDL, Kjarsgaard BA (2013) Unbedded diatreme deposits reveal maar-diatreme forming eruptive processes: Standing Rocks West, Hopi Buttes, Navajo Nation, USA. Bulletin of Volcanology 75:739
White JDL (1991) Maar-diatreme phreatomagmatism at Hopi Buttes, Navajo Nation (Arizona), USA. Bulletin of Volcanology 53:239-258