Learning from the gentry: Maars and Tuff Rings

Uni classes that specialize in intermediate - advanced volcanism are few & far between in my neck of the woods. Prior to the 4th year courses that dive into rheological properties of magmas and lavas, detailed structures of volcanic landforms, eruption dynamics, and mechanics of volcaniclastic deposition (at least according to the course outline), us undergrads get treated to repetitions of the usual volcanology basics as it pertains to other geoscience disciplines. It gets a bit tiresome going over the VEI for the umpteenth time, and retreads about the same types of volcanoes erupting the same types of lava. Variations within a single volcano don't exist before 4th year, and probing questions are usually met with the full-stop response "But for this course you only need to know...".

Diagram of the various zones and
facies of a maar-diatreme

So I've had to teach myself some ins and outs of volcanism to go beyond the basics. The Firefly guide was a good start, and had more in-depth technical detail than I expected. After looking at some sites in the High Lava Plains of Oregon, I became familiar with features called maars and tuff rings. Courtesy of Jessica Ball, I was pointed in the direction of certain papers written by Volker Lorenz, most notably on "Maar-Diatreme Volcanoes, their Formation, and their Setting in Hard-rock or Soft-rock Environments". It's an excellent read, and lays out the details of these structures as seen in varying locations, plus methods of emplacement during different periods of volcanism. Magma's interaction with various types of aquifers, and how each can produce a different maar-diatreme cross-section is also outlined.

The features themselves might lean towards the esoteric, but when trying to find places of interest that can offer something new to digest whilst hiking around, maars and tuff rings fit right in with tuya's, obsidian flows, lava domes, and other under-appreciated volcanic gems of the landscape. Some of the interesting, more explicit facts I discerned from Lorenz's paper include:

• Vast majority of maar-diatreme volcanic features occur in silica-poor (basaltic) volcanic fields
• Tuff Rings/Cones are thought of as the phreatomagmatic equivalent of rhyodacitic lava domes
• Posteruption, diatremes develop unique stratification of different facies (tuff, volcaniclastics, breccias, sediments, xenoliths)
• Maar lakes are generally short lived, as sediment fills in the shallow depression. If one exists, it indicates a geologically recent phreatomagmatic eruption
• The greater the volume of the Tuff Ring formed, the smaller the volume of lahars produced in the same phreatomagmatic eruption (subtraction of tuffaceous material from potential flow)
• Maars created within a jointed aquifer (hard rock that is heavily faulted) result in posteruptive diatreme pipes that are among the widest, due to block collapse of the hard rock edifice
• The center of Maars and Tuff Rings can have a measurable gravity anomaly when compared to the surrounding country rock
• The inclusion of irregularly distributed groundwater in the rising magma of an eruption can change the viscosity of the magma, and thus result in irregular eruptions and tephra deposition
• Diagenesis (a water-driven metamorphosis of sediments) is frequent in diatremes of kimberlite pipe origin near mid-ocean ridges, ie. olivine can hydrate into serpentine and thus shrink the diatreme pipe diameter

Those points are my interpretation, and I might be off (at least until year 4). The only experience I have with maars and tuff rings is from a trip into south-central Oregon's Lake county. Unfortunately, it was several years ago, prior to my budding interest in the geosciences. However, Fort Rock and Hole-in-the-Ground are excellent stops for marvelling at a standout tuff ring and maar not 10km from each other. They are part of a basin of primarily Quaternary alluvium eroded & transported from the High Lava Plains province/Steen's basalt group. During their diatreme eruptions in the Pleistocene, they were inland lakes, hence their genesis.

Hole-in-the-Ground maar (left) and Fort Rock tuff ring (right), both protected as Oregon state parks
43° 23.314'N 121° 7.919'W will place you smack between the two

Additional Info:

• Volker Lorenz - Maar-Diatreme Volcanoes, their Formation, and their Setting in Hard-rock or Soft-rock Environments
• Brennan T. Jordan - The Oregon High Lava Plains: Proof against a plume origin for Yellowstone?
• USGS - Maars and Tuff Cones
• J.D.L. White - Maar-diatreme volcanoes: A review