Post contributed by Dr. Sandro Rossato, Department of Geosciences, University of Padova, Padova, Italy

Figure 1: Terrestrial maars. (a) is a group of three maars filled with water in the Eifel region, Germany (rim-to-rim diameter ~0.5-1 km) (“Maare” by Martin Schildgen – Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons – https://commons.wikimedia.org/wiki/File:Maare.jpg#/media/File:Maare.jpg). (b) shows the Wabah maar, located in Saudi Arabia (rim-to-rim diameter ~2 km) (courtesy of Vic Camp, San Diego State University).

Terrestrial maar-diatremes are small volcanoes (see this previous post for a general description) which have craters whose floor lies below the pre-eruptive surface and are surrounded by a tuff ejecta ring 2-5 km wide (Figure 1) that depends on the size of the maar itself and on the depth of the explosion (Lorenz, 2003). Maar-diatremes constitute highly valuable sites for in situ investigations on planetary bodies, because they expose rocks at the surface from a great range of crustal depths and are sites which could preferentially preserve biomarkers.

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Post by Dr. W. Brent Garry¹, Dr. Jacob E. Bleacher², Dr. James R. Zimbelman³, and Dr. Larry S. Crumpler⁴

1. Planetary Science Institute, Tucson, AZ, 85719, USA
2. Planetary Geodynamics Laboratory, Code 698, NASA Goddard Space Flight Center, Greenbelt, MD, 20771, USA
3. Center for Earth and Planetary Studies, Smithsonian Institution, National Air and Space Museum, Washington, DC, 20013, USA
4. New Mexico Museum of Natural History and Science, Albuquerque, NM, 87104, USA

In volcanology, we are traditionally taught about basaltic lava flows advancing as toes of pāhoehoe or as channeled ‘a‘ā flows.  However, under the right emplacement conditions, some basaltic sheet flows will inflate (thicken) from only a few centimeters or meters to almost 20 meters in height.  This process occurs when lateral advancement of the flow is inhibited and liquid lava is injected underneath the solid crust of stalled sections of the flow field, causing the crust to uplift over an expanding liquid core.  The study of inflated lava flows on Earth reveals distinctive morphologic features related to this process including tumuli, inflated sheet lobes (Image 1), squeeze ups, and inflation-rise pits [1,2].  The McCartys lava flow (Image 2) is a 48-km-long, basaltic lava flow in El Malpais National Monument, near Grants, New Mexico that exhibits many of the complex morphologic features related to the process of lava flow inflation.  By studying the morphologic features that are characteristic of inflated lava flows on Earth, we can begin to identify this style of lava flow on other planetary bodies, including the Moon and Mars [3,4,5].

Image 1. Geologist Dr. Jake Bleacher stands on the edge of a 12 meter high inflated sheet lobe in the McCartys lava flow, New Mexico. This inflated lobe continues in the foreground and extends in the distance along the left side of the photograph. Cracks, up to 8 meters deep, have formed along the margin of the lobe as the brittle crust had to accommodate for the inflation. The lower elevation unit seen in the central part of this photograph is formed from breakouts along the margin of this inflated sheet lobe and has a hummocky and swale surface texture. Photograph by W. Brent Garry. Full Size Image.

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Post by Dr. Jim Zimbelman , Center for Earth and Planetary Studies, Smithsonian Institution, Washington DC, USA.  

Image 1. View of pseudocrater at Lake Myvatn, Iceland (JRZ, 8/25/10).

Pseudocraters are distinctive landforms generated when a lava flow moves across ground containing either water or ice; the heat from the lava causes the water or ice to flash to steam, generating localized explosions through the lava.  In the Eifel area of Germany, such explosion craters are often (but not always) later filled with water, and they are called maars.  Important aspects of their formation, which distinguishes them from cinder cones and other monogenetic volcanic vents, is the lack of a source for erupting lava beneath the lava flow or resulting crater; the explosive action is strictly the result of the sudden generation of steam resulting from the heat of the overlying lava flow.  Pseudocraters are typically much broader and shallower than cinder cones, and they may excavate through the entire thickness of the overlying lava flow, ejecting some materials from the rock beneath the flow.  A classic locality for pseudocraters is the Lake Myvatn area of northern Iceland, where a 2000-year-old lava flow moved across wet or icy ground, generating numerous rootless explosion craters that were subsequently surrounded by the shallow waters of Lake Myvatn (Images 1 and 2). (more…)