VOLCANIC SHATTER RINGS: A TYPE OF RAISED RIM DEPRESSION THAT FORMS OVER LAVA TUBES

Shatter rings are raised rim depressions that form over active lava tube on Earth, and may be preserved on the volcanic plains of Mars. This has important implications for planetary exploration. These inflationary features have been linked to fluctuations in discharge. If the flux of lava through the tube exceeds the tube’s cross-sectional area, lava will either break out from the tube and onto the surface, or the pressure within the tube will act to drive the roof upwards. Roof upheaval shatters the overlying crust, and repeated episodes can create a raised rim of shattered material tens of meters across, composed of blocks from the broken lava surface, that surrounds the mobile section of tube roof. Short-lived lava flows extrude from points around the exterior base of the shattered rim, forming a characteristic lava-matrix breccia and burying the surrounding lava surface. When discharge drops, or the tube is abandoned, the mobile section of tube roof drops, forming a depression often lower than the surface outside the shatter ring. Shatter ring dimensions are therefore controlled by the diameter of the tube over which they form. Shatter rings imply the presence of lava tubes and suggest that the eruption rate fluctuated. With their bowl-like depression, raised rim and surrounding radial flows, shatter rings share common features with impact craters. The volcano Olympus Mons on Mars displays raised rim depressions in association with ridges that likely are lava tubes. On Mars, lava tubes are suggested to exceed 100 m across. Impact craters of several hundred meter diameters are easily discernible in high resolution data from Mars and the Moon. Although it might seem unlikely that shatter rings would be confused with impact craters, erosion modification by aeolian, fluvial or space weathering processes might make difficult their distinction in remote sensing data. Therefore it is important to be aware of the existence and formation mechanisms for shatter rings to ensure that such features are not inadvertently included in counts of impact craters.