Mass Wasting Theme Overview
By Frank Chuang and Alfred McEwen
Explanation of the theme. "Mass Wasting" is a geologic term that encompasses the rapid downhill movement of rocks and fine particles due to the force of gravity. One of the most common and generic types of mass wasting features on Earth are landslides, but there are many others such as rock falls, debris flows, soil creep, and debris avalanches (Fig. 1). Landslides or any other mass wasting feature, require some type of triggering mechanism to induce the movement of particles under gravity. Some of these mechanisms include volume expansion of fractures (i.e. cracks) in rocks by freeze/thaw processes, increase in soil pore pressure (i.e. water content), undermining or removal of less-resistant material below a stronger material layer, and strong vibrational forces produced from above (e.g., meteorite impact) or below ground (e.g., volcanic eruption, earthquake). On Mars, two of the most common mass wasting features are landslides and dust avalanches (also referred to as slope streaks). Some of the most spectacular landslides in the solar system are found in the Valles Marineris canyon system on Mars (Fig. 2) and exhibit many of the classic characteristics of landslides on Earth. These characteristics include a semi-circular main scarp in the source region, a hummocky (i.e. irregular) or blocky surface in the upper portion of the deposit, surface ridges parallel to landslide flow direction in the middle portion of the deposit, and a lobate outer margin that has some significant thickness (e.g., tens to hundreds of meters). Dust avalanches are common on dune faces, crater interior walls, mesa slopes, and canyon scarps. The streaks are thought to occur when dust and/or other small particles on a sloped surface begins to move due to sublimation of a thin layer of water frost or by the oversteepening of slopes in localized dusty air fall deposits.
For more information about landslides on Earth, visit http://nationalatlas.gov/articles/geology/a_landslide.html
Major science questions for this theme.
What are the current and past rates for mass wasting in various terrains on Mars?
Do slope streaks involve water in their triggering and subsequent downslope movement?
What triggers large landslides (e.g., marsquakes, tectonic oversteepening of slopes, fluvial and/or eolian undercutting of slopes, weakening of rock materials from hydrothermal, physical, or chemical weathering)?
Can one type of mass wasting feature be clearly distinguished from another on the basis of boulder frequency and distribution?
Relationship to other science themes. If water is the main focus of the science question, then it would be best to select "fluvial processes" as the main science theme, or perhaps "hydrothermal processes". Viscous relaxation or flow of ice-rich materials is not considered mass wasting because it is much slower than typical mass wasting events and is better suited for "periglacial", "glacial", or "polar geology" themes. Mass wasting also occurs in the later stages of impact crater formation, and these should be considered for the "impact processes" theme. Mass wasting is an important component of landscape evolution, but if a suite of processes affecting landscape morphology is the goal, select the "landscape evolution" theme. Landslide scarps can create excellent exposures of bedrock, but if that is the main focus then use "sedimentary and layering processes", "geologic contacts/stratigraphy", or "volcanology" (if exposing volcanic rocks) as the main themes. Landslides are often closely associated with tectonic activity, but use "tectonic processes" if that is the main study goal. Slope streaks that are monitored to understand seasonal processes should be submitted to the "seasonal processes" theme. Mass wasting would be an appropriate secondary theme in any of the above examples.
Features of interest potentially visible at HiRISE scale. Boulders-- The sizes, shapes, sorting, colors, and distribution of boulders (~0.5 meters or larger in diameter) tell us a great deal about the transport process of mass wasting features. For slope streaks or other small mass wasting features, stereo coverage from HiRISE images may help resolve the topography or morphologies that are diagnostic of these processes. Ridges-- small ridges that can be seen in HiRISE images, but are too small or subtle to be seen in Mars Orbiter Camera (MOC) images, may be an indicator of a change in direction or rate of movement for landslide deposits or other larger mass wasting features. Faults-- small offsets in the deposit layers such as along fractures or faults, may be an indicator compressional flow of materials in parts of the mass wasting feature.
Figure 1. Examples of different types of mass wasting features (from USGS fact sheet 2004-3072).
Figure 2. Martian landslide deposits on the canyon floor of eastern Ganges Chasma.