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.
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