Biota and Topography

Topography: Definition by Substrate Cohesion and Physicochemical Agents

Earth surface topography is characterized by a substantial spatial heterogeneity at diverse spatiotemporal scales. Spatial and temporal distributions of topographic variations are not random. Landforms are dynamic features that result from self-organization processes of matter and energy, commonly revealing geometric and dynamic constants—or attractor domains—representing optimum physical solutions that minimize energy dissipation according to endogenous or exogenetic morphogenetic forces (e.g., tectonics, gravity, water flow, wind). For this reason, specific [Page 271]landforms are generally recognizable and can be distinguished from their surrounding environment—that is, from other landforms. It is possible to classify landforms based on their topography and complementary geomorphic characteristics such as sediment physicochemical properties. In many cases, the identification of the family of processes that are at work or, in the case of inherited landforms, were at the origin of the land-forms and their topography, is also possible. Erosional and depositional landforms on the Earth surface can be perceived as a characteristic signature of a certain combination of interactions between the material (e.g., sediments, rocks, ice) and the physical constraints applied on it (e.g., tectonics, gravity, water flow, wind, frost, chemical alteration). However, topographic signatures vary greatly because of the many possibilities of combinations between the fundamental and basic morphostructures and processes. The high variability of topography on the Earth's surface is also related to the frequency, the magnitude, and the timing of the physical constraints (i.e., the disturbance regime and its historical pathways) that is applied on the material.

The three main components that together define the variety of topographic features need to be distinguished:

• 1.
  The material
• 2.
  The endogenous geothermal force that is at the origin of tectonics and the uplift of landscapes
• 3.
  The exogenetic weathering agents that induce sediment erosion, transportation, and deposition on Earth's surface

The first component—the material—defines landform cohesiveness, that is, the ability of the substrate to resist deformation or desegregation when a mechanical or a chemical constraint is applied. The second component—tectonics—produces the most important variations in topography on the Earth through uplifting and deforming rocks and landforms, generating heterogeneity in topography at the global scale. The third component—sediment erosion, transportation, and deposition—redistributes matter in space and produces characteristic topographic signatures through the dissipation of kinetic energy associated with matter flows. For example, (a) wind produces dunes with a structural organization depending on a delicate balance between wind regime and sediment grain size; (b) water flow produces landforms such as hydrographic networks, rivers, or intertidal channels and alluvial fans (see alluvial bar photo); (c) gravity is at the origin of characteristic topographic signatures such as hill slopes, screes (accumulations of primarily angular clasts that lie at an angle of around 36° beneath exposed free rock faces or cliffs), solifluction lobes (tongue-shaped lobes that are the result of slow gravitational downslope movement of water saturated materials); and (d) frost and the freezing of water produce geometrical forms at the surface such as polygons delimited by stones.

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