Base level
In geology and geomorphology a base level is the lower limit for the vertical position of an erosion process.[1] The modern term was introduced by John Wesley Powell in 1875.[1] The term was subsequently appropriated by William Morris Davis who used it in his cycle of erosion theory.[1][2] The "ultimate base level" is the surface that results from horizontal projection of the sea level under landmasses (the geoid).[1] It is to this base level that topography tends to approach due to erosion, eventually forming a peneplain close to the end of a cycle of erosion.[3][4][5][6]
There are also lesser structural base levels where erosion is delayed by resistant rocks.[1] Examples of this include karst regions underlain by insoluble rock.[7] Base levels may be local when large landmasses are far from the sea or disconnected from it, as in the case of endorheic basins.[1] An example of this is the Messinian salinity crisis, in which the Mediterranean Sea dried up making the base level drop more than 1000 m below sea level.[8][9]
The height of a base level also influences the position of deltas and river terraces.[1] Together with river discharge and sediment flux the position of the base level influences the gradient, width and bed conditions in rivers.[10] A relative drop in base level can trigger re-adjustments in river profiles including knickpoint migration and abandonment of terraces leaving them "hanging".[11] Base level fall is also known to result in progradation of deltas and river sediment at lakes or sea.[12] If the base level falls below the continental shelf, rivers may form a plain of braided rivers until headward erosion penetrates enough inland from the shelfbreak.[12]
When base levels are stable or rising rivers may aggrade.[12] Rising base levels may also drown the lower courses of rivers creating rias. This happened in the Nile during the Zanclean flood when its lower course became, in a relatively short time, a large estuary extending up to 900 km inland from the Mediterranean coast.[9]
Base level change may be related to the following factors:
- Sea level change[1]
- Tectonic movement[1]
- River capture[1]
- Extensive sedimentation[13]
References
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- ^ Phillips, Jonathan D. (2002), "Erosion, isostatic response, and the missing peneplains", Geomorphology, Vol. 45, No. 3-4. Elsevier Archived 2010-01-24 at the Wayback Machine, 15 June 2002, pp. 225-241. Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value)..
- ^ Chorley, R.J. (1973). The History and Study of Landforms or The Development of Geomorphology. Vol. Two: The Life and Work of William Morris Davis, Methuen.
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