Development of topographic asymmetry: Insights from dated cinder cones in the western United States

Abstract

Topographic asymmetry, that is, differences in the morphology of landscapes as a function of slope aspect, can be used to infer ecohydrogeomorphic feedback relationships. In this study, we document the dependence of topographic gradients and drainage densities on slope aspect and time/age in four Quaternary cinder cone fields in Arizona, Oregon, and California. Cinder cones are particularly useful as natural experiments in geomorphic evolution because they begin their evolution at a known time in the past (many have been radiometrically dated) and because they often have simple, well‐constrained initial morphologies. North‐facing portions of cinder cones have steeper topographic gradients and higher mean vegetation cover (i.e., Normalized Difference Vegetation Index, or NDVI, values) under current climatic conditions compared with corresponding south‐facing portions of cones within each volcanic field. Drainage density is also higher on north‐facing portions of cones in three of the four volcanic fields. These differences in topography were not present initially but developed progressively over time, indicating that the asymmetry is a result of post‐eruption geomorphic processes. To test alternative hypotheses for the slope‐aspect control of topography, we developed a numerical model for cinder cone evolution and a methodology for estimating local paleovegetation cover as a function of elevation, slope aspect, and time within the Quaternary. The numerical model results demonstrate that rates of colluvial transport were higher on south‐facing hillslopes in at least three of the four cinder cones fields. Our paleovegetation analysis suggests that in the two Arizona volcanic fields we studied, higher rates of colluvial transport on south‐facing hillslopes were the result of greater time‐averaged vegetation cover and hence higher rates of sediment transport by floral bioturbation. Our results illustrate the profound impact that relatively small variations in solar insolation can have on landscapes via feedbacks among hydrology, vegetation cover, and sediment transport.

The Influence of Lithology on the Forms of Hillslopes and Drainage Basins in Soil Mantled Landscapes

“…While the configuration of drainage networks displays non-random components in a variety of landscapes surveyed here, it is only in sites in weak rocks where a strong north-south asymmetry in the density and length of low-order tributaries is observed. Past studies have focused on the role of contrasts in vegetation with aspect in driving asymmetry in channel networks (Istanbulluoglu et al, 2008;McGuire et al, 2014;Yetemen et al, 2015) and these certainly drive differences in landscape evolution. However, using remotely-sensed proxies for vegetation productivity, I demonstrate that all the surveyed sites are formed in climates that favor contrasts in vegetation with aspect.…”

“…Aspect-dependent differences in landscape morphology are not uncommon, and barring tectonic or geologic control are typically attributed to variations in the relative intensity of different geomorphic processes across the microclimatic gradients that arise on opposing slopes due to variations in incoming solar radiation (Bass, 1929;Istanbulluoglu et al, 2008;McGuire et al, 2014;West et al, 2014;Yetemen et al, 2015). Early work was divided on the cause of topographic asymmetry in the Gabilan…”

“…The production of asymmetric topography as a result of aspect-controlled microclimates might be viewed as the result of one of two general mechanisms. In one model, referred to here as locally driven asymmetry, differences in the relative intensity of different erosion and transport processes operating at the hillslope scale produce the majority of the asymmetry seen in landscape morphology (Bass, 1929;Istanbulluoglu et al, 2008;McGuire et al, 2014;West et al, 2014;Yetemen et al, 2015). Alternatively, these initial contrasts in erosion may yield aspect-dependent discrepancies in the flux of sediment to trunk channels, favoring lateral migration of trunk channels in a particular direction and greatly exacerbating asymmetry (Melton, 1960).…”

“…Climatic differences that arise due to aspect-dependent insolation can influence geomorphic process and, in turn, landscape forms (Bass, 1929;Emery, 1947;Melton, 1960;Dohrenwend, 1978;Burnett et al, 2008;Istanbulluoglu et al, 2008;West et al, 2014;McGuire et al, 2014). This 'micro-climatic' asymmetry is most commonly manifest in the northern hemisphere as north-facing slopes that are steeper than their south facing counterparts (Poulos et al, 2012).…”

“…Opposing aspects also commonly display differences in the degree of drainage dissection (Bass, 1929;Dohrenwend, 1978;Istanbulluoglu et al, 2008;McGuire et al, 2014), which likely measures the voracity of hillslope transport relative to channel incision (Perron et al, 2009). This may directly reflect aspect-dependent variations in surface hydrology or indirectly result from the differences in vegetation that arise from this hydrologic configuration (Bass, 1929;Emery, 1947;Melton, 1960;Dohrenwend, 1978;Istanbulluoglu et al, 2008;McGuire et al, 2014). Models that drive landscape evolution with processes whose intensity varies with aspect reproduce both local and continental measurements of topographic asymmetry .…”