Earth and Planetary Surface Processes

Self-organization of landscapes

Landscapes on Earth and other planets display many forms of self-organization. We use a variety of techniques to study how patterns and characteristic scales in topography emerge from the processes that shape a planet's surface.

 

Characteristic scales in drainage networks

Drainage networks are among the most widespread and recognizable signatures of erosion on Earth. In addition to serving as conduits for water and rock mass across the continents, drainage networks develop characteristic scales and structures that reflect the main factors that shape landscapes, including rock type, climate, tectonic deformation, and life. Our recent work has focused on the origin and interpretation of features such as evenly spaced valleys and the characteristic branching structure of tributary networks.

 

Formation and adjustment of wave ripples

Bedforms created by wave-generated oscillatory flows are a widespread and visually striking signature of the interaction of complex flows, sediment transport, and bed topography. Postdoc Justin Kao, graduate student Kim Huppert, and undergraduate Abby Koss are comparing field-scale experiments in a laboratory wave tank with numerical experiments to understand how defects in rippled beds accommodate adjustment of bedforms to changes in the driving flow. This is an ongoing collaboration with Paul Myrow (Colorado College) and John Southard (MIT).

 

Nonlinearity and uncertainty in landscape evolution

As opportunities to compare landscape evolution models with high-resolution topography become more abundant, it is important to assess the robustness of model outcomes. One recent effort culminated in an analysis of the legacy of initial conditions in landscape evolution (with Sergio Fagherazzi at BU). We are also developing improved numerical methods for modeling nonlinear geomorphic processes.

 

Topographic stresses and rock fracture

Graduate student Mirna Slim is collaborating with Steve Martel (U. Hawai'i) and Kamini Singha (Penn State), to test whether stresses generated by surface topography influence patterns of bedrock fracture.

Climate and landscape evolution

Despite the obvious importance of climate in shaping Earth's surface, a quantitative understanding of how climatic variables govern the long-term development of landscapes has been elusive. We are studying several natural experiments in which landscapes have evolved under varying climatic conditions while other factors have remained relatively constant.

 

Influence of rainfall gradients on erosion

Postdoc Ken Ferrier and graduate student Kim Huppert are exploring how steep rainfall gradients on ocean islands alter erosional processes and landscape evolution.

 

Microclimates and topographic asymmetry

Graduate student Paul Richardson is studying how microclimates create asymmetric hillslopes that lean away from the sun.

 

Evolution of Pleistocene coral reefs

Graduate student Michael Toomey, in collaboration with Andrew Ashton (WHOI), has developed a simple model that illustrates how Pleistocene sea level variations have created a wide variety of reef forms.

Planetary surface processes

One of the most exciting developments in geomorphology is the acquisition of imagery and topography for planetary surfaces. We use spacecraft observations, experiments, and simple models based on terrestrial theory to constrain the rates and histories of processes that shape planetary landscapes, especially processes driven by water or other liquids.

 

Fluvial processes on Titan

Titan, Saturn's largest moon, may be the only other solar system body with active rivers on its surface. In a recent analysis of the morphology of valley networks near the Huygens probe landing site, we found evidence that the valleys were incised by surface runoff, and calculated that the minimum methane rainfall rates required to form these features was similar to storms on Earth. Currently, Graduate students Ben Black and Yodit Tewelde, in collaboration with Devon Burr (U. Tennessee), are mapping drainage networks on Titan and using landscape evolution models to constrain the extent to which fluvial erosion has shaped the moon's surface.

 

Polar paleoclimate records on Mars

Layered deposits of ice and dust in Mars' polar caps are perhaps the most compelling climate records on the planet. Through a recent collaboration with Peter Huybers (Harvard), we discovered that the finest-scale beds in the polar layered deposits are often periodic, with a characteristic thickness of 1 to 2 meters. Graduate student Mike Sori, in collaboration with Huybers and Oded Aharonson (Caltech), is now exploring how different formation mechanisms might record changes in solar radiation due to long-term variations in Mars' orbit, and whether we can identify these changes by analysing images of the polar stratigraphy.

 

Ancient oceans on Mars

Several lines of evidence suggest that oceans might once have filled the northern lowlands of Mars, but topographic profiles along the margins of the lowlands do not follow surfaces of equal gravitational potential (i.e., sea level), as the shorelines of a standing body of water should. In a recent collaboration with Jerry Mitrovica (Harvard), Michael Manga and Mark Richards (Berkeley) and Isamu Matsuyama (U. Arizona), we showed that these long-wavelength topographic trends can be explained by deformation that occurred in response to true polar wander (TPW), a reorientation of the planet with respect to its rotation axis.

 

Experimental constraints on Mars megafloods

Mars has experienced huge floods that may have been the largest ever to occur in the solar system, but poorly constrained flow depths and velocities make it difficult to estimate the discharge of these ancient flows. Graduate student Hendrik Lenferink is conducting laboratory experiments to constrain the discharge of the floods based on the paths they followed as they emptied into the northern lowlands.