GEOG 440 Earth Surface Processes

Chris Fuhrmann Late: -1

GEOG 440 Earth Surface Processes

Fall 2006

Matlab Exercise 1

Figure 1 (Kg=0.4, Kh=0.0): Simulation of the evolution of geomorphology after an initial uplift.

Q1: What is the driving force for the geomorphologic evolution? How does the driving force affect geomorphologic evolution?

The simulation in Figure 1 represents a period of rapid uplift (t=0.1-0.2) followed by a period of rapid degradation/erosion of the landform (t=0.2-0.4) and then a period of more gradual degradation/erosion (t=0.5-0.8) until the land surface is converted to gentle uplands (t=0.9-1.0) with wide floodplains and minimal relief. Since major geomorphic transport processes are required to both lift and degrade the landscape this rapidly, it is likely that the driving force for the uplift is tectonic activity (e.g. the building of material through plate interaction) while the driving force for the degradation is likely major gravitational movements (e.g. mass wasting) due to rapid decreases in friction and internal cohesion (t=0.2-0.4) and infiltration from orographically enhanced precipitation. Over time, as the frictional coefficient and soil cohesion increase and the major rivers and hydrological systems mature and become larger, the landscape becomes a broadly sloped floodplain. More generally, the driving force behind these geomorphic processes is the weight of the soil itself, and the potential for rapid degradation following a major uplift period (or event) will be a function of the soil's ability, through its internal structure, to overcome the resisting forces. 3/3 Very good explanation.

(Kg=0.4, Kh=0.0)

(Kg=0.1, Kh=0.0)

Figure 2: Simulation of two geomorphologic structures with different soil properties.

Q2: Describe the effect of the diffusivity coefficient (Kg) on the geomorphologic evolution.

The diffusivity coefficient (or “creep coefficient”; Kg) is a dimensionless term that relates the rate of sediment transport (or the change in elevation) to the slope of the planar surface on which the material lies. Initially, we may assume a 1:1 ratio between the degree of slope and the rate of sediment transport. However, this ratio changes as the soil characteristics change. A higher (lower) rate of sediment transport over a similar slope angle will exhibit an increase (decrease) in Kg. In the first simulation in Figure 2 Kg=0.4, while in the second simulation we have decreased the diffusivity coefficient to 0.3. Also note that the initial rate of uplift was higher in the second simulation, which one may assume would lead to more rapid erosion. The characteristics of the soil in this particular simulation, however, were such that the rate of erosion was reduced markedly. In the first simulation we observe a much higher rate of sediment transport compared to the second simulation; at t=1.0 the landscape in the first simulation is now nearly flat as the entire uplift structure (in this likely a mountain) has essentially eroded. At the same time-step, the landscape in the second simulation has not fully eroded to a flat surface and exhibits a broadly sloping topography. With a smaller Kg, and thus a lower rate of sediment transport, we expect the landscape to degrade at a slower rate.

3/3

Q3: Plot two temporal profiles at x=0.5 Temporal profiles for different uplift sources. 2/3

Temporal Profile 1: Kg=0.3, Kh=0.0

Next time, please put proper labels on graphs.

Simulated Profile 1: Kg=0.3, Kh=0.0

Figure 3: Example of persistent tectonic uplift

Temporal Profile 2: Kg=0.3, Kh=0.0

Simulated Profile 2: Kg=0.3, Kh=0.0

Figure 4: Example of rapid tectonic uplift

Q4: Compare the two geomorphologic structures and explain why they are different.

Simulated profile 1 represents a condition of persistent tectonic lift. Such a condition may be caused by the collision of tectonic plates resulting in the gradual building of a broadly sloped mountain range as tectonic material accumulates from intermittent collisions. In this case the diffusivity coefficient is modest at 0.3, suggesting that the rate of sediment transport relative to the slope of the mountain is not sufficient for rapid erosion. Persistent uplift may also help suppress rapid erosion (actually not suppress but increases soil erosion rate). In the simulated profile 2 a major geologic event has occurred, resulting in the rapid building of a steeply sloped mountain range. In this case the collision of tectonic plates likely occurred at a subduction zone; the increased pressures below the lithosphere and the greater frequency of volcanic activity would explain such a condition. Similar to the simulated profile 1, the diffusivity coefficient is 0.3, although in this case there is rapid erosion/degradation of the landscape. Recall that this coefficient is a function of both the rate of sediment transport and the slope of the landscape. Therefore, since the slope in the second simulated profile is increased dramatically from the first profile, a higher rate of sediment transport would be required to keep the diffusivity coefficient at a modest value (see schematic below). Therefore, we can conclude that 1) a higher slope leads to a higher rate of sediment transport, assuming consistent soil properties, and 2) gradual, persistent uplift will suppress major degradation of the landscape.

3/3

Total: 10/12 = 8.3 Good job.