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Synthesizes concepts to understand how landscapes change over geological time scales.
If you could fast-forward time by 10 million years, would the mountains outside your window be taller, shorter, or completely gone? The answer lies in the invisible battle between tectonic uplift and the relentless power of water.
In the late 19th century, William Morris Davis proposed the Geographical Cycle, suggesting landscapes progress through stages like a living organism. He argued that after a rapid pulse of tectonic uplift, the land undergoes a predictable decay. The stages include Youth (steep slopes, V-shaped valleys, high energy), Maturity (rounded divides, integrated drainage), and Old Age. The ultimate end-point is the Peneplain, a nearly featureless, flat plain situated near sea level. This model is time-dependent, meaning the landscape's appearance is a direct function of how long erosion has been acting upon it since the last uplift event.
Quick Check
According to Davis, what is the primary factor that determines the shape of a landform?
Answer
The amount of time that has passed since the initial tectonic uplift.
Imagine a mountain range where the tectonic plates stop moving. 1. Davisian View: The mountains will slowly wear down over millions of years until they become a flat peneplain. 2. Dynamic Equilibrium View: As soon as uplift stops, the 'balance' is broken. The landscape will erode until the energy of the system reaches zero, but the focus is on the ratio of forces rather than a pre-destined life cycle.
Quick Check
In a steady-state landscape, if the rate of uplift is 2mm/year, what must the rate of erosion be?
Answer
2mm/year
Suppose a river basin has a slope and a drainage area . If a tectonic shift doubles the slope to : 1. Identify the variables: increases by factor of 2. 2. Apply the law: If , the erosion rate also doubles. 3. Result: The river will cut down into the valley twice as fast to try and return the system to equilibrium.
Climate change acts as a massive 'knob' turning the variables in our landscape models. Increased precipitation increases the drainage area variable () in the Stream Power Law, leading to higher erosion rates. However, it also increases vegetation, which can increase the resistance factor (). When change is rapid, landscapes may hit a geomorphic threshold—a tipping point where the entire drainage pattern reorganizes. For example, increased storm intensity can cause 'channel head migration,' where streams eat backward into hillsides faster than the soil can stabilize, permanently altering the topography.
Consider a semi-arid basin facing a 30% increase in annual rainfall due to climate change. 1. Initial Impact: Increased runoff () leads to higher stream power (). 2. Response: Rivers incise deeper, creating steeper hillslopes. 3. Threshold: The steeper slopes exceed the 'angle of repose' for the local soil, triggering widespread landslides. 4. New State: The landscape shifts from a water-dominated erosion regime to a landslide-dominated regime until a new steady state is reached.
Which term describes Davis's final, flat stage of landscape evolution?
In the Stream Power Law , what does the variable 'S' represent?
Dynamic Equilibrium theory suggests that a landscape's form is independent of how much time has passed since uplift.
Review Tomorrow
In 24 hours, try to sketch the difference between a 'Youthful' valley and a 'Steady State' valley, and write down the Stream Power Law from memory.
Practice Activity
Find a topographic map of a local mountainous area. Identify whether the valleys are V-shaped (Youthful/High Power) or wide and flat (Old Age/Low Power) and hypothesize which model best fits that region.