Page 461 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 14 Weathering, Karst Landscapes, and Mass Movement 425
▲Figure 14.1 Delicate Arch, Arches National Park, Utah. Resis- tant rock strata at the top of the structure helped preserve the arch beneath as surrounding rock eroded away. Note the person stand- ing at the base in the inset photo for a sense of scale. In the distance are the snow-covered La Sal Mountains, an example of a laccolith (a type of igneous intrusion) exposed by erosion. [Bobbé Christopherson.]
Hoodoos in the badlands of Alberta shown in Figure 14.2 are another striking example of horizontal rock strata that have eroded differentially. The two disc- shaped capstones, which are remnants of an extensive resistant layer of rock, have protected the softer, more easily eroded material directly beneath them. However, once the less resistant material became exposed, weath- ering processes were able to attack from the sides. As weathering continues, eventually there will be insuffi- cient strength in the remaining material to support the weight of the capstones, and these distinctive landforms will collapse.
As stated in earlier chapters, endogenic processes, such as tectonic uplift and volcanic activity, build landforms into initial landscapes, whereas exogenic processes tear landforms down, developing sequential landscapes characterized by lower relief, gradual change, and stability. However, these countering sets of processes happen simultaneously. Scientists have proposed several hypotheses to model denudation processes and to ac- count for the appearance of the landscape.
Dynamic Equilibrium Approach
to Understanding Landforms
A landscape is an open system, with highly variable in- puts of energy and materials. The Sun provides radiant energy that converts into heat energy that drives the hy- drologic cycle and other Earth systems. The hydrologic
cycle imparts kinetic energy through the mechanical motion of moving air and water. Chemical energy is available from the atmosphere and various reactions within the crust. In addition, uplift of the land by tectonic processes creates potential energy of position as land rises above sea level. Remember from Chapter 4 that potential energy is stored energy that has the capacity to do work under the right conditions, such as the pull of gravity down a hillslope.
As landscapes and the forces acting on them change, the surface constantly responds in search of equilibrium. Every change produces compensating actions and reactions. Tectonic uplift creates disequilib- rium, an imbalance, between relief and the energy required to maintain stability. The idea of landscape for- mation as a balancing act between
uplift and reduction by weathering and erosion is the dynamic equilibrium model. Landscapes in a dynamic equilibrium show ongoing adaptations to the ever-chang- ing conditions of local relief, rock structure, and climate.
Endogenic events, such as faulting or a volcanic eruption, or exogenic events, such as a heavy rainfall or a forest fire, may change the relationships between land- scape elements and within landscape systems. During or following a destabilizing event, a landform system some- times arrives at a geomorphic threshold, or tipping point, where the system lurches to a new operational level. This threshold is reached when a geomorphic system moves from the slow accumulation of small adjustments (as
▲Figure 14.2 Hoodoos in Dinosaur Provincial Park, Alberta. These landforms are products of denudation processes including weathering of rock strata at differential rates. Contemplate the empty space surrounding the hoodoos—space formerly occupied by material that has been removed by denudation. [Wayne Lynch/All Canada Photos/Getty Images.]
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