Exploring Geomorphic Thresholds
The concept of thresholds is not new to the scientific world and can be found in fields like environmental science, ecology, and geomorphology. It is also employed within the social sciences in a range of settings. Thus, it is important to understand the context in which the term is employed because its application is as varied as the fields that use it. Insert some comment about how the term is applied broadly to the field.
To understand the complexity of geomorphic thresholds you first need to understand what geomorphology actually is. In Bierman and Montgomery’s Key Concepts in Geomorphology, it is defined as “the study of the processes shaping the Earth’s surface and the landforms and deposits they produce” (p. 5). It is an inherently interdisciplinary field which draws upon methods of inquiry from a range of subjects. Geomorphologists, like any other scientists, might be interested in specific parts of landscapes or landforms and so the types of thresholds they consider will be influenced by that interest. Since geomorphology itself is such a varied field it then follows that geomorphic thresholds will have varied meanings.
Despite the variance of geomorphic thresholds, they do all share some core ideas. There are three core ideas that geomorphic thresholds generally adhere to: first, they are conceived as “points” where a certain process/event can or does occur; second, they are the “point” where rates of change become exponential or extreme compared to how they had been before; and third, they are used to describe certain aspects of processes or events and how they affect landscapes/landforms. Any geomorphic threshold is further characterized as being intrinsic or extrinsic. Geomorphologists “distinguish extrinsic thresholds that occur under the response of an external variable from intrinsic thresholds that occur without a change in an external variable” (Wohl, 2013). Implicit within these core concepts is the expectation that these changes can be observed within a reasonable, often very short, temporal scale (Muhs, 2013).
It should be noted that the concept of thresholds, as I have defined above, is adaptable and geomorphologists recognize different thresholds for different spatiotemporal scales and contexts. Adaptability is important for interdisciplinary studies because it leaves room for different lines of inquiry and applications. Insert a line to transition to examples of the varied application. Baynes et al. (2015) argue the “importance of landscape evolution, with significant landscape change occurring during extreme floods when a flow depth threshold has been surpassed” (p. 2359). They did this by exploring a particular geomorphic threshold related to bedrock erosional processes by looking at the morphology within Jökulsárgljúfur canyon. Jökulsárgljúfur canyon has large knickpoints that could not be caused by abrasion. The bedrock of the canyon is basaltic which requires significant flow depth and discharge to topple. Thus, the geomorphic threshold Baynes et al. were concerned with was the flow depth and discharge for basalt column toppling. It conforms to the first and third core idea of geomorphic thresholds and would be characterized as extrinsic because the threshold relies upon variables outside of the canyon, in this case, extreme flooding. A threshold might be in relation to a landform, like in the case of Baynes et al., or it might be in relation to specific geomorphic processes. For example, soil systems are very complicated, but many studies have been done on clay movement in calcareous parent material. The process requires a certain amount of leaching of carbonates to occur, which is an intrinsic threshold because it doesn’t require changes outside of the soil itself to occur. That being said, precipitation may speed up this process and the amount of precipitation it takes to do so would be an extrinsic threshold (Muhs 2013).