Course:GEOBrefGuide/Geomorphology/geomorphology introduction

Introduction to Geomorphology

Geomorphology is the science of understanding the processes and sequence of events that have shaped a given landscape. Because the focus is on processes happening at the surface of the Earth, geomorphology is closely related to the fields of hydrology, ecology and climatology, and it is rare that a landscape can be fully understood without considering processes from each of these disciplines. Future advances in geomorphology seem most likely to come from a careful consideration of how the various processes from each of these disciplines interact to control the spatial and temporal distribution and the intensity of land-shaping processes.

Given the long time-scales required for appreciable landscape change to emerge, much of the early work in geomorphology was speculative and descriptive. However, the emergence of advanced data collection technologies have made it possible to collect very precise data over a much larger area than was previously possible. The rapid development of computer power has also made it possible to model landscape processes over a suitably long time-scales.

One of the central concepts that guided early geomorphic thought was the notion that landscapes followed some sort of cycle. This cycle was driven by the initial uplift of a land mass, the development of a high-relief (relief refers to the difference between the highest mountain peak and the lowest valley bottom in a particular areas) topography comprising a series of valleys and mountain ranges, followed by the gradual wearing away of this topography. While there are several conceptual models describing this, the most well known and influential model is referred to as the Davisian Cycle, which identifies three distinct stages of landscape development:

• Youth: in this stage, uplift has occurred and/or is continuing to occur, but the processes of erosion have not yet caught up the the tectonic forces driving the uplift, and the landscape comprises extensive plateau surfaces that have not been significantly modified by erosion, and a series of river systems that are extending headwards into the plateau via a series of deep, steep-sided gullies.}
• Maturity: when the processes of erosion have erased the flat plateau surface, replacing it with a series of high mountain ranges and deep river valley, the landscape has reached maturity. While tectonic uplift may continue to occur, the rate of increase in relief is slowed down by the fact that erosion is occurring at appreciable rates everywhere in the landscape}
• Old-age: once tectonic uplift has halted, erosion will gradually wear down the mountain ranges, producing a series of lower hills with more gentle slopes and broad, low gradient valleys with abundant alluvial deposits. The relief of these landscapes is relatively low (and shrinking over time), and the erosion rates are also relatively low.

This cycle was proposed well before tectonic theory was articulated (see the Wilson Cycle), but is intrinsically linked to the cyclic transition between active tectonic margins and passive tectonic margins that is characteristic of the growth and collapse of major oceans with a periodicity of about 250 Ma.

Modern Geomorphology

Modern geomorphology can be divided into two branches:

• Historical Geomorphology, and
• Process Geomorphology

Both branches are becoming increasingly geophysical in nature, and require at least a basic familarity with mathematics, physics, chemistry, and geology.

Historical Geomorphology

This branch of geomorphology is concerned with reconstructing the history of a landscape using a combination of relative and absolute dating techniques.

• relative dating -- using stratigraphic position to work out the sequence of depositional events
• absolute dating -- using some technique to put an absolute age to a deposit (e.g. carbon-dating, cosmogenic dating)

Since the geomorphic history of a landscape is relatively short (compared with the geologic history), most historical geomorphology is restricted to the last 2 Ma, and is referred to as Quaternary Geomorphology. To do this effectively, it is necessary to first have a solid understanding of the processes that can act upon a landscape. For this reason, Quaternary Geomorphology is taught in the 3rd year (GEOB 308) in the Geographical Biogeosciences program and this course is a required prerequisite.

Process Geomorphology

Process geomorphology is the focus of GEOB 206: it refers to the application of the principles of chemistry and physics (and to some extent, ecology) to the understanding of geomorphic processes. This branch of the science relies heavily on measurements of contemporary geomorphic processes, development of geomorphic theory and various kinds of experimentation. This branch of the science began in earnest about 60 years ago, and is becoming increasingly quantitative in approach:

Reasons for quantifying processes:

• To improve predictions of future hazards (and to aid in managing them)
• To generate precise hypotheses that can be statistically tested
• To more precisely and consisely express our understanding of the system
• To facilitate numerical modeling of the system over time-scales beyond a single human lifetime

The Concept of Equilibrium

Since process geomorphology seeks to relate some causal factor (which we can call Governing Conditions) to some sort of geomorphic response (i.e. a change in process rate or a change in a landform), the concept of equilibrium becomes important. Unfortunately, most geomorphic systems respond in relatively complex ways and often more than one governing condition is changed at a time. The result is a complex, but (when we are lucky) understandable response to some sort of environmental change.

The reasons for this Complex Response include:

• Existence of thresholds (both for process occurrence and landform state)
• Non-linear response (e.g. a butterfly flaps it's wings....)
• Multiple response modes (more than one response is possible, often more than one occurs)
• Self-organizing behaviour (e.g. waves, dunes, meanders)
• Hysteretic relations between variables such that the relation $Y=f(X)$ is no longer unique.

In the end, the relation between any given landform and the conditions to which it is adjusted is rather imperfect. Small changes in the governing conditions often produce disproportionately large responses, while apparently large changes can produce no discernible response at all. Furthermore, the geomorphic history often plays a role in conditioning the response.