Geological structure is an important influence on coastal morphology, erosion rates, and the formation of cliff profiles. There are three important elements to geological structure, and each of these elements influences the coastal landscape and the development of landforms (they can even influence the specific lithology of the coast).
Structural geologists are concerned with features resulting specifically from deformation. In a coastal landscape, these include fractures, faults, folds, fissures, and dips, which we look at in more detail in this explanation.
What is geological structure in geography?
Geological structure refers to the arrangements of rocks in the Earth’s crust. Here are the main "elements" of geological structure:
- Strata (layers, bedding, deposition structures) refer to the different layers of rocks within an area and how they relate to each other.
- Deformation (folds) is the degree to which rock units have been deformed (either by tilting or folding) by tectonic activity.
- Faulting (fractures) refers to the presence of significant fractures that have moved rocks from their original position.
Fig. 1 - example of folding
Because geological structures influence the shape of landscapes, we need to know about them to determine the degree of landslide hazard or mass movement. In addition, they help us to understand what stresses the Earth went through in the past. This information is critical in understanding plate tectonics, earthquakes, mountains, metamorphism, and Earth resources.
What are the types of geological structures?
Let’s dive into some different types of geological structures.
Strata
In a coastal landscape, geological structure types produce two dominant types of coasts: concordant coasts (also known as Pacific coastlines) and discordant coasts (also known as Atlantic coastlines).
Concordant coasts (also known as a Pacific coastline)
A concordant coast forms when the rock layers are running parallel to the coast. The rock types can also be folded into ridges. The outer hard rock (i.e., granite) provides a protective barrier against the erosion of the softer rocks (i.e., clays) further inland. But sometimes, the outer hard rock is punctured, and this allows the sea to erode the softer rocks behind it, creating a cove.
A cove is typically a circle with a relatively narrow entrance from the sea.
Lulworth Cove in Dorset, the coast of Dalmatia, Croatia, and the southern fringes of the Baltic Sea.
Note that the southern fringes of the Baltic Sea are an example of a Haff coast. Haff coasts are long sediment ridges topped by sand dunes that run parallel to the coast. At a haff coast, you can see lagoons (a haff), which are created between the ridge and the shore.
Fig. 2 - Lulworth cove is an example of a concordant coastline
Discordant coast (also known as an Atlantic coastline)
A discordant coast forms when the rock layers run perpendicular to the coast. The different rocks each have differing levels of erosion, and this leads to coastlines dominated by headlands and bays. For example:
- A hard rock type like granite, which is resistant to erosion, creates a point of land that extends out into the sea (known as a promontory).
- A softer rock type like clay, which is easily eroded, creates a bay.
Swanage Bay, England, and West Cork in Ireland.
Deformation and faulting
Different aspects of geological structure influence cliff profiles at coastlines. Some of these aspects include
- where the rock is resistant to erosion,
- the dip of the strata in relation to the coastline, and
- joints (breaks), faults (major fractures), fissures (cracks), and dip.
Sedimentary rocks are formed in horizontal layers but can be tilted by tectonic forces. When dips are exposed on a cliff coastline, they have a dramatic effect on the profile of the cliff.
Joints
Joints are breaks in rocks, which are created without displacement. They occur in most rocks and often in regular patterns. They divide the rock strata into blocks with a formal shape.
- In igneous rocks, joints form when magma contracts as it loses heat (also known as cooling joints).
- In sedimentary rocks, joints form when rock undergoes compression or stretching by tectonic forces or by the weight of the overlying gemstone. When this happens, underlying rock is removed and underlying strata expand and stretch, creating unloading joints parallel to the surface.
Jointing increases erosion rates by creating fissures that marine erosion processes (such as hydraulic action) can exploit.
Check out our explanation on Subaerial Processes for more info on erosion processes at the coast.
Faults
Faults are major fractures in the rock caused by tectonic forces (rocks on both sides of the fault line are shifted by these forces). Faults represent a significant weakness within the rock layer. They are often large scale, extending many kilometres. Faults significantly increase the rate of erosion since zones of faulted rock are much more easily eroded. These weaknesses are often exploited by marine erosion.
Fissures
Fissures are narrow cracks that are a few centimetres long and are weaknesses in the rock.
To summarise: cliff profiles are influenced by their dips, joints, fractures, faults, fissures, and whether the rock is resistant to erosion.
Geological Structure - Key takeaways
- There are three important elements to geological structure: strata, deformation, and faulting.
- The geological structure produces two dominant types of coasts: concordant and discordant.
- A concordant coast is where the layers of differing rock types are folded into ridges that run parallel to the coast.
- Where bands of different rock types run perpendicular to the coast, you will see a discordant coastline.
- Cliff profiles are influenced by whether the rock is resistant to erosion, its dip, joints, fractures, faults, and fissures.
References
- Fig. 1: Folding (https://commons.wikimedia.org/wiki/File:Folding_of_alternate_layers_of_limestone_layers_with_chert_layers.jpg) by Dieter Mueller (dino1948) (https://de.wikipedia.org/wiki/Benutzer:Dino1948) licensed by CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en)
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