Geological Structure

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.

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.

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.

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.