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Geological Structure

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Geological Structure

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.

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.

Geological structure Lulworth Cove in Dorset StudySmarterLulworth Cove, Dorset, PD-Paste, Wikimedia Commons

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.

Frequently Asked Questions about Geological Structure

The three main categories of geological structure are fractures, folds, and faults.

Structural geology refers to the arrangements of rocks in the Earth’s crust, which are moved via tectonic processes.

Structural geologists are concerned with features resulting from deformation. In a coastal landscape, these include fractures, faults, folds, fissures, and dips

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 understand what stresses the Earth went through in the past. This information is critical in understanding plate tectonics, earthquakes, mountains, metamorphism, and Earth resources.

In a coastal landscape, the two main characteristics of geological structure are concordant and discordant coasts.

Final Geological Structure Quiz

Question

Name three critical elements of geological structure.

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Answer

Strata, deformation, and faulting.

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Question

Strata refer to what?

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Answer

The different layers of rocks within an area and how they relate to each other.


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Question

Add the missing words. Deformation is the degree to which rock units have been deformed, either by _____ or folding, by _____ activity.


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Answer

Deformation is the degree to which rock units have been deformed, either by tilting or folding, by tectonic activity.

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Question

Geological structure types produce two dominant types of coasts. What are they?


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Answer

Concordant and discordant coasts.

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Question

Complete the sentence. Concordant coasts form if the rock layers are running _____ to the coast.


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Answer

Concordant coasts form if the rock layers are running parallel to the coast.

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Question

A concordant coast is also known as what?


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Answer

Pacific coast.

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Question

What is a haff coast?

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Answer

Haff coasts are another type of concordant coast, e.g., the southern fringes of the Baltic Sea. Long sediment ridges topped by sand dunes run parallel to the coast just offshore, creating lagoons (haff) between the ridge and the shore.

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Question

Complete the sentence. Where bands of different rock types run _____, you will see a discordant coastline.


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Answer

Where bands of different rock types run perpendicular to the coast, you will see a discordant coastline.

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Question

What is another name for a discordant coast?


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Answer

Atlantic coast.

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Question

What is a prominent feature of a discordant coastline, and why?


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Answer

Headlands and bays. This is due to the rocks’ differing resistance to erosion. A hard rock type, which is resistant to erosion, will create a point of land that extends out into the sea, a promontory. A softer rock type is easily eroded, creating a bay.

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Question

What does dip mean when referring to a cliff profile?

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Answer

When referring to a cliff profile, dip means the angle of the rock strata in relation to the horizontal. For example, sedimentary rocks are formed in horizontal layers but can be tilted by tectonic forces.

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Question

What are joints? 


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Answer

Fractures in rocks created without displacement. They occur in most rocks, often in regular patterns, dividing rock strata into blocks with a normal shape.

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Question

Explain what faults are.

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Answer

Major fractures in the rock caused by tectonic forces; the rocks on both sides of the fault line will be shifted due to tectonic forces. They represent a significant weakness within the rock layer. 

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