What is Horst and Graben?
Horst Meaning- A horst is a raised fault block bounded by natural faults in physical geography and geology. A horst is a raised block of the Earth's crust that has lifted or stayed stationary while the land around it (graben) has sunk.
Horst and Graben are found together in an extensional environment. The graben is the block that has been downthrown, while the horst is the block that has been upthrown next to the graben.
What is the Meaning of Horst?
Horst meaning: Horst is a Dutch and German word that means heap and is similar to the English word "Hurst."
What is the Meaning of Graben?
A graben is a fault block that has been lowered relative to the blocks on each side without significant interference or tilting. Bordering faults, or fault zones, are usually of near-parallel strike and steeply dipping, with nearly equal vertical displacement. The relative movement of the blocks that define a graben Both of the blocks may have rotated according to their initial positions, but the middle block may have subsided further than the outer two. As a result, a true graben in its original surface shape is a linear structural depression. The scale of grabens varies widely, but the superiority of a long fault trough is a distinguishing feature.
Geomorphology of Horst
The cross-sections of horsts may be symmetrical or asymmetrical. The horst is likely to be symmetrical and approximately flat on top of the usual faults on either side have identical geometry and are moving at the same rate. The top of the horst would most likely be inclined and the entire profile would be asymmetrical if the faults on each side have different rates of vertical motion. In cross-section, erosion has a big influence on how symmetrical a horst looks.
[Image will be Uploaded Soon]
Horst and Graben Formation
Horst and graben are formed when opposite-dip natural faults with parallel strike lines occur in pairs. They are inextricably linked. Horsts and grabens may be as small as a few centimetres wide or as large as tens of kilometres wide, with vertical movement of several thousand feet. They're bordered on both sides by steeply dipping regular faults, which have virtually equivalent movement, resulting in blocks that are barely tilted. The faults that form horsts tend to dip away from each other, while those that form grabens tend to dip against each other. Two or more horsts and grabens can be found close together.
They're thought to be caused by lateral stress, which could be caused by regional uplift or salt dome formation; they're most common on dome crests or anticlines.
Hydrocarbon Exploration and Horsts
The vast majority of discovered hydrocarbons in many rift basins around the world are located in traditional traps associated with horsts. Most of the petroleum discovered in Libya's Sirte Basin (tens of billions of barrels of reserves) is located on large horst blocks like the Zelten Platform and the Dahra Platform, as well as smaller horsts like the Giallo High and the Bu-Attifel Ridge.
Examples of Horst and Graben Formation
Wallonia's Condroz and Ardennes regions are strong examples of horst and graben succession.
The Satpura Range is a horst in India, flanked to the north by the Narmada Graben and to the south by the much smaller but parallel Tapi Graben.
The Vosges Mountains in France and the Black Forest in Germany are examples of horsts, as are the Table, Jura, Dole Mountains, and the Rila - Rhodope Massif, which includes the well-defined horsts of Belasitsa (linear horst), Rila mountain (vaulted domed shaped horst), and Pirin mountain (a horst forming a large anticline situated between the complex graben valleys of Struma and that of Mesta.
FAQs on Horst and Graben
1. What are horst and graben in geography?
In simple terms, a horst is an uplifted block of land that lies between two parallel faults. In contrast, a graben is a block of land that has dropped down between two parallel faults, creating a valley or depression.
2. What is the main difference between a horst and a graben?
The key difference is their vertical movement and the resulting landform:
- A horst is the raised block, which often forms a feature like a block mountain or a plateau.
- A graben is the sunken block, which forms a valley or trough, sometimes referred to as a rift valley.
3. What kind of geological forces create horst and graben landforms?
These landforms are primarily created by tensional forces. These forces pull the Earth's crust apart, causing it to stretch and fracture. This process creates cracks known as normal faults, along which blocks of land either move up to form horsts or drop down to form grabens.
4. Can you provide some real-world examples of horst and graben topography?
Certainly. Classic examples can be found worldwide:
- Horst Examples: The Vosges mountains in France and the Black Forest mountains in Germany are well-known horsts.
- Graben Examples: The Rhine Valley in Europe, which separates the Vosges and Black Forest, is a famous graben. Death Valley in the USA is another excellent example.
5. What is another name for a mountain formed by a horst?
A mountain that is formed by the upward movement of a horst is called a block mountain. The horst itself constitutes the fundamental structure of this type of mountain.
6. How is a graben related to a rift valley?
A rift valley is essentially a very large-scale graben. While technically the same type of feature, the term 'rift valley' is usually reserved for major geological structures where continental plates are actively pulling apart, such as the East African Rift Valley. A graben can be a much smaller, more localised depression.
7. Why don't horsts and grabens form everywhere on Earth?
These features only form in specific geological environments. They require tensional stress, meaning the crust must be stretched. This happens at divergent plate boundaries, in areas of continental rifting, or in places where the crust is bending. They do not typically form where tectonic plates are colliding, as those areas are dominated by compressional forces that create fold mountains.
8. How can you tell the difference between the upward and downward blocks in a fault system?
You can identify them by their relative movement. The block that is pushed upwards between two faults is the horst. The block that sinks downwards is the graben. Geologists use terms like 'footwall' and 'hanging wall' to describe the blocks relative to the fault plane, which helps them analyse these complex movements.
