Folding and Faulting

Folding and Faulting

Key Words:

Term	Definition
Fold	A bend or curve in the rocks of the Earth's crust
Compression	When two continental plates collide, land is forced or folded upwards through the process of Compression
Orogeny	Refers to mountain building processes. This occurs due to tectonic movements that causes plates to buckle upwards, forming fold mountains.
Fault	A crack or fracture in the Earth's crust

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Geography

Folding and Faulting

Folding

• Anticline = Upfold
• Syncline = Downfold
• Limbs = Sides

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Folding and Faulting

Figure 1: Types of Folding

Periods of Folding:

Caledonian Orogeny

• 400 Million Years ago

• Examples include Wicklow Mountains, Highlands of Scotland and the Appalachian Mountains in North America.

• Took place when the North American and Eurasian plates collided.

• Folding has a South West-North East trend therefore mountains were created in North West and Leinster region.

• They are the oldest mountains in Ireland.

• The rock strata (layers) were folded to such extremes that cracks/fault lines appeared in the rocks.

• This erosion has also exposed some of the Igneous and Metamorphic rocks that lie well below the surface.

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Folding and Faulting

Geography

• These mountain ranges were once as high as the Alps or Himalayas but are much older. Therefore, their height has been drastically reduced by the effects of weathering and erosion (denudation).

Armorican Orogeny

• 250 Million Years Ago

• Examples include the Paris Basin in France and the Munster Ridge and Valley (Case study)

• This fold mountain period occurred when the African and Eurasian plates collided.

• Horizontal strata of shale, sandstone and limestone were buckled and folded into anticlines and synclines.

• This resulted in a 'ridge and valley' landscape that runs from Kerry through Cork and into West Waterford.

• Over millions of years the overlying Limestone and chalk have been removed from the ridges by denudation, revealing old red sandstone.

• The valley floors are still covered in Sedimentary rock such as limestone and shale.

Munster Ridge

• Created when the African and Eurasian continental plates collided, causing the land to buckle upwards

• 380 MYA Ireland had a desert climate due to its location near the equator – Old Red Sandstone developed here

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Folding and Faulting

• The area consists of Sedimentary rock laid down in strata

• 350 MYA, Ireland laid below a warm, shallow sea which allowed Limestone to form followed by other rocks such as shale and chalk

• The compression caused by the colliding plates forced the rock to bend into anticlines and synclines which included mountains such as the MacGillycuddy Reeks, Comeraghs and Galtees

• Due to weathering and erosion, over time the outer layers of sedimentary rock were removed leaving the more resistant Old Red Sandstone on anticlines

Results:

• Trellised drainage pattern as the rivers navigate through the synclines e.g. Blackwater, Bandon

Figure 2: Drainage pattern in the Munster Ridge and Valley

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Geography

Folding and Faulting

Faulting

• A Fault is a moving fracture or break in the Earth's surface created by pressure and tension exerted on layers of rock.

• Faults usually occur at places of weakness in the crust.

• There are three main types of faults:

  • Normal fault
  • Reverse fault
  • Tear fault

Normal Fault

Figure 3: Normal Fault and the features created

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Geography

Folding and Faulting

• Occurs when one side of the fault slips down (vertical movement) in relation to the other.
• It is caused by tension from plates pulling apart (divergent plate boundaries).
• It creates a steep, exposed slope called a scarp eg. Ben Bulben.

Figure 4: Ben Bulben

• If two fault lines run parallel to each other and there is a vertical movement of the piece of land in the middle.
• This land slips down creating a rift valley or graben.
• If the land between the fault lines is compressed upwards, a block mountain or horst is formed eg, Ox Mountains in Sligo.

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Folding and Faulting

Reverse Fault

• Example: Pollatomish, Co. Mayo.
• This occurs when the land is compressed/pushed together at a convergent plate boundary.
• The land is pushed upwards - it is the opposite of what happens at a normal fault. This type of fault can also create block mountains.
• A thrust fault is a type of reverse fault that occurs in the same way but is not as prominent.
• Eg. A thrust fault runs from Killarney to Mallow.

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Folding and Faulting

Tear Fault

• Example: San Andreas Fault in California
• Also known as Transform faults
• Occur when there is a vertical fault in the Earth's crust and one plate slides past the other
• Movement is horizontal
• Leannan Fault in Co. Donegal is also a Tear Fault but has significantly less seismic activity than the San Andreas Fault

Case Study: African Rift Valley

• Stretches from the Red Sea to norther Mozambique in Eastern Africa – 4,800km long, formed 20 MYA
• Huge parallel fault lines exist due to hot spots that have weakened the Earth's crust, creating a rift valley that includes Mount Kilimanjaro

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Folding and Faulting

Geography

Causes:

• Rock strata are weakened by plumes of magma entering the crust and pulling the rock apart, creating fault lines

• The land has subsided to form a large rift valley – steep sides, flat floor.

• The floor can be as much as 153m below sea level

• It is between 40km and 100km wide, and this is increasing by 4m per year

• Eventually this part of Africa will completely break away from the African continent and form a new divergent plate boundary

Figure 7: African rift Valley

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Folding and Faulting

Doming:

• Found in sedimentary rocks as they are easier to deform – looks like an upside-down bowl

• Occurs when large volumes of molten magma rise towards the service without causing the strata of rocks to crack

• Earth's surface bulges upwards as a result

• Examples include Slieve Bloom mountains, Co. Laois/Offaly and Mourne Mountains, Co. Down