CHAPTER 5 Folding and Faulting

Folding and Parts of a Fold Mountain Fold mountains are areas of rock that have been pushed high above the surrounding landscape. There are three parts of a fold mountain: the anticline, syncline and limb. The anticline is the rock which has been folded upwards. The rock in between the anticlines, which has been folded downwards, is known as a syncline. The slopes that separate the anticlines and synclines are known as limbs.

Types of Folds There are three main types of folds: symmetrical, asymmetrical and overturned. The type depends on the amount of pressure exerted from each of the plates that are colliding. Symmetrical are evenly shaped (i.e. both sides match) Asymmetrical are lopsided (i.e. one side has pushed over more than the other) An overturned fold is created when one plate has folded over on itself

Location of Fold Mountains Fold mountains are evidence that plate tectonics are or were active in that region at some point in the past. Folding occurs at two different types of convergent plate boundaries: continental-oceanic and continental-continental.

Location of Fold Mountains The convergence of continental-oceanic plates creates fold mountains along the coastlines of the continental plate. When a continental plate and an oceanic plate collide, the heavier oceanic plate subducts into the mantle before melting. As the oceanic plate subducts underneath the continental plate, pressure causes the edges of the continental plate to buckle upwards. This creates a series of fold mountains that run along the full edge of the plate boundary. The Andes Mountains in South America have been created in this way as the oceanic Nazca Plate is subducting underneath the South American Plate

Location of Fold Mountains The convergence of continental-continental plates also create series of fold mountain ranges. When two continental plates collide, neither plate subducts as they are much lighter than oceanic plates. As convection currents push the plates together, the increased pressure eventually causes layers of sedimentary rock to buckle upwards, creating anticlines and synclines. The Macgillycuddy’s Reeks in Co. Kerry were formed by the collision of the African and Eurasian Plates.

Periods of Folding There have been three main orogenies in recent geological history. These are: • The Caledonian folding period (450–400 million years ago) • The Armorican folding period (270–250 million years ago) • The Alpine folding period (60 million years ago to present)

Monoclines Monoclines are mountains and other upland structures formed from smaller earth movements. They form when sedimentary rock is uplifted slightly and faulted due to compression. The rocks uplifted in this way generally tilt slightly in one direction. This uplifting of land can lead to the formation of plateaus such as Ben Bulben in Co. Sligo. These structures are broken down by weathering, erosion and mass movement. As some rocks are more resistant to weathering and erosion than others, a stepped landscape can develop.

Doming A dome refers to a round, or an oval, anticline structure similar in shape to that of an upturned bowl. It has a high peak in the centre and slopes down to its base. Doming is commonly cause by pressure from rising magma underneath the crust. As the magma rises upwards, pressure causes the overlying sedimentary rock to bulge upwards. Weathering and erosion break down the overlying material, exposing older rock layers in the centre of the dome. Slieve Bloom in Co. Laois/Co. Offaly is an example of a dome.

Faulting and its process When rocks are put under too much strain, they fracture and break, forming faults. Faulting is caused by compression, tension or shearing of rock.

Faulting and its process Compression occurs when rock is pushed together as pressure is exerted from both sides. When this occurs, the main movement of the crust is upwards. Compression is the key process involved in the formation of a reverse fault. Tension occurs when rocks are pulled apart as they are strained from both sides. Generally, this causes the crust to sink downwards. Tension is the key process involved in the formation of a normal fault. Shearing occurs when sections of rock are moved laterally in opposite directions. This causes the crust to tear apart. Shearing is the process associated with the formation of a transform fault.

Parts of a Fault A scarp, or escarpment, is the cliff formed by the vertical displacement of rock. The throw is the amount of vertical displacement of the rock. The heave is the horizontal displacement of the rock. The section of crust above the fault is called the hanging wall and the section of rock that is under the fault is the footwall. The hanging wall slides along the footwall.

Types of Fault There are three types of fault that occur, each formed by a different type of movement: Normal fault Reverse fault Transform fault.

Types of Fault (Normal Fault) Normal faults are created by the stretching of crust. The strong tension caused by this stretching is causes rock to pull apart. As the crust pulls apart, tension causes the crust to get. The stress causes the rock to fracture and pull apart. When the rocks fracture, one side of the fault sinks downwards under the influence of gravity, leaving the other section above. The East African Rift Valley is an example of a normal fault. An Irish example is Killary Harbour.

Types of Fault (Transform Fault) Transform faults occur when two sections of crust move in opposite directions. This eventually leads to the formation of a passive plate boundary. The San Andreas Fault is the largest transform fault on Earth. It lies between the North American and Pacific Plates. The movement of the crust in opposite directions puts pressure on layers of rock within the crust. Eventually, the pressure becomes too great and the rock fractures, causing the crust to tear apart.

Types of Fault (Reverse Fault) A reverse fault occurs when the crust is compressed together. The pressure from the compression causes the crust to buckle upwards, forming an anticline. As the pressure continues, the rock is put under increased pressure, eventually causing it to fracture along its anticline. When this occurs, one section of rock slips upwards on top of the other. This leads to the formation of an escarpment. The Killarney–Mallow thrust fault was created in this way 250 million years ago, during the Armorican folding period. As the African Plate collided with the Eurasian Plate, sandstone was buckled upwards. Continued pressure eventually caused the Old Red Sandstone to fracture, creating a thrust fault.