The creation of fold mountains Learning Objective: To learn where fold mountains are and how they are created

STARTER: How do you think these mountains have been formed?

Describe the location of fold mountains.

Fold mountains Young fold mountains form in many parts of the world and they form along the plate margins where great Earth movements have taken place. They are found at destructive plate margins and places where there used to be destructive margins e.g. the West coast of North America. They are found where a continental and oceanic plate collide (e.g. the Andes in South America were formed this way). They form where two continental plates collide e.g. the Himalayas in Asia were formed this way.

Collision plate boundary Collision boundaries occur when two plates of similar densities move together (i.e. a continental plate and a continental plate). This causes the material between them to buckle and rise up, forming fold mountains. The Himalayas are an example of a chain of fold mountains. They have been formed by the African plate colliding into the Eurasian plate.

Collision plate boundary

Fold mountains are formed when tectonic plates collide and the plates fold up and are forced upwards into mountains. There were long periods of quiet between Earth movements during which sedimentary rocks, thousands of metres thick, formed in huge depressions called geosynclines. Rivers carried sediments and deposited them into the depressions. Over millions of years the sediments were compressed into sedimentary rocks such as sandstone and limestone. These sedimentary rocks were then forced upwards into a series of folds by the movement of the tectonic plates. Sometimes the folds were simple upfolds (anticlines) and downfolds (synclines). In such places fold were pushed over on one side, giving overfolds.

Fold mountains have been formed at times in the Earth’s geographical history called mountain-building periods. Recent mountain-building movements have created the Alps, the Himalayas, the Rockies and the Andes, some of which are still rising. For this reason many of these ranges are called young fold mountains. http://www.bbc.co.uk/schools/gcsebitesize/geography/natural_hazards/fold_mountains_video.shtml

Rivers deposited huge quantities of sediments in depressions called geosynclines

Over millions of years the sediments were compressed into sedimentary rocks such as sandstone and limestone

The plates moved together (a compressional boundary) forcing the sedimentary rocks upwards into a series of folds.

Ring of Fire http://blip.tv/scripts/flash/blipplayer.swf?autoStart=false&file=http://blip.tv/file/get/Gatm-RingOfFireKS3556.flv?source=3 San Andreas Fault http://blip.tv/scripts/flash/blipplayer.swf?autoStart=false&file=http://blip.tv/file/get/Gatm-SanAndreasFault634.flv?source=3 Plate Boundaries http://www.gatm.org.uk/?p=139

Ocean Trenches The majority of ocean trenches are located around the sides of the Pacific Ocean. They are associated with destructive plate margins. The subduction zone is an ocean trench. One wall is formed by subducted ocean plate (the Nazca plate), the other by the overriding continental plate (the South American plate). Ocean trenches are very deep, typically 5000-10 000 metres and narrow. Ocean trenches are inaccessible to humans. The ocean area of greatest importance is the continental shelf, the shallow zone less than 200 metres deep off the coast. The main opportunities here are for fishing and drilling for oil and gas.

3000m

5000m

Deepsea Challenger 11,000m

11,000m

Mid-Ocean Ridges

A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges (chains), typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading center, which is responsible for seafloor spreading. The uplifted seafloor results from convection currents which rise in the mantle as magma at a linear weakness in the oceanic crust, and emerge as lava, creating new crust upon cooling.

A mid-ocean ridge demarcates the boundary between two tectonic plates, and consequently is termed a constructive plate boundary. The mid-ocean ridges of the world are connected and form a single global mid-oceanic ridge system that is part of every ocean, making the mid-oceanic ridge system the longest mountain range in the world. The continuous mountain range is 65,000 km (40,400 mi) long (several times longer than the Andes, the longest continental mountain range), and the total length of the oceanic ridge system is 80,000 km (49,700 mi) long.

Rift Valleys A rift valley forms where the Earth’s crust, or outermost layer, is spreading or splitting apart. This kind of valley is often narrow, with steep sides and a flat floor. Rift valleys are also called grabens, which means “ditch” in German. While there is no official distinction between a graben and a rift valley, a graben usually describes a small rift valley.

It happens when a continental plate starts pulling itself apart It happens when a continental plate starts pulling itself apart. The most famous rift valley is the Great African Rift Valley. Here the African plate is basically ripping in half along a fault that runs from Syria in the north down to Mozambique in the south. The rift valley is over 6,000km in length. As the plates pull apart the land in between the plates collapses creating a rift valley. Overtime this valley may fill up with lakes e.g. Lake Malawi and Lake Tanganyika and then sea e.g Red Sea. Eventually the East of Africa will be separated from mainland Africa by a new sea.

As the plates pull apart magma is also able to escape from the mantle below, creating volcanoes e.g. Mount Kenya and Mount Kilimanjaro. Sometimes the rift valleys are called grabens and the remaining higher land horsts or block mountains. When the African plate finally rips in half the two plates will be known as the Somalian plate and the Nubian plate.

Mid-Ocean Ridges

Island Arcs The journey of a plate sinking into a trench is a long and complex affair, and the effects of this journey are apparent on the face of the earth. Firstly, as the cold, stiff plate begins its arduous descent, a continuous series of earthquakes is created. The plate then starts to heat up and at a depth of about 75 miles, certain magmas are melted and rise toward the surface. Eventually these magmas make their way up into the leading edge of the overriding plate, where they add material to the crust and build volcanoes above it. If the upper plate is oceanic, the volcanoes pile up until they poke through the surface of the ocean and form an elegant arc. Examples of island arcs created in this way are the Aleutians, the Kuriles, Japan, the Ryukyus, and the Philippines, found clustered around the northern and western borders of the Pacific Plate like a necklace.

Island Arcs Island Arcs Excuse the spelling, I did not make this video