3.1 Rocks and weathering: plate tectonics Flashcards
Recognise the global pattern of plate boundaries
What are divergent (constructive) plate boundaries
These are found where two plates are moving apart.
Sea floor spreading is the sideways/lateral expansion/extension of some ocean floors as the oceanic plates move apart.
The Mid-Atlantic ridge is a very long, high submarine mountain chain. It stretches through the centre of the Atlantic between Africa and Euorpe to the east and North and South America to the west. It is composed of volcanic lava and it varies in both width and height. High volcanic peals appear above the ocean surface forming the islands of the Azores, Ascension Island, Tristan da Cunha and Iceland. The Mid-Atlantic ridge is very irregular, with huge pieces of the plates offset at an angle sliding past eacher other to form transform faults, creating a series of short zigzags to accommodate the movement of the plates.
Conservative plate boundaries
These are fond were plates slide past each other and commonly produce earthquakes, but land is neither destroyed nor created along these margins – for example, where the North American and Pacific plates move past each other. As no subduction occurs, there is no melting of the crust and so there is no volcanic activity associated with this type of plate margin.
Convergent (destructive) plate boundaries
They are found where two plates are moving towards each other and are classified by referring to the type of the plates colliding.
- Continental-oceanic margins are seen when the denser oceanic plate subducts a continental plate, triggering earthquakes and forming volcanoes. This leads to the formation of ocean trenches. An example of an ocean trench is the Mariana Trench, which as 10,994 metres deep is the deepest part of the world’s oceans. Located in the western Pacific Ocean and to the east of the Philippines, it is about 2550km long and only 69km wide (on average).
- Oceanic-oceanic margins are found where two convergent oceanic plates meet. At the point of impact or subduction a deep ocean trench may be formed where the oceanic crust moves downwards. The heavier, melting subducting oceanic plate rises through the thinner, lighter oceanic plate above it, upwelling magma and forming island arcs which are chains of volcanic islands, as in the Caribbean islands.
- Continental-continental margins (collision plate margins) are found where two lighter continental plates collide with each other. This leads to the formation of fold mountains as neither can sink into the denser rocks below. Instead. They are crushed, crumpled and forced upwards, usually folding in the process.
What is sea floor spreading
Sea floor spreading is the sideways/lateral expansion/extension of some ocean floors as the oceanic plates move apart
What is subduction
Subduction is the sideways and downward movement of the edge of a plate of the earth’s crust into the mantle beneath another plate.
Describe and explain fold mountains
These are linear or arcuate chains – sometimes called ‘orogenic mountain belts’- are associated with convergent boundaries, and formed on land. Mountain building is often associated with crustal thickening, deformation, and volcanic activity, although in the case of the Himalayas volcanic activity is relatively unimportant.
Where an ocean plate meets a continental plate, the lighter, less dense contintental plate may be folded and buckled into fold mountains eg Andes:
- The Andes are the highest mountain range in the Americas, with 49 peaks over 6000m high and contains many active volcanoes.
- They were formed as a result of the subduction of oceanic crust under continental crust.
- Before about 250 million years ago, the western margin of South America was a passive continental margin.
- Sediments accumulated on the continental shelf and slope.
- With the break-up of Pangaea, the South America plate moved westward, and the eastern-moving oceanic lithosphere began subducting beneath the continent.
- As subduction continued, rocks of the continental margin and trench were folded and faulted and became part of an accretionary wedge along the west coast of South America.
- Subduction also resulted in partial melting of the descending plate, producing andesitic volcanoes at the edge of the continent.
- The Andes mountains comprise a central core of granitic rocks capped by andesitic volcanoes. To the west of this central core, along the coast, are the deformed rocks of the accretionary wedge; and to the east of the central core are sedimentary rocks that have been intensely folded. Present-day subduction, volcanism, and seismicity indicate that the Andes mountains are still actively forming.
Where two continental plates meet, both may be folded and buckled eg Himalayas:
- The Himalayas is the highest mountain range on Earth, with peaks such as Mt. Everest (8,848m tall). It stretches for over 2,900km along the border between the Tibetan Plateau and India.
- After Pangea broke up (around 200 million years ago), India was a large island close to the continental islands of Australia, and Antarctica.
- The northward movement of the Indian plate began about 71 million years ago, moving at around 9m every 100 years.
- 40 million years ago it rammed into the Eurasian plate.
- The rate of northward drifting of the Indian plate naturally slowed soon after it collided with the Eurasian plate, to around 4.5m every 100 years.
- The Indian and Eurasian plate thus formed a continental-continental convergent plate boundary, in which both slabs have equal density, so they both buckle at the plate boundary, and crust is pushed upwards.
- In the 40-50 million years since the Indian plate has collided with the Eurasian plate, the crust that forms the Himalayas has been uplifted more than 9km, and has not stopped. Even today, these two plates are still converging and pushing up the crust, adding to the height of the Himalayas at a rate of about 1cm per year.
Describe and explain rift valleys/oceanic ridges
If the boundary is found between two continental plates, you are left with a rift valley. If two oceanic plates begin moving away from each other it creates a mid-oceanic ridge.
The longest linear, uplifted features of the Earth’s surface are to be found in the oceans. They are giant submarine mountain chains with a total length of more than 60,000km, between 1,000-4,000km wide, and have crests that rise to 2-3km above the surrounding ocean basins, which are 5km deep. The average depth of water over their crests is thus about 2500 metres. These features are mid-ocean ridges.
Mid-ocean ridges occur along divergent plate boundaries, where new ocean floor is created as the Earth’s tectonic plates spread apart. As the plates separate, molten rock rises to the seafloor, producing enormous volcanic eruptions of basalt. The speed of spreading affects the shape of a ridge – slower spreading rates result in steep, irregular topography while faster spreading rates produce much wider profiles and more gentle slopes.
Two well-studied mid-ocean ridges within the global system are the Mid-Atlantic Ridge and the East Pacific Rise. The Mid-Atlantic Ridge runs down the centre of the Atlantic Ocean, slowly spreading at a rate of 2-5cm per year and forming a rift valley that is about the depth and width of the Grand Canyon. In contrast, the East Pacific Rise is spreading quickly, at rates of 6-16cm per year. Due to the fast spreading rates, there is no rift valley in the Pacific, just a smooth volcanic summit with a crack along the crest that is much smaller than the Atlantic rift valley.
Describe and explain ocean trenches
At the subduction zone, deep-sea ocean trenches are found. Deep-sea trenches are long, narrow depressions in the ocean floor with depths from 6,000m to 11,000m. Trenches are found adjacent to land areas and associated with island arcs worldwide. They are more numerous in the Pacific Ocean. The trench is usually asymmetric, with the steep side towards the land mass. Where a trench occurs off a continental margin, the turbidites (sediments) from the slope are trapped, forming a hadal plain on the floor of the trench.
Subduction zones: form where an oceanic lithospheric plate collides with another plate, whether continental or oceanic. The density of the oceanic plate is similar to that of the astenoshpere, so it can be easily pushed down into the upper mantle. Subducted (lithospheric) ocean crust remains cooler, and therefore denser than the surrounding mantle, for millions of years; so once initiated, subduction carries on, driven, in part, by the weight of the subducting crust. As the Earth ha not grown significant in size – not enough to accommodate the new crustal material created at mid-ocean ridges – the amount of subduction roughly balances the amount of production at the constructive plate margins.
Subduction zones dip mostly at angles between 30-70 degrees, but individual subduction zones dip more steeply with depth. The dip of slab is related inversely to the velocity of convergence at the trench, and is a function of the time since the initiation of subduction. The older the crust, the steeper it dips. Because the downgoing slab of lithosphere is heavier than the plastic asthenosphere below, it tends to sink passively; and the older the lithosphere, the steeper the dip.
Describe and explain volcanic island arcs
Island arc systems are formed when oceanic lithosphere is subducted beneath oceanic lithosphere. They are consequently typical of the margins of shrinking oceans such as the Pacific, where the majority of island arcs are located. They also occur in the western Atlantic, where the Lesser Antilles (Caribbean) and Scotia arcs are formed at the eastern margins of small oceanic plates.
Ocean-ocean subduction zones tend to be simpler than ocean-continental subduction zones. In a typical ocean-ocean subduction zone, there are a number of characteristic features:
- Ahead of the subduction zone, there is a low bulge on the sea floor (know as the trench outer rise) caused by the bending of the plate as it subducts. One of the best-known features is the trench that marks the boundary between the two plates. In the Eastern Caribbean, the trench associated with the subduction zone is largely filled with sediment from the Orinoco River. These sediments, more than 20km thick, have been deformed and folded into the Barbados Ridge, which emerges above the sea at Barbados.
- The outer slope of the trench is generally gentle, but broken by faults as the plate bends. The floor of the trench is often flat and covered by sediment (turbidites) and ash. The trench inner slope is steeper and contains fragments of the subducting plate, scraped off like shavings from a carpenter’s plane. The subduction complex (also known as accretionary prisms) is the slice of the descending slab and may form significant landforms – for example in the Lesser Antilles, the islands of Trinidad, Tobago, and Barbados are actually the top of the subduction complex.
- Most subduction zones contain an island arc, located parallel to a trench on the overriding plate. Typically they are found some 150-200km from the trench. Volcanic island arcs such as those in the Caribbean, including the islands from Grenada to St Kitts, are island arcs above sea level.
