GCSE Geology Plate Tectonics: Features and Processes

Transcription

1 GCSE Geology Plate Tectonics: Features and Processes A) DIVERGENT BOUNDARIES: 1. O-O Divergence, Example: The Mid-Atlantic Ridge, Iceland The North American and Eurasian Plates are moving away from each other along the line of the Mid Atlantic Ridge. The Ridge extends into the South Atlantic Ocean between the South American and African Plates. The ocean ridge rises to between 2 to 3 km above the ocean floor, and has a rift valley at its crest marking the location at which the two plates are moving apart. The Mid Atlantic Ridge, like other ocean ridge systems, has developed as a consequence of the divergent motion between the Eurasian and North American, and African and South American Plates. This creates crustal tension and thinning of the crust. As the mantle rises towards the surface below the ridge the pressure is lowered (decompression) and the hot rock starts to partially melt. This produces basaltic volcanoes when an eruption occurs above the surface (Eyjafjallajökull in Iceland) and characteristic basalt pillow lava in underwater eruptions. In this way, as the plates move further apart new ocean lithosphere is formed at the ridge and the ocean basin gets wider. This process is known as sea floor spreading and results in a symmetrical alignment of the rocks of the ocean floor which get older with distance from the ridge crest. Evidence for this process comes from the magnetic properties of the erupted basalt. The Earth s magnetic field has been shown to flip occasionally so that the North and South magnetic poles reverse with time. Basalt contains minute magnetic minerals that take on the direction of the Earth s magnetic field at the time of eruption. These polarity reversals are therefore recorded in the rocks forming at the Mid Atlantic Ridge every time the Earth s magnetism changes from a normal field (as it is today) to a reversed field (as it was about 0.78 million years ago). When these variations in the magnetic rocks are mapped on the ocean bed they are seen to line up in a series of alternating magnetic stripes, rather like barcodes. The significance of the magnetic stripes was only revealed when their ages were discovered from dating magnetic reversals in volcanic rocks accessible on land. The dates revealed that the Atlantic Ocean was opening by seafloor spreading from the Mid Atlantic Ridge at a rate of about 0.02 metres per year. 1

2 This means that North America and Europe are moving away from each other at about the rate it takes for your fingernails to grow. B) CONVERGENT BOUNDARIES 2. O-C Convergence, Example: The Andes Mountains & Volcanoes S. America The Nazca Plate is moving eastwards, towards the South American Plate, at about 79mm per year. Where the two plates meet, the denser oceanic lithosphere of the Nazca Plate is forced down and under the more buoyant continental lithosphere of the South American Plate, descending at an angle into the mantle in a process called subduction. This is marked on the ocean surface by the presence of the Peru-Chile (or Atacama) Trench. The friction between the plates prevents the subducting oceanic plate from sliding smoothly. As it descends, it drags against the overlying plate, causing both to fracture and deform. This results in frequent shallow focus earthquakes that get deeper as the ocean plate descends further, defining an inclined zone of earthquake foci known as a Benioff zone. The effect of the collision of the two plates deforms the leading edge of the South American Plate by folding the rocks. This crustal shortening increases the vertical thickness whilst reducing the width of the lithosphere in the collision zone (imagine a car hitting a solid wall) and so produces the fold mountains of the Andes. Continued subduction of the Nazca Plate brings sea water, locked in the ocean crust, deep into the mantle. As the plate heats up the water is liberated (lost to surrounding rocks), lowering the melting point of the mantle and causing partial melting of the mantle wedge above the subducting plate. This produces magma, which rises and may be erupted explosively from grey volcanoes, as andesite at the surface. Andesitic magma is less dense than the surrounding material, and can have a temperature of 1000 o C. It is viscous, trapping gases as it rises. The water and gases in andesitic magma account for the explosive activity of andesitic volcanoes, which typically lie dormant for many hundreds or thousands of years. These volcanoes typically produce ash and pyroclastic flows, as well as small amounts of andesitic lava. Andean volcanoes such as the stratovolcano Láscar, in northern Chile, are a good example of this type of activity. Láscar erupted ash and pyroclastic flows in 1993 and was still active in

3 3. C-C Convergence, Example: The Himalayas The Himalayan mountain range and Tibetan plateau have formed as a result of the collision between the Indian Plate and Eurasian Plate which began 50 million years ago and continues today. 225 million years ago (Ma) India was a large island situated off the Australian coast and was separated from Asia by the Tethys Ocean. The supercontinent Pangea began to break up 200 Ma and India started a northward drift towards Asia. 80 Ma India was 6,400 km south of the Asian continent but moving towards it at a rate of between 9 and 16 cm per year. At this time Tethys Ocean floor would have been subducting northwards beneath Asia and the plate margin would have been a Convergent oceanic-continental one just like the Andes today. Between 40 and 20 Ma the rate of northward drift slowed as the two continental plates collided and the former Tethys Ocean closed. Neither continental plate could be subducted due to their low density/buoyancy (2.7g/cm 3 ). This caused the continental crust to thicken due to folding and faulting by compressional forces. Large scale folds called Nappes can be seen in the rocks today as well as many thrust faults. The continental crust here is twice the average thickness at around 75 km. The thickening of the continental crust marked the end of volcanic activity in the region as any magma moving upwards would solidify before it could reach the surface. The Himalayas are still rising by more than 1 cm per year as India continues to move northwards into Asia, which explains the occurrence of shallow focus earthquakes in the region today. However the forces of weathering and erosion are lowering the Himalayas at about the same rate. The Himalayas and Tibetan plateau trend east-west and extend for 2,900 km, reaching the maximum elevation of 8,848 metres (Mount Everest). Thrust Faults Nappes Large Folds 3

4 4. O-O Convergence, Example: The Caribbean Islands The South American Plate is moving westwards due to sea floor spreading at the Mid Atlantic Ridge. Where it meets the Caribbean Plate, it descends (subducts) beneath it. This is because the oceanic lithosphere of the South American Plate is cooler, denser and older than that of the Caribbean Plate. The subduction causes low density ocean floor sediment to be scraped off the surface of the South American Plate and thrust onto the Caribbean Islands as accretionary wedges, in a process called obduction. The line of subduction is marked by the deep sea Puerto Rico Trench. As the South American Plate descends, it drags against the overlying plate, causing both to fracture and deform. This results in frequent shallow focus earthquakes that get deeper as the ocean plate descends further, defining a zone of earthquake foci known as a Benioff zone. Continued subduction of the South American Plate brings sea water, locked in the ocean crust, deep into the mantle. As the plate heats up the water is liberated, lowering the melting point of the mantle and causing partial melting of the mantle wedge. This produces magma, which rises and may be erupted explosively as andesite at the surface. Andesitic magma is less dense than the surrounding material, and can have a temperature of 1000 o C. It is viscous, trapping gases as it rises. The water and gases in andesitic magma account for the explosive activity of andesitic volcanoes (grey volcanoes), which typically lie dormant for many hundreds or thousands of years. These volcanoes typically produce ash and pyroclastic flows, as well as small amounts of andesitic lava. The eruptions on Montserrat during the 1990s are a good example of this type of activity. The Caribbean volcanic islands form a curved linear chain or volcanic island arc parallel and to the west of the Puerto Rico Trench. C) INTRA-PLATE FEATURES 5. C-C divergence (Rifting) forming a Rift, Example: The East African Rift Valley Along this part of East Africa, the African plate is being pulled apart and thinned by the crustal tension that results due to the plate movement. Tension causes normal faulting and produces a rift valley by the process of rifting. This thinning crust allows fractures called fissures to open up and less pressure on the mantle means that it rises to fill the gap. As a result melting occurs in the region where the thinning takes place and magma rises and is extruded through the cracks. Famous volcanoes such as Kilimanjaro occur here and sit within the rift valley. Rift Valley 4

5 6. Hotspots, Example: Hawaii Whilst most volcanic activity happens at plate margins, there are cases of volcanoes erupting in the middle of plates. The Hawaiian Islands are formed by volcanic activity, despite the nearest plate margin being 3,200 km away. Some geologists have suggested that a 'hot spot' in the mantle, which remains stationary as the Pacific Plate moves over it, explains the existence of the island chain. The hot spot may represent the top of a mantle plume which originated deep down at the outer core - lower mantle boundary. The plate moves in a north westerly direction due to sea floor spreading along the East Pacific Rise. As oceanic lithosphere moves away from the hot spot, volcanic activity ceases and it cools, becomes denser, and slowly subsides. As new oceanic lithosphere is positioned over the hot spot, a new island will begin to form above. The islands extend for around 2,400 km, forming a chain that is Mauna Loa Basalt Flows progressively older from the south east end to the north west end. The volcanoes are often very wide, with gently sloping sides comprising many thin (1 to 5 metres thick) basaltic lava flows. These are referred to as 'shield volcanoes'. Kilauea and Mauna Loa on Big Island are currently active examples. The next island to appear in the Hawaiian chain has already been identified, and named as Lo ihi. It is currently 975 metres below sea level, and is estimated to emerge above sea level in the next 10,000 to 100,000 years. 5

6 D) CONSERVATIVE BOUNDARIES 7. Example: The San Andreas Fault Zone The San Andreas Fault marks the junction between the North American and Pacific Plates. The fault is 1300 km long, extends to at least 25 km in depth, and has a north west-south east trend. It is classified as a right lateral (dextral) strike-slip fault. Although both plates are moving in a north westerly direction, the Pacific Plate is moving faster than the North American Plate, so the relative movement of the North American Plate is to the south east. The Pacific Plate is being moved north west due to sea floor spreading from the East Pacific Rise (divergent margin) in the Gulf of California. The North American Plate is being pushed west and north west due to sea floor spreading from the Mid Atlantic Ridge (divergent margin). NB: Both plates move north! Movement along the fault is not smooth and continual, but sporadic and jerky. Frictional forces lock the blocks of lithosphere together for years at a time. When the frictional forces are overcome, the plates slip suddenly and shallow focus earthquakes are generated. Landscape and manmade features (eg rivers, fences and roads) are displaced across the fault as movement occurs. San Francisco has historically suffered significant earthquakes, notably in 1906 and As the movement is jerky, sometimes it involves some vertical displacement of the fault too causing a fault scarp (after weathering and erosion) after the movement has ceased. Fault Scarp The average rate of movement along the San Andreas Fault is between 30mm and 50mm per year over the last 10 million years. If current rates of movement are maintained Los Angeles will be adjacent to San Francisco in approximately 20 million years. 6