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Course Outline GEOG 3 WORLD CITIES TECTONICS Plate Theory • Earth Structure • Plate Movement • Theory and Evidence (Alfred Wegener) Plate Margins Earth Structure Earth Structure Crust • Thin, outer-layer of the earth. • It is as thin as the skin of an apple is to its flesh. • Oceanic Crust: a 6-10 km thick layer composed of mainly basalt. At its hottest point it is up to 1200C • Continental Crust: up to 70 km thick. Separated from the mantle by the moho discontinuity. Earth Structure – Mantle • Composed mainly of silicate rocks, which contain a lot of magnesium and iron. • It extends to a depth of 2900km. • Temperatures can reach 5000 degrees Celsius, which generate convection currents. • This is the ridge top layer of the mantle. Together with the crust these 2 layers make up the lithosphere. • asthenosphere, this is the lower semi molten state of the mantle. Earth Structure - Core • Contains iron and nickel. • Situated approximately 6371km below the surface. • Temperature is around 5500 degrees Celsius. • outer core – semi molten • inner core – Solid The Theory: AW Plate tectonics grew out of a theory that was first developed in the early 20th century by the meteorologist Alfred Wegener. In 1912, Wegener noticed that the coastlines of the east coast of South America and the west coast of Africa seemed to fit together like a jigsaw puzzle. Plate Movement – Sea Floor Spreading Paleomagnetism What is Paleomagnetism? A volcano erupts underwater, the lava which contains many different elements including that of iron is heated and forced through the gap in the crust of the earth. These Iron particles within the lava have become magnetic they now start to align themselves with the earths polarity. (which changes every 400,000 years) Due to this change in polarity we can see by the direction of the particles how long ago the eruption was. Theory: Jigsaw • Further examination of the globe revealed that all of the Earth's continents fit together somehow and Wegener proposed an idea that all of the continents had at one time been connected in a single supercontinent called Pangaea. He believed that the continents gradually began to drift apart around 300 million years ago - this was his theory that became known as continental drift. Theory: Geology • Rocks that are similar in origin and age can be found in both Brazil and South Africa, this suggests that originally these two were touching and closer than they are today. • The trends of the mountains in the Eastern USA and Europe are similar • Glacial deposits from Antarctica, South America and India now 1000s of kilometres apart share some similar trends. • Striations which show the same orientation are present when you compare Brazil to west Africa. Theory: Fossil / Living • Again similar fossil formations are found on both sides of the Atlantic • The Mesosaurus is only found in South America and South Africa. • Evidence for humid swaps which later formed coal fields have been found in Antarctica but also in India • Marsupials (kangaroos and koalas) only survived in Australia because it had drifted away from the super continent before predators could wipe them out. Theory: Climate • Places that are today, far apart contain the same coal deposits suggesting that at one point they were close together and in a different climate area. (Antarctica, North America, Svalbard ( a Norwegian island) and the UK) These places are no longer in tropical climates that are needed for the “coal” swaps and so they must have drifted apart Plate Margins • Constructive • Destructive - Oceanic to continental - Oceanic to Oceanic - Continental to Continental • Conservative Plate Margins Plate Margins Plate Margins Plate Margins Plate Margins Constructive Also known as a divergent margin. Plates move away from each other, for example, N. American and Eurasian plates, at oceanic plates this creates mid-ocean ridges such as the Mid Atlantic Ridge and at continental plates they produce rift valleys. The space between the diverging plates is filled with basaltic lava upwelling from below. Constructive margins are therefore some of the youngest parts of the Earth's surface, where new crust is being continuously created. Constructive - Ocean Ridges • Oceanic ridges are the longest continuous uplifted features on the surface of the planet, and have a total length of 60,000km. In some parts they rise 3,000m above the ocean floor. Their precise form appears to be influenced by the rate at which the plates separate: • A slow rate (10-15mm per year), as seen in parts of the mid-Atlantic ridge, produces a wide ridge axis (30-50km) and a deep (3,000m) central rift valley with inward-facing fault scarps • An intermediate rate (50-90mm per year), such as that on the Galapogos ridge (pacific), produces a less well-marked rift (50-200m deep) with a smoother outline • A rapid rate (>90mm per year), such as on the east Pacific rise, produces a smooth crest and no rift • Volcanic activity occurs along the ridge, forming submarine volcanoes, which sometimes rise above sea level, e.g. Sutsey to the south of Iceland. These are volcanoes with fairly gentle sides because of the low viscosity of basaltic lava. Eruptions are frequent but relatively gentle. • As new crust forms and spreads, transform faults occur at right angles to the plate boundary. The parts of the spreading plates on either side of these faults may move at differing rates, leading to friction and ultimately to earthquakes. These tend to be shallow-focus earthquakes, originating near the surface. Constructive – Rift Valleys At constructive margins in continental areas, such as east Africa, the brittle crust fractures as sections of it move apart. Areas of crust drop down between parallel faults to form rift valleys. The largest of these features is the African rift valley which extends 4,000km from Mozambique to the Red Sea. In some areas the inward-facing scarps are 600m above the valley floor and they are often marked by a series of parallel step faults. The area is also associated with volcanic activity (for example the highest mountain in Africa, Mt Kilimanjaro). The crust here is much thinner than in neighbouring areas, suggesting that tension in the lithosphere is causing the plate to thin as it starts to split. The line if the African rift is thought to be an emergent plate boundary, the beginning of the formation of a new ocean as eastern Africa splits away from the rest of the continent. Destructive Oceanic to Continental Where the two plates meet, the denser oceanic lithosphere plate is forced down and under the more buoyant continental lithosphere of the continental Plate. The friction between the plates prevents the sub-ducting 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. See Case Study Source Oceanic to Oceanic The more dense plate will be subducted under the other during the collision. At the subduction zone a very deep trench is formed in the ocean floor. Oceanic and oceanic plate convergence result in the formation of volcano chains and island arcs. The crust that is pulled under or subducted melts to form magma. This magma rises to the top of the overriding oceanic plates and erupts on the ocean floor. Over millions of years, the lava and debris from the volcanic eruptions pile up on the ocean floor until the volcano rises above sea level to form a volcanic island. These types of islands are usually formed as chains called island arcs, which run parallel to the trench at the subduction zone. Oceanic-oceanic plate convergence experience similar powerful earthquakes to oceanic-continental convergences. Source Continental to Continental Continental to Continental Case Study When continental plates collide neither can be subducted due to their low density/buoyancy. This causes the continental crust to thicken due to folding and faulting by compressional forces. The continental crust in the Himalayas is twice the average thickness at around 75 km. The thickening of the continental crust marks the end of volcanic activity in the region as any magma moving upwards would solidify before it could reach the surface. At continental plate boundaries fold mountains such as the Himalayas are formed. Source Conservative • Conservative margins are also known as transform faults. Transform faults are mainly found on the ocean floor, where they offset mid ocean ridges and enable to ocean to spread at different rates. It was through the work of John Tuzo Wilson that these faults were recognised as the connection between the ocean ridges (divergent margins) and ocean trenches (convergent margins). • At conservative margins, plates slide past each other, so that the relative movement is horizontal, and classified as either sinistral (to the left) or dextral (to the right). Lithosphere is neither created nor subducted, and whilst conservative plate margins do not result in volcanic activity, they are the sites of extensive shallow focus earthquakes, occasionally of considerable magnitude. SOURCE Plate Margin Case Studies • • • • • • Conservative margin – San Andreas Fault Oceanic/ Continental plate convergence – Nazca Rift Valley – E.Africa Rift Valley Ocean Ridge – Mid Atlantic Rift Ocean Trench – Tonga Trench Fold Mountains- Himalayas Conservative San Andreas Fault Although both plates are moving in THE SAME 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). 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 The average rate of movement along the San are overcome, the plates slip suddenly and Andreas Fault is between 30mm and 50mm per shallow focus earthquakes are generated. year over the last 10 million years Oceanic/Cont. CONVERGENCE NAZCA PLATE Oceanic/Cont. CONVERGENCE NAZCA PLATE • The Nazca Plate is moving eastwards, towards the South American Plate, at about 79mm per year. • 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. • Andesitic magma is less dense than the surrounding material, and can have a temperature of 1000oC. 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 2012. RIFT VALLEY – East Africa The East African Rift System (EARS) is one the geologic wonders of the world, a place where the earth's tectonic forces are presently trying to create new plates by splitting apart old ones. In simple terms, a rift can be thought of as a fracture in the earth's surface that widens over time, or more technically, as an elongate basin bounded by opposed steeply dipping normal faults. The exact mechanism of rift formation is an on-going debate among geologists and geophysicists. One popular model for the EARS assumes that elevated heat flow from the mantle (strictly the asthenosphere) is causing a pair of thermal "bulges" in central Kenya and the Afar region of north-central Ethiopia. SOURCE RIFT VALLEY – East Africa A hot spot seems to be situated under central Kenya, as evidenced by the elevated topographic dome there (Figure 1). This is almost exactly analogous to the rift Ethiopia, and in fact, some geologists have suggested that the Kenya dome is the same hotspot or plume that gave rise to the initial Ethiopian rifting. SOURCE Ocean Ridge – Mid Atlantic Rift 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. 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. Ocean Ridge – Mid Atlantic Rift Ocean Trench – Tonga Trench Underwater mountains are being dragged westward on the Pacific plate and subducted into the Tonga Trench . submarine trench in the floor of the South Pacific Ocean, about 850 miles (1,375 km) in length, forming the eastern boundary of the Tonga Ridge; the two together constitute the northern half of the Tonga-Kermadec Arc, a structural feature of the Pacific floor completed to the south by the Kermadec Trench and Ridge. The Tonga Trench has an average depth of 20,000 feet (6,000 m) and a width of about 50 miles (80 km); it reaches a maximum depth of 35,702 feet (10,882 m). FOLD MOUNTINS – Himalayas Continental Collision Boundaries fold mountains fold as the material is crumpled against each other. The Example for fold mountains is the Himalayas in India. There is no volcanic activity but there are intense earthquakes like Sichuan 2008 and NE India in 2001. FOLD MOUNTINS – Himalayas The Himalayas, which stretch some 2,900 kilometres between India, Pakistan, China, and Nepal, is the world’s tallest mountain range. In addition to Mount Everest, the world’s tallest mountain by peak elevation standing at 8,848 meters tall, the range also features several other mountain peaks over 8,000 meters. It is the only mountain range to boast mountains over 8,000 meters—the runner-up is a mountain range in South America, whose tallest peak is just 6,962 meters tall. Millions of years ago, these mountain peaks didn’t exist. The Asian continent was mostly intact, but India was an island floating off the coast of Australia. Around 220 million years ago, around the time that Pangea was breaking apart, India started to move northwards. It travelled some 6,000 kilometres before it finally collided with Asia around 40 to 50 million years ago. Then, part of the Indian landmass began to go beneath the Asian one, moving the Asian landmass up, which resulted in the rise of the Himalayas. It’s thought that India’s coastline was denser and more firmly attached to the seabed, which is why Asia’s softer soil was pushed up rather than the other way around. FOLD MOUNTINS – Himalayas The mountain range grew very rapidly in comparison to most mountain ranges, and it’s actually still growing today. Mount Everest and its fellows actually grow by approximately a net of about a centimetre or so every year. That’s in comparison to the Appalachian Mountains, which developed some 300 million years ago or more, which is actually decreasing in peak elevations as it erodes. The continued growth in the Himalayas is likely due to the Indian tectonic plate still moving slowly but surely northward. We know the plate is still moving in part because of the frequent earthquakes in the region. Now, if you do the math, you’d find that if the Himalayas had been growing at the current rate for 40 million years, they should be about 400 km tall! Once the infrastructure was in place, this would have given us a much cheaper way to put things into low Earth orbit and beyond. (For reference, the International Space Station typically orbits at between 300 km to 400 km.) So what happened? In part, the rate of vertical growth has varied over time, including in favor of more horizontal growth. And, of course, gravity and erosion having limited the mountains’ growth significantly. FOLD MOUNTINS – Himalayas India merging into Asia became the accepted theory about how the Himalayas were formed around 1912. That’s when Alfred Wegener, a German meteorologist, came up with the “Theory of Continental Drift” which gave us our first ideas about Pangea, tectonic plates, and the thought that continents were moving away from or closer to each other. What does the future for the Himalayas look like? Undoubtedly, the mountains will continue to grow, though at the same time eroding too; but the net is expected to continue to grow as the Indian tectonic plate doesn’t look like it’s going to slow down any time soon. That means more earthquakes and, over time, slightly taller mountains to climb. Hot spots associated with plumes of magma and their relationship to plate movement. Some volcanoes do not occur at plate margins For example the Hawaiian Islands which are volcanic in origin, were formed in the middle of the pacific plate. 3200km from the nearest plate boundary. Hot spots are the result of plumes of magma deep in the mantle which are super heated by radio active decay which occurs within the core. Volcanoes erupt over the hotspots and new islands are formed. The plate is moving past the hot spot and so the islands formed will also over time drift past the hotspot. This creates an Island Arc. Hotspots HOTSPOTS Hawaii • Before 2009, The theory of Hotspots hadn’t been proven, however in December 2009 scientists and geologists were able to prove the existence of the Hawaii Hotspot (HHS) • After two years of research and analysis the PLUME group released the first image of the HHS which stretches a depth of 1500km • The Pacific Plate is moving at a rate of 5-10cm a year. And so the newly created islands are slowly moved away from the hotspot. • The next volcano is the Lo’ihi Seamount which is forming on the sea floor, however it will not form an island on the surface for about the next 200,000 ISLAND ARCS • The Island of Hawaii is the youngest island in the chain. The most southeastern part of the Island presently sits on top of the hot spot. The active submarine volcano Lö'ihi, will form the next volcanic island. With the possible exception of Maui, the other Hawaiian islands have moved northwest beyond the hot spot and have become cut off from the sustaining magma source and are no longer volcanically active. • The progressive northwesterly drift of the islands from their point of origin over the hot spot is well shown by the ages of the principal lava flows on the various Hawaiian Islands from northwest (oldest) to southeast (youngest), given in millions of years: Ni'ihau and Kaua'i, 5.6 to 3.8; O'ahu, 3.4 to 2.2; Moloka'i, 1.8 to 1.3; Maui, 1.3 to 0.8; and Hawai'i, less than 0.7 and still growing. SOURCE