Features Related with Plate Boundaries
- 1. FEATURES RELATED TO PLATE BOUNDARIES Mustafa Ahmed Wazeer Department of Geology, University of Karachi.
- 2. TECTONICS Tectonics is the study of the origin and arrangement of the broad structural features of Earth’s surface, including not only folds and faults but also mountain belts, continents, and earth-quake belts. Plate tectonics has come to dominate geologic thought today because it can explain so many features. The basic idea of plate tectonics is that Earth’s surface is divided into a few large, thick plates that move slowly and change in size.
- 3. Intense geologic activity occurs at plate boundaries where plates move away from one another, past one another, or toward one another. The eight large lithosphere plates, plus a few dozen smaller plates, make up the outer shell of Earth (the crust and upper part of the mantle). The concept of plate tectonics was born in the late 1960s by combining two preexisting ideas—continental drift and seafloor
- 4. Continental drift is the idea that continents move freely over Earth’s surface, changing their positions relative to one another. Seafloor spreading is a hypothesis that the sea floor forms at the crest of the mid-oceanic ridge, then moves horizontally away from the ridge crest toward an oceanic trench.
- 5. THE MAJOR PLATES OF THE WORLD
- 6. WHAT ARE PLATES ? A plate is a large, mobile slab of rock that is part of Earth’s surface. The plates are composed of the relatively rigid outer shell of Earth called the lithosphere. The surface of a plate may be made up entirely of sea floor (as is the Nazca plate), or it may be made up of both continental and oceanic rock (as is the North American plate). Some of the smaller plates are entirely continental, but all the large plates contain some sea floor.
- 8. HOW DO PLATES MOVE ? Below the rigid lithosphere is the asthenosphere, a zone of low seismic-wave velocity that behaves in a ductile manner because of increased temperature and pressure. The ductile asthenosphere acts as a lubricating layer under the lithosphere, allowing the plates to move.
- 10. HOW DO WE KNOW THAT PLATES MOVES ? In the 1960s, two critical tests were made of the idea of a moving sea floor. These tests involved marine magnetic anomalies and the seismicity of fracture zones. These two, successful tests convinced most geologists that plates do indeed move. Alternating positive and negative anomalies form a stripe like pattern parallel to the ridge crest confirm the plate movement. Earthquakes, occur only on the section of the fracture zone, the transform fault.
- 11. TYPES OF PLATE BOUNDARIES Plate boundaries are of three general types, based on whether the plates move away from each other, move toward each other, or move past each other. A divergent plate boundary is a boundary between plates that are moving apart. A convergent plate boundary lies between plates that are moving toward each other. A transform plate boundary is one at which two plates move horizontally past each other.
- 13. At transform boundaries, where one plate slides horizontally past another plate, the plate motion can occur on a single fault or on a group of parallel faults. Transform boundaries are marked by shallow- focus earthquakes in a narrow zone for a single fault or in a broad zone for a group of parallel faults. First-motion studies of the quakes indicate strike- slip movement parallel to the faults. The name transform fault comes from the fact that the displacement along the fault abruptly ends or transforms into another kind of displacement.
- 14. The most common type of transform fault occurs along fracture zones and connects two divergent plate boundaries at the crest of the mid-oceanic ridge (4.18B). When two oceanic plates begin to diverge, the boundary may be curved on a sphere. Mechanical constraints prevent divergence along a curved boundary, so the original curves readjust into a series of right-angle bends. The ridge crests align perpendicular to the spreading direction, and the transform faults align parallel to the spreading direction. The boundary will then readjust into a series of transform faults parallel to the spreading direction.
- 16. THE SAN ANDREAS FAULT The San Andreas fault is a 1,100-kilometer-long rupture marking the border between the Pacific and North American plates in California. But only about one-third of the approximately 2,000 kilometers of total slip between the two places, the biggest plates on Earth, has taken place along the fault during its 25 to 30-million- year history. Its motion is right-lateral strike-slip (horizontal). The slip rate along the fault ranges from 20 to 35 mm (0.79 to 1.38 in)/yr.
- 20. Divergent plate boundaries, where plates move away from each other, can occur in the middle of the ocean or in the middle of a continent. The result of divergent plate boundaries is to create, or open, new ocean basins. Oceanic divergent boundaries create what are known as mid-ocean ridges, such as the Mid- Atlantic Ridge. Rising convection currents in the athenosphere press upward on the thin oceanic plates, causes the plates to bulge upwards.
- 21. As these currents reach the plates, they also spread outward, pulling the plates apart. As the plates are stretched thin by the upward and outward forces, they fracture. These fractures are quickly filled by solidifying magma and the process begins again. This process produces sub-surface mountain ranges, fissure eruptions, shallow earthquakes, new seafloor and a widening of the ocean basin. This divergent process is characterized by slow and steady expansion approximately 2.5cm a year.
- 23. MID-OCEAN RIDGES A mid-ocean ridge (MOR) is an underwater mountain system formed by divergence of plates. It consists of various mountains linked in chains, typically having a valley known as a rift running along its spine. This type of oceanic mountain ridge is characteristic of 'oceanic spreading center', which is responsible for seafloor spreading. Mid-ocean ridges are geologically active, with continuing volcanism and seismicity.
- 24. Two of the most well-known mid-ocean ridges are the Mid-Atlantic Ridge and the East Pacific Rise. As you might guess, most of the Mid-Atlantic Ridge is in the Atlantic, and most of the East Pacific Rise is in the Pacific. The Mid-Atlantic Ridge is spreading one to two inches a year, and the East Pacific Rise is spreading at two to six inches per year. The mid-Atlantic Ridge begins in the Arctic Ocean extends across Iceland all the way to the bottom of the Atlantic near Africa before dividing into two.
- 26. CONTINENTAL DIVERGENT ZONES Continental plates are much thicker than oceanic plates. The force produced by upward currents in these divergent boundaries is not strong enough to create a single break through the entire plate. Instead, the plate bulges upward as it is stretched and fault lines develop on each side of the crest. When these faults fracture, intense earthquakes are produced and the center block drops, forming a rift-like structure. This continental divergent process is much choppier than the smoother oceanic divergence, and is characterized by more sudden, irregular and intense shifts in the rift structure. The East Africa Rift Valley is a classic example of this type of plate boundary.
- 27. EAST AFRICA RIFT VALLEY
- 31. At convergent plate boundaries, two plates move toward each other (often obliquely). The character of the boundary depends partly on the type of plates that converge. A plate capped by oceanic crust can move toward another plate capped by oceanic crust, in which case one plate dives (subducts) under the other. If an oceanic plate converges with a plate capped by a continent, the dense oceanic plate subducts under the continental plate. If the two approaching plates are both carrying continents, the continents collide and crumple, but neither is subducted.
- 32. OCEAN-OCEAN CONVERGENCE Where two plates capped by sea floor converge, one plate subducts under the other (the Pacific plate sliding under the western Aleutian Islands is an example). Ocean-ocean convergence forms a subduction zones, trench, and volcanic island arc.
- 33. SUBDUCTION ZONES Subduction is a geological process that takes place at convergent boundaries of tectonic plates where one plate moves under another and is forced or sinks due to gravity into the mantle. Regions where this process occurs are known as subduction zones. Subduction zones are sites that usually have a high rate of volcanism and earthquakes. Rates of subduction are typically in centimeters per year, with the average rate of convergence being approximately two to eight centimeters per year along most plate boundaries.
- 34. MAKRAN SUBDUCTION ZONE The 1000-km long Makran subduction zone extends from southern Pakistan to Iran, with the Himalayas to the east and the Zagros orogenic belt to the west (Fig.1a). This subduction zone is bounded by the left- lateral Ornach Nal Fault to the east and the right-lateral Minab–Zendan Fault to the west. Along this plate boundary, the Arabia and Ormara Plates subduct northward underneath the Eurasia Plate with a 3 to 4cm/yr convergence.
- 36. Map of the Makran subduction zone, where the Arabia and Ormara plates subduct under Eurasia.
- 37. TRENCHES Oceanic trenches are topographic depressions of the sea floor, relatively narrow in width, but very long. These oceanographic features are the deepest parts of the ocean floor. Oceanic trenches are a distinctive morphological feature of convergent plate boundaries, along which lithospheric plates move towards each other at rates that vary from a few millimeters to over ten centimeters per year. A trench marks the position at which the flexed, subducting slab begins to descend beneath another lithospheric slab. Trenches are generally parallel to a volcanic island arc, and about 200 km (120 mi) from a volcanic arc. Oceanic trenches typically extend 3 to 4 km (1.9 to 2.5 mi) below the level of the surrounding oceanic floor. The greatest ocean depth measured is in the Challenger Deep of the Mariana Trench, at a depth of 11,034 m (36,201 ft) below sea level.
- 38. THE MARIANA TRENCH The Mariana Trench or Marianas Trench located in the western Pacific Ocean approximately 200 kilometers (124 mi) east of the Mariana Islands, and is the deepest known point in the world's oceans. The Mariana trench contains the deepest part of the world's oceans, and runs along an oceanic- oceanic convergent boundary. It is the result of the oceanic Pacific plate subducting beneath the oceanic Mariana plate. It is a crescent-shaped trough in the Earth's crust averaging about 2,550 km (1,580 mi) long and 69 km (43 mi) wide.
- 39. VOLCANIC ISLAND ARC An island arc is a curved line of volcanoes that form a string of islands parallel to the oceanic trench (figure 4.25). The distance between the island arc and the trench can vary, depending upon where the subducting plate reaches the 100-kilometer depth. If the subduction angle is steep, the plate reaches this magma-generating depth at a location close to the trench, so the horizontal distance between the arc and trench is short. If the subduction angle is gentle, the arc-trench distance is greater.
- 41. OCEAN-CONTINENT CONVERGENCE If an oceanic plate converges with a plate capped by a continent, the dense oceanic plate subducts under the continental plate. Ocean-continent convergence forms a trench, a magmatic arc, and a young mountain belt on the edge of the continent. Examples of this type of boundary are the subduction of the Nazca plate under western South America and the Juan de Fuca plate under North America.
- 42. MAGMATIC ARC The magma that is created by ocean-continent convergence forms a magmatic arc, a broad term used both for island arcs at sea and for belts of igneous activity on the edges of continents. The surface expression of a magmatic arc is either a line of andesitic islands (such as the Aleutian Islands) or a line of andesitic continental volcanoes (such as the Cascade volcanoes of the Pacific Northwest). Beneath the volcanoes are large plutons in thickened crust.
- 43. YOUNG MOUNTAIN BELT The hot magma rising from the subduction zone thickens the continental crust and makes it weaker and more mobile than cold crust. Crustal thickening causes uplift, so a young mountain belt forms here as the thickened crust rises iso-statically. Another reason for the growth of the mountain belt is the stacking up of thrust sheets on the continental side of the magmatic arc.
- 45. CONTINENT-CONTINENT CONVERGENCE If the two approaching plates are both carrying continents, the continents collide and crumple, but neither is subducted. The collision of two continents forms a young mountain belt in the interior of a new, larger continent. The most famous example of continent- continent collision is the collision of India with Asia (formation of Himalayas).
- 46. Mountain Belt (Himalayas) The Himalayas , or Himalaya form a mountain range in Asia , separating the plains of the Indian subcontinent from the Tibetan Plateau. The Himalayan range has many of the Earth's highest peaks, including the highest, Mount Everest . The Himalayas include over fifty mountains exceeding 7,200 meters (23,600 ft) in elevation, including ten of the fourteen 8,000-metre peaks. By contrast, the highest peak outside Asia ( Aconcagua, in the Andes) is 6,961 meters (22,838 ft) tall.
- 47. The Himalayas are spread across five countries : Nepal , India , Bhutan , China and Pakistan . The 6,000-kilometre-plus journey of the India landmass (Indian Plate) before its collision with Asia (Eurasian Plate) about 40 to 50 million years ago. Its formation is a result of a continental collision or orogeny along the convergent boundary between the Indo-Australian Plate and the Eurasian Plate. The Himalayas rising by about 5 mm per year,
- 49. CONCLUSION Now u can know about the importance of plate boundaries that it controls many geologic features. Plate tectonics has come to dominate geologic thought today because it can explain so many features.
- 50. THANK YOU