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Constructive or divergent boundaries are places where plates are coming apart. As shown in the drawing above, when Earth's surface layer (the lithosphere) is pulled apart, it breaks along faults and they tilt slightly outward from each other. As the plates separate along the boundary, the block between the faults cracks and drops down into the soft, plastic interior (the asthenosphere). The sinking of the block forms a central valley called a rift. Magma (liquid rock) seeps upward to fill the cracks. In this way, new crust is formed along the boundary. Earthquakes occur along the faults, and volcanoes form where the magma reaches the surface.
Where a divergent boundary crosses the land, the rift valleys, which form, are typically 30 to 50 kilometres wide. Examples include the East Africa rift in Kenya and Ethiopia, and the Rio Grande rift in New Mexico. Where a divergent boundary crosses the ocean floor, the rift valley is much narrower, only a kilometre or less across, and it runs along the top of a mid-oceanic ridge. Oceanic ridges rise a kilometre or so above the ocean floor and form a global network tens of thousands of miles long. Examples include the Mid-Atlantic ridge and the East Pacific Rise. Plate separation is a slow process. For example, divergence along the Mid Atlantic ridge causes the Atlantic Ocean to widen at only about 2 centimetres per year.
Jai Mehta 11AY
Conservative or transform Boundaries are places where plates slide past each other. Since the plates on either side of a transform boundary are just sliding past each other and not tearing or crunching each other nothing happens to the land. Instead, transform boundaries are marked in some places by linear valleys along the boundary where rock has been ground up by the sliding. In other places, transform boundaries are marked by features like streambeds that have been split in half and the two halves have moved in opposite directions.
The most famous transform boundary in the world is the San Andreas Fault, shown in the drawing above. The slice of California to the west of the fault is slowly moving north relative to the rest of California. Since motion along the fault is sideways and not vertical, Los Angeles will not crack off and fall into the ocean, as popularly thought, but it will simply creep towards San Francisco at about 6 centimeters per year. In about ten million years, the two cities will be side by side!
Although transform boundaries are not marked by spectacular surface features, their sliding motion causes lots of earthquakes. The strongest and most famous earthquake along the San Andreas Fault hit San Francisco in 1906. Many buildings were shaken to pieces by the quake, and the fires that followed destroyed much of the rest of the city. More than 600 people died as a result of the quake and fires. Recent large quakes along the San Andreas include the Imperial Valley quake in 1940 and the Loma Prieta quake in 1989.
Jai Mehta 11AY
There are two types of destructive boundaries, two continental plates colliding or a continental plate and an oceanic plate colliding. Firstly I will do continental and oceanic then continental and continental.
Convergent Boundaries or destructive costal boundaries are places where plates crash or crunch together. Plates only move a few centimetres each year, so collisions are very slow and last millions of years. Even though plate collisions take a long time, lots of interesting things happen. For example, in the drawing above, an oceanic plate has crashed into a continental plate. The two plates colliding are like looking at a single frame in a slow-motion movie of two cars crashing into each other. Just as the front ends of cars fold and bend in a collision, so do the "front ends" of colliding plates. The edge of the continental plate in the drawing has folded into a huge mountain range, while the edge of the oceanic plate has bent downward and dug deep into the Earth. A trench has formed at the bend. All that folding and bending makes rock in both plate’s break and slip, causing earthquakes. As the edge of the oceanic plate digs into Earth's hot interior, some of the rock in it melts. The melted rock rises up through the continental plate, causing more earthquakes on its way up, and forming volcanic eruptions where it finally reaches the surface. An example of this type of collision is found on the west coast of South America where the oceanic Nazca Plate is crashing into the continent of South America. The crash formed the Andes Mountains, the long string of volcanoes along the mountain crest, and the deep trench off the coast in the Pacific Ocean.
Jai Mehta 11AY
Destructive collision boundaries, these are places where two continental plates collide causing fold mountains, earthquakes, and volcanoes. Millions of people live in and visit the beautiful mountain ranges being built by plate collisions. For example, the Rockies in North America, the Alps in Europe, the Pontic Mountains in Turkey, the Zagros Mountains in Iran, and the Himalayas in central Asia these were formed by plate collisions. Each year, earthquakes and volcanic eruptions in those mountains kill thousands of people. Occasionally, big eruptions or earthquakes kill large numbers of people. In 1883 an eruption of Krakatau volcano in Indonesia killed 37,000 people. In 1983 an eruption caused mudslide on Nevada del Ruiz in Columbia and it killed 25,000 people. In 1976, an earthquake in Tangshan, China killed an astounding 750,000 people. On the other hand, earthquakes and volcanoes occurring in areas where few people live and therefore harm no one. If we choose to live near destructive collision boundaries, we can build buildings that can resist earthquakes, and we can evacuate areas around volcanoes when they threaten to erupt.