Plate Boundaries Help (page 2)
Convection causes plates to meet and separate. When this happens, there are three main types of plate boundaries that form. These boundaries include:
- Convergent boundaries – plates clash and one is forced below the other, pulling older lithosphere to the depths of the mantle (see trenches below).
- Divergent boundaries – plates pull apart and move in opposite directions making room for new lithosphere to form at the lip from out-pouring magma (see ridges below).
- Transform fault boundaries – plates slide past each other parallel to their shared boundaries.
Since the Earth’s core is hot in the extreme, roughly 6000°C, the malleable mantle beneath the brittle crust is always hot too. This constant heat production by the core keeps the cycle of convection going. The heat transfer from rising and sinking convection currents provides the power that moves plates around the globe.
The rate of plate movement varies a lot depending on location. In Africa, there is very little movement from year to year, while the active Pacific plate has sections that move as much as 10 cm/year relative to the hot spots.
A long, thin ocean valley, sometimes less than 100 km wide, with steep sides and caused by the descent of a plate’s edge back into the mantle, is called an ocean trench . Some of the deepest points on Earth are found within ocean trenches. The Java trench in the West Indies and the Mariana trench in the Pacific average between 7450 m and 11,200 m.
A trench is formed along the convergent boundary of two plates. Subduction digs ocean trenches when one plate collides with another, pushing it down underneath the first and causing a deep trench. The front edge of the top plate is crumbled and pushed up like snow in front of a snow plow. The clashing forces and constant pushing action along the border between two plates, form towering mountain ranges parallel to the trench like the Andes range along the Peru–Chile trench.
Before the idea of global plate tectonics was accepted, marine geologists were stumped over the formation of ocean trenches. They didn’t understand what was causing the ever deepening valleys in the ocean floor. They kept trying to figure out why the core or lower mantle seemed to be pulling down the asthenosphere. They didn’t know much about convection currents at that point and so had no energy source for the movement of the landmasses.
Because most subduction zones are found in the Pacific Ocean, the edges of the Pacific plate, where surface rock is constantly being pulled down and destroyed, has the most deeply grooved trenches. The Pacific Ocean is ringed by these trenches because of the constant plate action of the Pacific oceanic plates against the North American, Eurasian, Indian-Australian, Philippine, and Antarctic plates.
Trenches are found at both continental margins and at ocean–ocean convergence zones along island plate lines. The Java trench, also known as the Sunda trench is a deep depression in the Indian Ocean, 305 km from the coasts of the islands of Sumatra and Java, Indonesia. The trench is 2600 km long and is the deepest point in the Indian Ocean, the site of the massive Dec. 26, 2004 earthquake (9.0) and tsunamis, that killed over 200,000 people.
Twenty-two trenches have been identified though not all are major trenches. Of these, 18 are in the Atlantic and one (Java trench) is in the Indian Ocean. The depths of the major trenches are greater than 5.5 km deep and between 16 and 35 km in width. The deepest trench is the Challenger Deep (11 km deep) found in the Marianas trench. The Peru–Chile trench, off the coast of South America, is the longest trench at 1609 km in length, while the Japan trench at 241 km is the shortest.
A rift or upgrowth of the ocean floor, where plates are slowly edged apart by the filling of hot magma, is known as an ocean ridge . Ridges are formed along divergent boundaries where plates move slowly away from each other. Magma then rises into the crack between them, filling it, and hardening into rock. Figure 4-6 shows how this seafloor growth takes place.
Fig. 4-6. Upswelling magma adds to seafloor spreading at divergent boundaries.
Most of the magma exiting the mantle today is found at ridges in the ocean floor and along plate edges. When magma pours out of cracks in the ocean floor, they build up a lip along the crack and form mid-ocean ridges. Ridges thousands of miles long can be found in the Atlantic Ocean, and around the plate borders of the Pacific plate.
Cooled magma (lava) also flows horizontally forming more ocean floor and piling up around vertical vents to form volcanic cones and islands like the Hawaiian Islands and the Galapagos chain. These are hot spots. The unending creation of solidified magma (rock) creates new seafloor and widens the ocean basins, a process called seafloor spreading .
When British geologists, Drummond Matthews and Fred Vine sampled rocks along the edges of ocean ridges, they found that the farther away they were from the ridge crest, the older the rocks. When this information was added to the idea of continental drift and seafloor spreading, it helped explain the puzzling increase of crustal landmass and supported the plate tectonics theory.
Nearly all of the ocean ridges are at the bottom of the oceans, but the Mid-Atlantic Ridge that stretches up the center of the Atlantic Ocean, emerges in a few places including Iceland, where geologists can measure its growth and characteristics.
Below the waves, photographs from submarines at great ocean depths show that rocks near ridge edges are clean and sharp. As the distance from the ridge increased, rocks became covered with sediment. At about 10 km (6 miles) from a ridge, the rocks are completely obscured from sight by layer upon layer of sediment dusted over them for millions of years. We will take a closer look at the hardening of sediment into rock.
Practice problems of this concept can be found at: Plate Tectonics Practice Test
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