By using the magnetic reversal information preserved in the minerals of the mid-ocean ridge basalts, scientists were able to prove that the plates were moving, and that new crust was being formed and old crust was being destroyed in a continuous process that had been going on for most of Earth's history. The oldest crust in the ocean dates back to the early Cretaceous , million years ago, which is relatively recent in geologic time.
If this is the case, where did all the rest of the crust go? This leads us to the second type of plate boundary, which is called a convergent boundary or subduction zone. These are plate margins where one plate is overriding another, thereby forcing the other into the mantle beneath it. These boundaries are in the form of trench and island arc systems. All the old oceanic crust is going into these systems as new crust is formed at the spreading centers.
Convergent boundaries also explain why crust older than the Cretaceous cannot be found in any ocean basin-- it has already been destroyed by the process of subduction. Subduction zones are the location of very strong earthquakes, which occur because the action of the down going slab interacts with the overriding slab.
The "Ring of Fire" around the margins of the Pacific Ocean is due precisely to the subduction zones found around the edges of the Pacific plate. Subduction also is the cause of the volcanic activity in places like Japan: as the downgoing slab goes deeper beneath the overriding plate, it becomes hotter and hotter because of its proximity to the mantle. This causes the slab to melt and form magma, which moves upward through the crust and eventually forms volcanoes island arcs in oceanic crust or huge intrusive masses plutons and batholiths in continental crust.
The Aleutian Islands are another example of a surface expression of subduction. Sometimes, when there is a convergent boundary between two continental plates, subduction cannot occur.
Since continental crust is more bouyant, or less dense, than oceanic crust, one plate does not easily override the other. When the pressure is released the ruler will return to its original position because it is a rigid but elastic material. Imagine trying this experiment with plasticine.
The flexural bulge is a common feature of ocean-trench systems and is marked by a small increase in free-air gravity, while the trench itself is marked by a large decrease in free-air gravity Figure 17a. Such gravity variations imply that arc systems are out of isostatic equilibrium - the negative anomaly over the trench reflects a mass deficit, meaning that the crust in the trench must be being held down, while the increase over the flexural bulge implies that it is underlain by dense material beneath the plate.
The interpretation is that as the plate flexes upwards it 'pulls in' the asthenospheric mantle beneath. The isostatic imbalance, with the trench held down and the bulge supported, is due largely to the rigidity of the subducting plate.
While many ocean trenches are particularly deep, others are not. However, they are still characterised by a strongly negative free-air gravity anomaly, implying that they are filled with low-density material.
As a plate ages, it accumulates a veneer of deep-sea sediments made up of clays and the remains of micro-organisms in the oceans. At the subduction zone, this sedimentary cover is partly scraped off against the overriding plate to form huge wedges of deformed sediment that can eventually fill the trench system.
This material is often known as an accretionary prism. Not all the sediment is removed, however, and some remains attached to the descending oceanic plate and may become attached to the base of the overlying plates or even be carried into the upper mantle. Behind many island-arc systems, especially those in the western Pacific, small ocean basins open up between the arc and the adjacent continent.
Typical examples include the ocean basins immediately to the west of the Tonga and Marianas island arcs. Various lines of evidence show that these regions of ocean crust are very young and characterised by active spreading centres. Such features are known as back-arc basins. Does the existence of young oceanic crust suggest an extensional or a compressional tectonic regime in back-arc basins? An extensional tectonic regime.
New ocean crust is only produced when lithospheric plates move apart. The presence of an extensional regime in the back-arc region basin may appear counter-intuitive because where two plates are converging the dominant tectonic regime should be compressional.
The mechanisms that give rise to back-arc tension may relate to convection in the asthenosphere underlying the back-arc region. Alternatively, it has been suggested that old, dense slabs may subside into the mantle at a faster rate than the plate is moving, causing the trench to migrate towards the spreading centre euphemistically called 'slab roll-back'.
This gives rise to an extensional regime not only in the back-arc basin but also across the whole arc, even to the extent of suggesting that back-arc basins may originate as arcs that have been split by extension as a consequence of slab roll-back.
All of the major features of an oceanic destructive boundary are included in Figure 18, which is an idealised cross-section through an oceanic island-arc system. As you work through this course you will need various resources to help you complete some of the activities. Making the decision to study can be a big step, which is why you'll want a trusted University. Take a look at all Open University courses. If you are new to University-level study, we offer two introductory routes to our qualifications.
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One of the most famous transform plate boundaries occurs at the San Andreas fault zone, which extends underwater. Natural or human-made structures that cross a transform boundary are offset—split into pieces and carried in opposite directions. Rocks that line the boundary are pulverized as the plates grind along, creating a linear fault valley or undersea canyon.
Earthquakes are common along these faults. In contrast to convergent and divergent boundaries, crust is cracked and broken at transform margins, but is not created or destroyed. Virgin Islands. Home Ocean Exploration Facts What are the different types of plate tectonic boundaries?
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