Plate Tectonics II: Subduction Zones, Hot Spots & Triple Junctions
Goals
Introduce the transform fault and fracture zone.
Use geologic cross sections through the oceanic crust to demonstrate the nature of different plate boundaries.
Examine the origin of the arc-trench gap in subduction zones.
Demonstrate the relationship between sea floor spreading and ocean-floor topography.
Introduce hot spots and demonstrate their importance in understanding plate dynamics
Introduction

In addition to discovering the midocean ridge system, the oceanographic surveys of post-World War II revealed several other interesting features of the ocean floor. Marine geologists recognized large linear structures associated with the ridges, deep-sea trenches typically fringing the continents, long chains of extinct volcanoes and isolated undersea mountains. At first, these features were difficult to explain using conventional geologic theories. However, in the mid-1960s, each was recognized as playing an important role in the process of plate tectonics. As such, the understanding of each greatly aided the development of this important revolution in geologic thinking.



Fig. 1: Subduction Zones of the World
The linear features were most often oriented at right angles to the midocean ridge system and offset the ridge. Some of these structures extend nearly halfway across the ocean basins. Among their unusual features were
  1. Significant differences in sea-floor elevation across them
  2. Termination at continental margins
  3. Confinement of seismic activity to the area between the ridge crests (a small fraction of the total length of the linear zone)
  4. Offset of magnetic lineations across the structures
  5. Significant differences in ocean floor age on adjacent sides of the structure.

The major conceptual breakthrough that explained the origin of these structures and radically altered our interpretation of the oceanís physiographic features was the concept of the transform fault. This theory, proposed in 1965 by J.Tuzo Wilson, suggested that transform faults were a class of faults with characteristics very much different from those associated with faults on land. Wilson's concept of the transform fault was key in the development of the theory of plate tectonics and was important in explaining how plates interact.
The deepest spot on Earth is located in the Marianas Trench near the Philippines and is 11,021 m (36,160 ft) below sea level. Deep-sea trenches, like the Marianas, were found to occur along the coasts of many continents and within the ocean basins. Trenches near land were filled with thick wedges of sediment, whereas those far removed from the continents lacked significant amounts of sediment. Trenches extend for thousands of kilometers, are 40 to 120 km (25 to 75 mi) wide, and are sites of high volcanic activity and earthquakes. Such features are difficult to explain as the result of simple vertical movement of crust. Their origins were not adequately explained until the concept of subduction zones was outlined by Robert Coats in 1962.
Another interesting feature of the ocean floors is the many submarine mountains. Some of these mountains have simple conical shapes, whereas others have flat tops. In addition, they occur as isolated structures or in long linear chains. When found in chains, there is a linear progression in age along the chain. In all instances, these submarine mountains are mostly volcanic in origin. With the development of the plate tectonic theory, it was realized that these structures are either hot spots or the products of hot spots. As with the other features, they only make sense within the framework of plate tectonics. Thus three seemingly unrelated physiographic features of the ocean basins are, along with the midocean ridge system, key in developing the concept of plate tectonics.

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