The rocks of the ocean floor differ from the continental crust not only in composition, but also in age. Continental rocks have been dated as old as 3.5 billion years and ages of 1.5 billion are not uncommon. In contrast, the oldest ocean floor material is less than 200 million years old and the rocks of the oceanic rises are younger. This startling data is explained by the spreading sea floor and plate tectonics concepts. Oceanic crust is being formed continuously from intrusion of mantle materials at the oceanic rises creating new, young ocean floor. As the plates move apart, the inflow of molten lava forms new basaltic sea floor. The other sides of the plate are shear fault zones of transcurrent and transform faulting. The older oceanic crust is drawn downward with the upper mantle as the lithosphere subducts in the deep ocean trench regions. This does not happen with the continental crust. Because of its lower density, it rides upward and the continents are progressively built up while the ocean floor is always in the process of being renewed and destroyed.
The features and changes in the earth that were proposed by adherents of continental drift and spreading sea floor are part of the plate tectonics theory -- the changes are in the mechanisms, plates and their boundaries, and the lithosphere concept. The driving force is still convection current circulation, but additional thought has been given as to how the plates are moved.
About 200 million years ago, the continental land masses were joined as one major continent . There were movements prior to this which can be traced, but because of limited outcrops, the records are incomplete. The convergent movement is reflected in the record of major mountain building. The latest episode of plate movement resulted in the spreading apart of the continents to their present position -- a movement that is still in progress. We know that the changes in movement patterns occurred, but do not know the causes.
The Atlantic Ocean, the Caribbean Sea, and the Indian Ocean are new ocean basins. The Mediterranean Sea is a remnant of the Tethys Sea and is still undergoing closure. The Pacific Ocean is bounded by subduction zones and it is closing as the Atlantic grows. The oldest oceanic crust is found in the northwestern corner of the Pacific because it is an ocean where crust has been destroyed while new crust was forming in the other oceans. The Americas have drifted west, while those continents around the Indian Ocean have moved north. Antarctica has remained almost stationary, as has Eurasia, but Eurasia is undergoing clockwise rotation in which Europe has moved north and China southward. The last major event in this movement was the detachment of Australia from Antarctica in the Eocene - about 55 million years ago. Assuming a continuation of the present movement, we can predict a pattern of distribution for a 50 million year future world.
If we use the 1000 meter contour on the continental slope to define the edges of the continental blocks, there is an extraordinary fit of South American to Africa and North America to Eurasia, India, Australia, and Antarctica fit to show the form of Pangaea. This fit is reinforced by using paleomagnetic data in the reconstruction, and we can see the fit of tectonic and stratigraphic trends that are older than 200 million years.
The origin of today's oceans lies in the pattern of breakup and movement of the plates. Since the movements create strains and release at the boundaries, the plate boundaries are marked by earthquake activity. Earthquake distribution is not random and the zones of activity extending through the Mediterranean, Middle East, Northern India, around the Pacific, and along the ocean rises mark principal plate boundaries.
Differences in earthquake focus and intensity allow us to distinguish different types of boundaries. Under the median valley of the ocean rise where the plates diverge, earthquakes are relatively shallow and because of less crustal rigidity and states of tension, are of relatively low intensity. Where the plates slide past one another along transform faults -- as on the north coast of Turkey -- and the ocean rise offsets, the earthquakes are shallow to intermediate depths. On the rises, the intensities are not severe, but where the trace cuts through continental edges, major earthquakes can occur. Intermediate and deep focus earthquakes are restricted to convergent boundaries where subduction occurs.
The ocean rise looks like a sinuous mountain chain some 1000 km wide. The axis has straight crestal/median valley segments that are cut and offset by transform faults normal to the median valley. This pattern results from the spreading process which adapts itself to the shapes of the retreating continents. Rifting of the median valley is related to the rising convection cell. The warmer convecting mantle creates a zone of upward pressure which forces the walls of the crest upward and outward and linear fissures open a valley flow. Magma wells up through these fissures and erupts to form small hills of pillow lava. These have been photographed in the mid Atlantic rift valley. In the crust below the eruption, the magma cools, forming a dike of volcanic rock (basalt). The next split pulls the dike apart and more magma is injected along the axis of the previous dike. Each of these injected dikes is given a magnetic polarity signature as it cools. Each successive injection adds younger material as the older dikes are carried sideways and bilaterally away from the injection axis.
The immediate magma source is an intermediate chamber within the oceanic crust along the axis. This chamber is refueled from within the inner mantle. As the plates move apart, the chamber walls are carried sideways and molten rock solidifies against the walls. These plutonic bodies cool slowly forming coarse grained basic igneous rocks. These are layer 2 and layer 3 of the oceanic crust. Layer 1 is the accumulation of sedimentary layers that are deposited over the new crust.
These are simply strike-slip faults between plates. The kind of motion is changed -- transformed -- at the ends of the active part of the fault. Transform faults connect convergent and divergent plate boundaries in various combinations. Where segments of the oceanic ridge have been offset, a transform fault connects the two divergent plate boundaries and creates a major topographic feature called a fracture zone. Beyond this zone, the plates on either side of the fracture are moving in the same direction and rate, and can be considered to be linked together.
The oceanic ridge is not being offset by motion along the transform fault; it was offset previously and may represent an old line of weakness in the rifted continental crust that preceded the development of the oceanic crust. It may also represent different cells of convection or plumes in the mantle.
Within the craters of Hawaiian volcanoes, lava lakes show a ridge-forming process . As molten lava cools, a solid crust forms over the lake, but the crust splits into sheets and moves. Because it is cooler and denser than the underlying liquid, large slabs of the crust break up, split, and sink. Molten lava rises from below and creates a zone of new cooling lava. Spreading ridges, transform faults, and subduction zones are all observed. From this perspective, it is more accurate to think of the plates and mantle as forming a single, though complex system, with each portion of the system affecting the other.
Oceanic crust meets oceanic crust in the Pacific and it seems to be a matter of chance as to which plate is subducted. Along the Tonga-Kermadec Trench, the Pacific plate moves under the Indo-Australian lithosphere but in the nearby Solomon basin along the same plate boundary, the Indo-Australian plate is subducting. At both sites, chains of volcanic islands form above the descending plate and the subduction zone is marked by earthquakes.
When two plates collide in a zone of convergence and one plate (oceanic) passes under the other, the layer of sedimentary rock on the oceanic plate is scraped off and accumulates as an island arc or against a continental margin. The lithosphere (oceanic crust - upper mantle) slab of some 100 km thickness subducts at an angle of some 30 degrees with melting of the surface due to friction and pressure. At depths of 100 to 300 km (asthenosphere layer), the lighter molten rocks force their way upward behind the subduction zone, forming a volcanic chain . The slab of subducted lithosphere moves downward causing earthquakes until it finally breaks up at a depth of about 700 km. This whole sloping surface is an area of shallow to deep earthquake activity of major intensity.
The Marianas Trench is separated from Asia by the Philippine Sea which is a case of oceanic crust colliding with a marginal sea bordering a continent. Upwelling magma from the subducting oceanic crust is trapped between Asia and the Pacific. The marginal back-arc sea floor is active and local spreading centers force the crust toward the Pacific plate.
When oceanic crust meets continental crust, the oceanic plate subducts. As the Nacza plate moves under South America, earthquakes and volcanic activity are associated with the Andes. Subduction destroys oceanic crust and reduces the oceanic area. The final ocean extinction occurs when the continental blocks meet in collision . The collision of India and Asia 30 million years ago uplifted continental areas into the Himalayas and the approach of Africa to Europe will soon (less than 50 million years) eliminate the Mediterranean, Baltic, and Black Seas and then form another great mountain chain.
Numerous lines of data and evidence lend support to the concept of a mobile lithosphere. Many of these have already been mentioned. Single items or several, and indeed all of the points can be refuted or explained in other ways. However, as in a job of police detection, it is the fact of accordance and the weight of the evidence as each bit adds more support which gives credence to the theory of plate tectonics. As presented, the plate tectonics concept makes a neat package, providing a framework for interpreting geological processes that works.
The basic movements of lithospheric plates and tectonic features resulting from the movement have been described in detail, but no explanation has been given for epirogenic events in the platforms, block faulting, and other non-marginal features. Geologists have described regularities and cyclic events in the history of the earth, and the concept of plate tectonics may be a random, incidental occurrence of tectonic activity. There is an inherent limitation of application of plate tectonics that is neglected by its adherents. Processes within the earth may generate various modes of tectonic behavior -- one of which is the plate response. An unfortunate tendency has been selectivity in choice of data used, attributing all features to drift patterns, to base broad interpretations on limited data, and worst of all, to be woefully ignorant of basic geological knowledge in many of the model areas.
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| plate tectonics - you try it |