The Andes Mountains of South America and the Cascade Mountains of North America are examples of volcanic arcs. They are unlikely to form and would quickly become oceanic-oceanic divergent margins as seafloor spreading occurred. Continental-continental divergence causes a continent to separate into two or more smaller continents when it is ripped apart along a series of fractures. The forces of divergence literally tear a continent apart as the two or more blocks of continental crust begin slowly moving apart and magma pushes into the rift formed between them. Eventually, if the process of continental rifting continues, a new sea is born between the two continents. Rifting between the Arabian and African plates formed the Red Sea in this way.

S edge is on the same plate as the adjacent ocean, and it is along passive margins that accretionary wedges form. Technological advances during World War II made possible the accumulation of significant evidence that gave rise to modern plate tectonic theory, which explained how continents could move. The theory of plate tectonics gained widespread acceptance only in the late 1960s through the early 1970s.

Transform margins are less spectacular than convergent and divergent ones, and the type of plates involved is really of no significance. As two rock plates slide past one another at a margin, a crack or fault develops. The energy generated by the movement is often released in the form of an earthquake. The best known example of a transform plate margin is the San Andreas Fault in California, where the Pacific and North American plates are in contact.

SEAFLOOR SPREADING:

A constructive plate margin occurs when two plates move away from each other. It is marked by a mid-oceanic ridge where basaltic material wells up from the mantle to form new oceanic crust, in a process known as sea-floor spreading. The production of new crust at constructive plate margins is compensated for by the destruction of material along a destructive plate margin. Along these margins, which are also known as subduction zones and marked by an oceanic trench, one plate is forced to plunge down beneath the other . The crust becomes partially melted and rises to form a chain of volcanoes in the upper plate parallel to the trench. When two continental plates collide the compression results in the formation of mountain chains.

Forms of particles or energy include alpha particles , beta particles , or gamma rays, which occupy the highest energy level in the electromagnetic spectrum. The upper layer of Earth’s interior, including the crust and the brittle portion at the top of the mantle. The uppermost division of the solid earth, representing less than 1% of its volume and varying in depth from 3-37 mi.(5-60 km). A form of stress produced by the action of equal and opposite forces, the effect of which is to reduce the length of a material. (Compression is a form of pressure.) As for shear, this is a kind of stress resulting from equal and opposite forces that do not act along the same line.

It is heated up, and while there isn’t enough heat to melt the subducting crust, there is enough to force the water out of some of its minerals. This water rises into the overlying mantle where it contributes to flux melting of the mantle rock. The mafic magma produced rises through the mantle loveagain to the base of the crust. There it contributes to partial melting of crustal rock, and thus it assimilates much more felsic material. That magma, now intermediate in composition, continues to rise and assimilate crustal material; in the upper part of the crust, it accumulates into plutons.

Tracking subduction through geological time and dating the onset of plate tectonics on Earth

At triple points where three plates converge (e.g., where the Philippine sea plate merges into the North American and Pacific plate subduction zone), the situation be-comes more complex. Also, mid-plate stresses can exist due to forces acting on plate boundaries and may result in bowing, fracturing, or mid-plate earthquakes such as the 1812 New Madrid earthquake, which devastated part of what is now the state of Missouri. At an ocean-continent or ocean-oceanconvergent boundary, oceanic crust is pushed far down into the mantle (Figure 4.4b).

EARTH’S TECTONIC PLATES

“It’s responsible for mediating the climate on long geological time scales, making sure the climate is more or less temperate for life,” says Roger Fu, a geophysicist at Harvard University. The geometric method gives scientists the spreading direction to go with the spreading speed. If you look at a spreading ridge on a map, it has a stair-step pattern of segments at right angles. If the spreading segments are the treads, the transforms are the risers that connect them. With plate speeds and directions, you have velocities that can be plugged into equations. One assumption behind plate tectonics is that the lithosphere is rigid, and indeed that is still a sound and useful assumption.

They melt and the new magma rises, to be erupted anew out of volcanoes. Scientific revolutions in other fields, new ideas and interpretations do not invalidate earlier observations. On the contrary, the theories of seafloor spreading and plate tectonics offer for the first time a unified explanation for heretofore seemingly unrelated observations in the fields of geology, paleontology, geochemistry, and geophysics.

Specifically, they did not see how continental rock could plow through the much denser rock that makes up oceanic crust. Wegener could not explain the force that drove continental drift, and his vindication did not come until after his death in 1930. By 1915, after having published a first article in 1912, Alfred Wegener was making serious arguments for the idea of continental drift in the first edition of The Origin of Continents and Oceans. In that book (re-issued in four successive editions up to the final one in 1936), he noted how the east coast of South America and the west coast ofAfrica looked as if they were once attached. Furthermore, when the rockstrata of the margins of separate continents are very similar it suggests that these rocks were formed in the same way, implying that they were joined initially.