Plate Tectonics(Part-II)
Revival of the Continental Drift Hypothesis
During the 1940s and 1950s, great advances were made in our knowledge of the sea floor and in the magnetic properties of rocks. Both of these fields of study provided new evidence to support continental drift.
Activity 1.6 The Location of Plates (Grades 4-6)
Activity 1.6 is a jigsaw puzzle. The plate tectonic map of the world has been cut along plate boundaries, and the pieces (plates) randomly arranged. Print the activity and ask students to cut out the pieces and reassemble the tectonic map. Describe the major geologic features that correspond to plate tectonic boundaries, i.e., mid-ocean ridges, trenches, and mountain ranges. Sea-Floor Spreading
In 1962, a geologist presented an explanation for the global rift system. Harry Hess proposed that new ocean floor is formed at the rift of mid-ocean ridges. The ocean floor, and the rock beneath it, are produced by magma that rises from deeper levels. Hess suggested that the ocean floor moved laterally away from the ridge and plunged into an oceanic trench along the continental margin.
A trench is a steep-walled valley on the sea floor adjacent to a continental margin. For example, ocean crust formed at the East Pacific Rise, an oceanic ridge in the east Pacific, plunges into the trench adjacent to the Andes Mountains on the west side of the South American continent. In Hess' model, convection currents push the ocean floor from the mid-ocean ridge to the trench. The convection currents might also help move the continents, much like a conveyor belt.
As Hess formulated his hypothesis, Robert Dietz independently proposed a similar model and called it sea floor spreading. Dietz's model had a significant addition. It assumed the sliding surface was at the base of the lithosphere, not at the base of the crust.
Hess and Dietz succeeded where Wegener had failed. Continents are no longer thought to plow through oceanic crust but are considered to be part of plates that move on the soft, plastic asthenosphere. A driving force, convection currents, moved the plates. Technological advances and detailed studies of the ocean floor, both unavailable during Wegener's time, allowed Hess and Dietz to generate the new hypotheses.
Layers of the Earth
The Earth is divided into three chemical layers: the core, the mantle and the crust. The core is composed of mostly iron and nickel and remains very hot, even after 4.5 billion years of cooling. The core is divided into two layers: a solid inner core and a liquid outer core. The middle layer of the Earth, the mantle, is made of minerals rich in the elements iron, magnesium, silicon, and oxygen. The crust is rich in the elements oxygen and silicon with lesser amounts of aluminum, iron, magnesium, calcium, potassium, and sodium. There are two types of crust. Basalt is the most common rock on Earth. Oceanic crust is made of relatively dense rock called basalt. Continental crust is made of lower density rocks, such as andesite and granite.
The outermost layers of the Earth can be divided by their physical properties into lithosphere and asthenosphere. 
The lithosphere (from the Greek, lithos, stone) is the rigid outermost layer made of crust and uppermost mantle. The lithosphere is the "plate" of the plate tectonic theory. The asthenosphere (from the Greek, asthenos, devoid of force) is part of the mantle that flows, a characteristic called plastic behavior. It might seem strange that a solid material can flow. A good example of a solid that flows, or of plastic behavior, is the movement of toothpaste in a tube. The flow of the asthenosphere is part of mantle convection, which plays an important role in moving lithospheric plates. Testing the Sea-Floor Spreading Hypothesis
Before being widely accepted, a new hypothesis must be tested. One test for the sea-floor-spreading hypothesis involved magnetic patterns on the sea floor. 
In the late 1950's, scientists mapped the present-day magnetic field generated by rocks on the floor of the Pacific Ocean. The volcanic rocks, which make up the sea floor have magnetization because, as they cool, magnetic minerals within the rock align to the Earth's magnetic field. The intensity of the magnetic field they measured was very different from the intensity they had calculated. Thus, the scientists detected magnetic anomalies, or differences in the magnetic field from place to place. They found positive and negative magnetic anomalies. Positive magnetic anomalies are places where the magnetic field is stronger than expected. Positive magnetic anomalies are induced when the rock cools and solidifies with the Earth's north magnetic pole in the northern geographic hemisphere. The Earth's magnetic field is enhanced by the magnetic field of the rock. Negative magnetic anomalies are magnetic anomalies that are weaker than expected. Negative magnetic anomalies are induced when the rock cools and solidifies with the Earth's north magnetic pole in the southern geographic hemisphere. The resultant magnetic field is less than expected because the Earth's magnetic field is reduced by the magnetic field of the rock. 
When mapped, the anomalies produce a zebra-striped pattern of parallel positive and negative bands. The pattern was centered along, and symmetrical to, the mid-ocean ridge.
A hypothesis was presented in 1963 by Fred Vine and Drummond Matthews to explain this pattern. They proposed that lava erupted at different times along the rift at the crest of the mid-ocean ridges preserved different magnetic anomalies.
For example, lava erupted in the geologic past, when the north magnetic pole was in the northern hemisphere, preserved a positive magnetic anomaly.
In contrast, lava erupted in the geologic past, when the north magnetic pole was in the southern hemisphere, preserved a negative magnetic anomaly.
Lava erupting at the present time would preserve a positive magnetic anomaly because the Earth's north magnetic pole is in the northern hemisphere.
Vine and Matthews proposed that lava erupted on the sea floor on both sides of the rift, solidified, and moved away before more lava was erupted. If the Earth's magnetic field had reversed (changed from one geographic pole to the other) between the two eruptions, the lava flows would preserve a set of parallel bands with different magnetic properties. The ability of Vine and Matthews' hypothesis to explain the observed pattern of ocean floor magnetic anomalies provided strong support for sea floor spreading. Subduction
If new oceanic lithosphere is created at mid-ocean ridges, where does it go? Geologists had the answer to this question before Vine and Matthews presented their hypothesis. In 1935, K. Wadati, a Japanese seismologist, showed that earthquakes occurred at greater depths towards the interior of the Asian continent. Earthquakes beneath the Pacific Ocean occurred at shallow depths. Earthquakes beneath Siberia and China occurred at greater depths. After World War II, H. Benioff observed the same distribution of earthquakes but could not offer a plausible explanation. 
The movement of oceanic lithosphere away from mid-ocean ridges provides an explanation. Convection cells in the mantle help carry the lithosphere away from the ridge. The lithosphere arrives at the edge of a continent, where it is subducted or sinks into the asthenosphere. Thus, oceanic lithosphere is created at mid-ocean ridges and consumed at subduction zones, areas where the lithosphere sinks into the asthenosphere. Earthquakes are generated in the rigid plate as it is subducted into the mantle. The dip of the plate under the continent accounts for the distribution of the earthquakes. Magma generated along the top of the sinking slab rises to the surface to form stratovolcanoes.
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