Volcanoes are also created by plate tectonics. Lets take the Hawaiian volcanoes, which are situated off the plate boundaries as an example. Hawai'i is one of the places where volcanism occurs away from a plate boundary. This is because of two reasons. Firstly, Hawai'i is situated at a point where the crust thins to about 5 km; secondly, beneath the Hawai'ian island is a rising plume known as a hotspot. Although most textbooks theorize that these hotspots remain fixed in position, there is evidence that the Hawai'ian hotspot has moved slightly. Nonetheless, the theory still applies. The Pacific plate moves over the hotspot, and as it does a volcanic island is formed. But the plate continues to move on, so a chain of volcanoes is generated, the oldest becoming extinct and ultimately being eroded away into a form of seamount. In the case of Hawai'i, the Pacific plate has moved northwest, and the oldest (dormant) volcano is Kuaui. The youngest is Hawai'i, and the latest active volcano is Loihi, off the coast of the main island.
When plates slide across each other there is a mass amount of friction and the two plates will then jerk about pushing against each other. This can create earthquakes and destroy large buildings.
On January 25, 1999 a 6.2 magnitude quake near the city of Armenia, Columbia, for example, killed an estimated 1,185 people and injured some 4,750. About 60% of all buildings were destroyed. Many burned to the ground. We are trying to stop structural damages as repairs cost millions by installing large shock absorbers. One cost-effective strategy for limiting the effects of a major quake would be to provide earthquake protection kits, seismic upgrades and retrofits to older buildings. Nickel-containing stainless steels resist corrosion, are fire-resistant and have a higher tensile strength and greater elongation than ordinary steel, all of which renders them well suited to solving problems related to earthquakes. For example, the outstanding plastic deformation capability of stainless steel and its ability to absorb a tremendous amount of energy has proved advantageous when used in space trusses, bracing, column bases and base-isolated structures.
We know plates move as all of the continents seem to be like a large jigsaw and the same animals and plants are found where they used to be connected. Scientists believe that plates move due to convection currents in the magma of the earth. This is quite similar to lava lamps. The drawing below shows that the hotter areas in the magma are rising, and then fall as they cool.
Wegener was browsing in the university library when he came across a scientific paper that listed fossils of identical plants and animals found on opposite sides of the Atlantic. Intrigued by this information, Wegener began to look for, and find, more cases of similar organisms separated by great oceans. Orthodox science at the time explained such cases by postulating that land bridges, now sunken, had once connected far-flung continents. But Wegener noticed the close fit between the coastlines of Africa and South America. In 1915 the first edition of The Origin of Continents and Oceans, a book outlining Wegener's theory, was published; expanded editions were published in 1920, 1922, and 1929. About 300 million years ago, claimed Wegener, the continents had formed a single mass, called Pangaea (from the Greek for "all the Earth"). Pangaea had rifted, or split, and its pieces had been moving away from each other ever since. Wegener was not the first to suggest that the continents had once been connected, but he was the first to present extensive evidence from several fields.
Above is a map of Pangea, which was all of the continents, joined together about 300 million years ago. No one at first believed him. Part of the problem was that Wegener had no convincing mechanism for how the continents might move which Wegener thought that the continents were moving through the earth's crust, like icebreakers ploughing through ice sheets. There were scientists who supported Wegener: the South African geologist Alexander Du Toit supported it as an explanation for the close similarity of strata and fossils between Africa and South America, and the Swiss geologist Émile Argand saw continental collisions as the best explanation for the folded and buckled strata that he observed in the Swiss Alps.
Wegener's theory found more scattered support after his death, but the majority of geologists continued to believe in static continents and land bridges. Since Wegener's day, scientists have mapped and explored the great system of oceanic ridges, the sites of frequent earthquakes, where molten rock rises from below the crust and hardens into new crust. We now know that the farther away you travel from a ridge, the older the crust is, and the older the sediments on top of the crust are. The clear implication is that the ridges are the sites where plates are moving apart. The model of the Earth developed by the seismologists, at this time, was a liquid iron core surrounded by a solid mantle with no convection movements. When Elsasser and Bullard (1965) developed their geomagnetic field theory, postulating that there are convective motions in the fluid iron core, there was no real objection by the seismologists since the core did not transmit s-waves, indicating it is a classical fluid. It was not until the development of paleomagnetism that there was new evidence for continental drift, then later on, geophysical measurements of the ocean floor swept away most of the doubts geophysicists had about continental drift. This now constitutes part of the subject called plate tectonics.
Many theories on the mechanism for plate movement have been developed. The most popular and widely held view is that convection currents below the lithospheric plates, in the mantle, are responsible for their movement. This involves hot spots and subduction zones. The most radical view was that that developed by Carey (1954), Heezen (1959) and others, that the earth is expanding causing the continents to break up and form plates.
