The elastic rebound theory: Before the theory of continental drift there were many beliefs to why and how earthquakes occurred. Following the great San Francisco earthquake Henry Fielding Reid examined the displacement of the ground surface around San Andrea's fault. He concluded that the earthquake must have been the result of the elastic rebound of previously stored elastic strain energy in the rocks on ether side of the fault.
On January 6 1912 Alfred Weger presented to the German geological society his hypotheses that all the continents were once together in a super continent called Pangaea. Pangaea came from the Greek word " all lands". He had a lot of evidence. Similar plant and animal fossils are found around the shores of different continents suggesting that they were once joined. The discovery of fossils of the land reptile Lystrosaurus in rocks of the same age at locations in Africa, India and Antarctica. Even living evidence of animals alive today and being found in two continents. Some earthworm families are found in South America and Africa. Wide spread distributions of permo-carboniferous glacial sediments in South America, Madagascar, Arabia, India, Antarctica, and Australia was one of the major pieces of evidence for the theory of continental drift. The puzzle-like arrangement of the facing sides of South America and Africa is clear. In millions of years of continental drift will further separate and rotate the two continents. It was this that got Weger to study what he defined as continental drift. The theory of continental drift was not accepted for many years. The first problem was that a driving force was missing. Also Weger was not a geologist but a meteorologist. One thing Weger was not able to figure out was what the nature of the forces moving the plates was.
British geologist Arthur Holmes proposed in 1931 that the earths mantle contained convection cells that dissipated radio active heat and moved the crust at the surface. It is now known that there are two kinds of crust, continental crust and oceanic crust. Continental crust is inherently lighter and its composition is different from oceanic crust, but both kinds reside above a much deeper plastic mantle. Oceanic crust is created at spreading centers and this along with subduction drive the system of plates in a crazy manner resulting in continuous isostatic imbalance. In geological terms a plate is a large rigid slab of solid rock . The word 'tectonics' comes from the Greek word ' to build '.The theory of tectonics explains all this including the movements of the continents. The Himalayas, the worlds tallest mountain range, are assumed to have been formed by the collision of two major plates. Before uplift they were covered by the Tethys Ocean. New oceanic crust is formed when new magma rises and erupts along the spreading ridges to form new crust. In effect, the ocean basins are being 'recycled' with the creation of new crust and the destruction of old oceanic crust occurring simultaneously. Therefore the new concepts neatly explain why the earth does not get bigger with sea floor spreading, why there is so little sediment accumulation on the sea floor and why oceanic rocks are much younger then continental rocks.
Earthquakes, Volcanic activity, mountain building and oceanic trench formation occur along plate boundaries. Tectonic plates can create mountains, mid-oceanic ridges depending on which way the plates are moving:
plates moving together- earthquakes, mountains and volcanoes
plates moving apart- earthquakes and trenches
plates moving side to side- earthquakes
A Geophysicist named J. Tuzo Wilson came up with an idea to explain why there was volcanic activity out in the middle of the pacific ocean, in the middle of the pacific plate. During this time scientists thought that volcanoes only happened at plate boundaries, but no one knew why there was volcanic activity in the middle of the pacific plate. Dr.Wilson said that there were hot spots under the earths crust. These are called hot spots because they are places where there is lots of heat concentrated in one area. The heat causes the rock to melt. Since magma is liquid and is lighter then the surrounding rock it floats to the surface and forces its way out to the surface. Once magma is through the surface it is called lava. After a long time the constant out pouring of lava can form a sea mound or island volcano if the hot spot is under the sea floor like the Hawaiian islands. The hot spot never moves but the plate does and as it moves over the hot spot new volcanoes form on the plate each time.
If Dr Wilson theory was right it explained why a chain of sea mounds and volcanoes had formed as the plate moved. If this was true then the volcanoes should be different ages so Dr Wilson took rock samples from each Hawaiian island and tested to see how old they were. He found that the oldest rocks were in the more northern island and were the most weathered rocks. The youngest rocks were found in the biggest island of the chain Hawaii which is the most south of the islands. Even now there is a new volcano forming on the sea floor south of Hawaii called Loihi. It is just a sea mound but some day it will be a new island.
