The Ever-changing Earth
In the 1960's, the new theory of plate tectonics began a revolution in earth sciences. Since the theory came into play, it has been refined and verified by scientists everywhere. We now have a better understanding of the plates and how the influence all things. We now view our world differently everyday.
We all benefit from the forces and are at the mercy of consequences of plate tectonics. With little warning, an earthquake or volcanic eruption can bring wraith down upon us. We have no control over these forces, but we now have the knowledge of how they work. The more we learn the better we become at preventing disasters and utilizing this for our benefit.
Introduction to plate tectonics by Dr. James Beard (wav 479kb)
About 225 million years ago the continents on our planet started out as a supercontinent, called Pangaea (meaning all lands in Greek). The earth then evolved into the Triassic period, around 200 million years ago. During this period Pangaea broke into two supercontinents called Laurasia and Gondwanaland. After the Triassic period came the Jurassic period (135 million years ago), in which Gondwanaland and Laurasia split into many continents forming the beginning of what we know today. Next, in the Cretaceous period (65 million years ago) the continents further split apart and were almost where they are today. Today we are in the Cenozoic period. The continents appear as you see them on the globe now.
In Alfred Wegener's mind, the continental drift theory not only explained the only matching fossil occurrences but the evidence of major climate changes as well. For example, the discovery of coal deposits(fossils of tropical plants) in Antarctica pointed out the conclusion that Antarctica was previously near the equator where dense climate vegetation could grow.
The theory would soon become a controversial issue that started a different way of how we look at the Earth. At the time when the theory was introduced by Wegener, most scientists viewed the theory to be absurd and said that the oceans and continents were permanent features of the Earth's surface. His theory was rejected by most, although it did support the given evidence at the time. The community still persisted in knowing what forces could move such large masses of rock. Wegener told them that the continents just simply plowed through the ocean floor. Harold Jeffreys disagreed and stated correctly that it was impossible for such a large mass of rock to simply plow through the ocean floor without crumbling itself.
For the rest of his life Wegener looked for additional evidence to back up his theory and to satisfy his critics. He died in 1930, frozen to death during an expedition crossing the icecaps of Greenland. After his untimely death other evidence rose up to help support his theory. This evidence, gathered from ocean floor exploration, led to the theory of plate tectonics.
Not only did the continents fitting together support Wegener's theory of continental drift, but so did the evidence of matching fossils and plants on matching coastlines. The animals that lived then could not have swam across the huge oceans. The continents were at one time joined for such a phenomenon to occur. More obvious evidence includes the (tropical plant) fossils found on Antarctica which shows that Antarctica was once located somewhere warmer. Also evidence is presented by the fact of the marsupial. There are various types of these 'pouched' mammals in Australia but in North America there is the marsupial the opossum. According to the theory of evolution, for opossums to have gotten to North America, Australia and North America had to be one time connected.
Scientists began to discover seafloor spreading around 1961 when they saw that the ocean floor was being pulled apart at mid-ocean ridges. Magma from deep in the earth is pushed to the top of the ridge to form new crust. Then as more magma pushes to the top of the ridge the crust splits and spreads outward, thus making the ocean floor. This process over millions of years has helped to form the 50,000km long system of mid-ocean ridges. Other evidence for oceanic spreading includes the following; (1)The rocks are younger at the top of the ridge, but as you move farther away from the ridge the rocks get older. (2) The Glomar Challenger was built for marine geology study after World War II. The Glomar Challenger embarked on an expedition to date petroleum deposits in the ocean. They took samples of the petroleum and used paleontology dating to finally prove the oceanic spreading theory.
There are three main types of plate boundaries. These plate boundaries trigger many disasters such as earthquakes and volcanoes. Divergent boundaries are caused when new crust is produced as the plates pull away from each other. An example of a divergent boundary would be the Mid-Atlantic Ridge. The Mid-Atlantic ridge is an underground mountain chain that moves 2.5cm annually. Convergent boundaries occur when two plates collide into each other. Two plates can either crash head on into each other or one of the plates can be taken under by the other, usually an oceanic plate gets subducted by the bigger continental plate. When a continental plate crashes with another continental plate mountains are formed. An example of that convergence would be the Indian plate colliding into the Eurasian plate forming the Himalayan mountain chain. An example of an oceanic plate being subducted under a continental plate is the oceanic Nazca plate and the continental South American plate. This convergence is creating the very deep Peru-Chile trench and the towering Andes on South America. When two oceanic plates collide they create deep trenches. An example of this convergence is the oceanic Pacific plate and the oceanic Philippine plate. These two plates are creating the Marinas trench and small volcano arcs in the ocean. The last boundary called transform boundaries are produced when two plates slide past each other. An example would be the San Andreas fault in California.
The plates can not float across the earth's surface. There is a very powerful force moving the plates, into, under, and side by side other plates. Hot, molten material fuses and creates convection currents, below the plates and forces them to move. Plates move at different rates per year. Scientists find out how much the plates move a year by using a GPS satellite and a ground receiver. Such places as the Arctic ridge move about 2.5cm a year others move more rapidly such as the East Pacific Rise near Easter Island moves about 15cm a year. Satellites have been the most useful in recording plate motion. During large earthquakes plates have been known to move more. The biggest move made by an earthquake in history was 33 feet.
Ring of Fire
The Ring of Fire is the western coastal area of the North and South American continents, the south cost of Alaska, the eastern coast of Asia and Japan, Indonesia and Oceania. The Ring of Fire is the major region of volcano and earthquake activity in the world. Around the whole rim of the Pacific plate, there is a large amount of activity. Most of the worlds major earthquakes have happened here, as with the world's volcanoes. We learn a lot from this region due to the numerous observatories and scientific monitoring posts located here. Every earthquake that happens is recorded from these sites, which help us also to see warnings of possible new volcanoes.
The tectonic plates do not just move around the earth when they feel like it, they are driven by an unexplained, but definite force. Scientists can not fully understand or explain these forces. Most of the scientists believe that the shallow lithosphere forces work with unseen forces deep within the earth. This theory was originally proposed by Arthur Holmes who influenced Harry Hess to draw it up. His theory actually states that the mobile rock beneath the rigid plates moves in a circular manner like a pot of thick soup boiling. The soup rises to the top, spreads and cools. It then sinks back to the bottom where it is reheated and then it rises again. This cycle is repeated over and over to make what he called a convection cell or convection flow. You can see this process easily in a pot of soup but it is harder to grasp when dealing with the earth. We know the process in the Earth is much slower, but there are still many unanswered questions: How many of these cells exist? How are they made up? Where do their originate? How?
Links for more info
- USGS "The Dynamic Earth"
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(All images on this page provided by United States Geological Survey)