Ever since the beginning on time, Humans believed the ground is solid and immobile. But this is not true whatsoever. The Earth is every-changing and continually in motion. The stability of the Earth is not at all what we think it is. Thinking about the rotational axis of the Earth, and possibly of what the Earth may become at a certain point in time, has a great influence on understanding all aspects of living things, either in the past, present, or future. The study of Plate tectonics is accredited to most of the creations of Mountain Ranges, the drifting of continents, earthquakes, and volcanic activity.
Plate tectonics and mountains also play a big part in the geological features of our planet or any planet for that matter. Geophysics, which studies the physics of the Earth, has led to many important findings about the Earth and how it is made. Seismologic studies of planet Earth have revealed new information about the inside of the Earth that has helped to give new openings in understanding plate tectonics. The Earth is made of several significant layers. Each one of these layers has its own properties. The crust is the outermost layer of the Earth. The crust is made up of the oceans and continents.
The crust has a fluctuating thickness, being thirty to seventy-five kilometres thick in the continents and ten to fifteen kilometres thick in the ocean basins. The crust is made up mainly of alumino-silicates (Fowler p472). The layer underneath the crust is the mantle, which is made up mainly of ferro-magnesium silicates. The mantle is approximately two thousand, nine hundred kilometres thick, and is separated in to the upper and lower mantle. It is in the mantle where most of the centralized heat of the Earth is located. Big convective cells in the mantle disperse heat and produce the plate tectonic processes.
The core is the last layer of the Earth, which is broken down into the liquid outer core and the solid inner core. The inner core is about thirteen hundred kilometres thick and the outer core is about twenty-three hundred kilometers thick. A nickel-iron alloy mixture makes up the outer core, and the inner is almost all composed of iron. The Earth is separated in layers based on composition and mechanical properties. The top layer is the lithosphere, which is comprised of the solid upper mantle and the crust. It is divided into plates that move due to tectonic forces. The lithosphere
floats on top of a semi-liquid layer that is called the asthenosphere. The asthenosphere allows the lithosphere to move around since it is much weaker (Tarbuck p605). Early scientist believed that one huge supercontinent existed over two hundred million years ago. The name for this supercontinent is Pangaea. Pangaea was broken in to several pieces, and each piece was a part of the lithosphere. They believed that the pieces of Pangaea formed the continents that we know of in present day geology. When Pangaea existed, the rest of the Earth was covered by an ocean called Panthalassa.
Eventually, Pangaea split into two land masses, Laurasia to the north and Gondwanaland to the south. The theory of plate tectonics does in fact have an explanation for the movement of the Earth’s crust. The fact that Pangaea did exist could be quite plausible. Scientist also believes that as the Pacific Ocean is closing, a supercontinent may form in millions of years to come. In present day geology, we can consider Eurasia as a supercontinent because the Ural Mountains separate Europe from Asia and make a line of compression and change where the two continents smashed in to each other (Tarbuck p606).
In 1620, Sir Francis Bacon wrote in his book Novan Organum and noted that the coasts of the Atlantic Ocean seemed to be parallel to one another. However, the plate tectonic theory really started to begin in 1915 when Alfred Wegener proposed the theory of continental drift. Alfred Wegener believed that the continents bulldozed through crust on the ocean basins, which would explain why most of the coastlines look like they could fit together. Wegener was not the first one to realize that the continents looked like they could fit together because Magellan and other early explorers noticed this also.
However, he was the first person to realize that the Earth’s surface has changed over time, and that continents that are not together now could have possibly been joined together at some point in the past (Twiss p532). Many people were against Wegener’s theory because he did not really have an explanation for why the continents moved. During that time, geologist believed the Earth possessed these features because the planet went through periods of cooling and heating. Anti-mobilists were people that were against Wegener’s theory.
People who were in favor of Wegener were known as mobilists because they had seen proof of continental movement in the Alps (Fowler p475). In a few short years, Wegener’s theory was denied. However, his theory was the first time the idea of continental movement was announced to the science community. His theory laid the foundation for the advancement in twenty-first century plate tectonics. Years would pass and more evidence became available to support the idea that plates were in fact in motion and changed over a period of time (Fowler p476).
After the Second World War, more information was discovered which supported the theory of plate tectonics. In the sixties, a bunch of seismometers were installed to collect data on nuclear bombs, and these instruments sparked curiosity among geologists. It showed that volcanoes, earthquakes, and other features were along the Pacific Ocean and ran along the continents edges for the most part. It turned out that all of these edges became known as tectonic plates (Kearey 2009). Further studies showed a pattern of magnetic fields in the ocean basins. The rock basalt contains a lot of magnetic minerals called magnetite.
The lava forms and cools and the magnetic minerals align with the North Pole. This proves that the Earth has gone through several magnetic reversals; this would not be possible if the lithosphere was not in motion. Since all of this has been discovered, plate tectonics has gained acceptance as the Earth processes (Kearey 2009). Plate tectonics is made up of the study the motion and change in the Earth’s crust. This is based on the theory that the lithosphere is divided into seven major plates and several minor plates, and they all move in accordance with each other.
They also move in relation to hot spots, which is where mantle material comes up. The plate tectonic theory tries to tell us that the Earth’s crust moves over a period of time. The crust moves in a rigid way, which explains the change that we see. The theory is based on a few beliefs. New material is made by the spreading of the ocean floor and eventually become part of a plate, and motion of plates occurs only at plate boundaries. Plates are rocks that pretty much float on top of the asthenosphere. The crust has two types, the oceanic and continental; they both differ because of composition.
The continental crust is made up of mostly granite. This brings us to the conclusion that the rocks have a lot of quartz and feldspars. However, most of the oceanic crust is made up of basalt. Basalt usually has other minerals like olivine and mafic minerals (Fowler p477). There are three types of plates, divergent, convergent, and conservative. Wide places of change are usually around plate boundaries because of the two plates colliding. We know that these boundaries exist because of their motions. One sort of plate is the divergent boundary. At this boundary two plates move apart.
As they move apart it creates a crack in the crust and magma comes in to the ocean and cools. When the plates move, more and more crust is formed. Divergent boundaries are believed to be the reason for the plates moving. The formation of the new crust pushes the two plates apart, this is apparent in the mid-ocean ridge, which helps to move Europe and North America further and further apart. Mid-ocean ridges are mountains under water. They can even be as tall as mountains that are on land, this process is known as convection. Magma is pushed up by convection currents.
Some magma erupts through the crust and some moves under the crust away from the ridge crest. The magma flows and helps move the plates away from each other to allow more crust to be created and to grow; this is called convection cells. We know this to be sea-floor spreading. The mid-ocean ridge plays a big part in the plate tectonic theory because of the minerals uniqueness within the basalt. It contains a lot of magnetic minerals that align with the Earth’s magnetic field when it crystallizes. In the past, scientist has known the Earth’s magnetic field to change.
When the magnetic minerals align scientist can use it to date the crust. This plays an important role in the theory because it is first proof that plates were in fact moving and have been for almost Earth’s existence. We can use the magnetic information to prove that the plates are moving, and we can also determine that new crust is being formed and that the old crust was erased in a continuous process that has been going on for all of Earth’s past. The oldest crust ever dated is approximately one hundred million years old, which is quite recent in geologic time.
This may lead you to question, where did all of the old crust go (Fowler p478). This brings me to my next discussion which is convergent boundary. A convergent boundary is when another plate overrides another plate, causing one plate to go underneath it. Most of the boundaries can be found in island systems and trenches. Most of the old crust goes in to these systems as new crust is formed at spreading centers. This explains why scientist cannot find any crust that date past the Cretaceous period. The old crust was destroyed by the process of subduction. Earthquakes are very active in subduction zones.
The earthquakes occur because one plate slides under another. Although this movement is not visible, it has very strong effects on the Earth. The outer edges of the Pacific Ocean are referred to as the “Ring of Fire” because the subduction zones go all around the Pacific Ocean. Volcanic activity is also caused by subduction zones because when one plate goes under another it gets hotter. The reason it gets hotter is because it is closer to the mantle. When the old crust gets close to the mantle, it melts and forms in to magma. The magma eventually runs up through the crust and forms volcanoes.
One good example of a subduction zone surfacing is the Aleutian Islands off the coast of Alaska (Twiss p536). When two plates collide, subduction zones do not always occur. Continental crust is less dense than oceanic crust, and when they collide, they do not run over each other. Instead, they kind of plow in to each other and create mountains. This type of boundary is called a collisional boundary. An example of two plates colliding is the Himalayas in India. The third type of boundary is the transform boundary. It is called this because plates are not destroyed or created, but they slide past each other.
A good example of a transform boundary would be the San Andreas Fault in California. In California, the Pacific Plate and the North American Plate are sliding past each other. This is why many earthquakes occur in that region. The earthquakes occur because of the strain that the two plates exert as they slide past each other (“Historical perspective). The Earth changes in many ways, however there are three forces the cause change within the Earth. When these forces act, they create stress and they change the volume and shape of a material. The three main types of stress are shear, compressional, and tensional.
Stress puts a lot of strain on the Earth which causes the change in rocks and the Earth’s crust. Compressional forces may cause a rock to compress or shorten. Tensional forces may cause a rock to become longer and pull apart. Shear forces can cause rocks to slip past each other (“Historical perspective). Faults are places where rocks have been broken and have been changed. There are three big types of faults. These are strike-slip, normal, and reverse. The stresses caused by motion of the plates build up over a course of time and ultimately cause the Earth’s crust to break when the rocks rub past each other.
Usually when a fault happens, an earthquake occurs. Every plate boundary has some characteristic of a type of fault. Normal faulting can be affiliated with crustal extension. Normal faults can usually be found at divergent boundaries. Crustal shortening can be affiliated with reverse faulting. Reverse faults can usually be found at convergent boundaries. Strike-slip faulting is affiliated with sideways movement of the crust. These faults usually form at transform boundaries (Twiss p538). Even today, we are constantly reminded that the plates are in motion.
One recent catastrophe caused by plate tectonics was the earthquake that happened in Haiti. Studies suggest that the earthquake occurred due to a strike-slip fault. The Caribbean sits on its own little plate and is surrounded on three sides by the bigger North and South American plates. Scientists believe that the North and South American plates are moving westward at approximately two to three centimeters per year. Based on the recordings of the earthquake, the Haitian quake seems to have occurred close to the Enriquillo Fault. The Enriquillo Fault is a big strike slip fault that runs across the southern border of Haiti.
Scientists presume this is the fault that most likely ruptured because it is closest to the epicenter of the rupture. Although this was a big catastrophe for human life on the island of Haiti, it was not really unusual given the plate tectonic activity in that area. Unfortunately for Haiti, it is one of the most poorest and underdeveloped countries in the world. Its government was not really in the position to have any preparations in line for such a huge earthquake, and this caused thousands of people lost their lives (Kearey 2009).
Courtney from Study Moose
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