FALL, 2000




Martha Figueroa
Fall, 2000


Essay Topic: Research the science of Plate Tectonics. Explain the role of plate tectonics in earthquakes. Define the two different types of faults and explain why some earthquakes are more destructive than others. Choose 2 earthquakes, one caused by a slip-slide fault and the other caused by a thrust fault. Compare and contrast the consequences of each.

The earth's surface is covered by a series of crusted plates. In geologic terms a plate is a large, rigid slab of solid rock. The word tectonics comes from the Greek root "to build." Putting these words together, we get the term plate tectonics, which refers to how the earth's surface is built of plates.

Plate tectonics is a new theory that was introduced some 30 years ago. This theory has changed radically the way geologists think about the earth. Plate tectonics is the new structural design that geoscientists use to explain the dynamics of the earth's system. (Kious and Tilling, 1996). According to the theory of plate tectonics, the earth's crust is like the shell of a cracked egg. The surface of the earth is broken into seven large and many small plates. These plates are approximately 50 miles thick and hundreds or thousands of kilometers wide. The plates move one towards the other very slowly. The movement of the plates is approximately 5cm each year. As a result, the edges of these plates cause intense geologic activity, such as earthquakes, volcanoes, and mountain building.

Continental Drift and Sea Floor Spreading

Plate tectonics is a combination of two early ideas: Continental Drift and Sea Floor Spreading. Continental drift is the movement of continents over the earth's surface. Sea floor spreading is the creation of a new oceanic crust at mid ocean ridges and the movement of the crust away from the mid ocean ridges.

In the early 1900's the emergence of this theory of plate tectonics started a revolution in the earth sciences, and it helped scientists to describe the ways in which the tectonics process affects the daily lives of people everywhere.( Kious and Tilling, 1996).

Alfred Wegener was a scientist whose ideas brought a new perspective about plates. In the early 1900's, he began to study the earth's atmosphere and he wanted to find out why there were many different climates in the world. During his research he found that tropical plants once grew in icy Greenland and that glaciers had covered warm Africa and Brazil.(Kious and Tilling, 1996). His theory was that at one time all the continents may have been united as one continent (called a supercontinent pangaea) and may have shared a similar climate. He said that the section which was once one big continent had slowly drifted apart to form the continents as they are today. Wegener's ideas served to formulate the development of the theory of plate tectonics three decades later; this theory was called Continental Drift.

At present geologists have found clues that the continents move. The first clue is the shape of the continents; they seem to be like puzzle pieces. The second clue comes from rocks. The geologists compared the rocks on different continents and they found that some rocks have similar traces of plant and animals that lived long ago. These fossils were found on the coast of South America and Africa which are now widely separated by the Atlantic Ocean. It is evident that these animals and plants couldn't swim or be transported across the huge oceans. Also some animal fossils found under the ice in Antarctica once lived in warm climates. This research demonstrates that Weneger's theory was right; once the big continent broke into smaller continents, it created the plates.

In the 1950's new evidence emerged around the ocean floor. A new theory called "Sea Floor Spreading" showed that great mountain ranges were found on the sea floor . (Kious and Tilling, 1996). These mountains were found in the central Atlantic. These are called the global mid-ocean ridges . In some places the mid-ocean ridges show above the ocean surface. These mountains are more than 50,000 Km. Long, more than 800 Km. across the continent and approximately 4,500 meters above the sea floor. This mountain is wider than the Himalayas in size. In the mid-ocean ridges there are also many mountains peaks within the same mountain change. Most of them were once active volcanoes under the sea.

Plate Boundaries

The ocean floor continues to move, spread, and shake at the edges and continues to regenerate. Oceanic ridges and continental drift lead to three fundamental types of plates boundaries: Divergent, convergent, and transform-fault. Divergent boundaries occur when two plates are moving apart under the ocean floor. They leave a crack between them where the magma, or melted rock, rise up through the crack, forming a mid ocean ridge. Some of the magma make a new ocean floor on both sides of the crack. This process is known as sea-floor spreading that forms plates. Convergent boundaries move toward each other and collide. When a continental plate collides with an oceanic plate, the oceanic plate tips down and it gets under the continental plate forming a deep ocean trench. This kind of movement occurs in the oceanic plate Nazca and continental South. This plate is found in Ocean Pacific and South America front in coast of countries Ecuador, Peru and Chile. Transform-fault boundaries occur when two plates move horizontally to one another. The San Andreas Fault zone is a good example of this type of movement.

The basic concept of plate tectonics has helped science come up with a better explanation on the movement of strong big pieces of the earth's crust which affect certain areas of the earth. These concepts have also helped us to understand the formation of mountains and the distribution of earthquakes and volcanoes. The connection between plates tectonics and earthquakes is evident. The effect that produces the plate collisions, where plates move apart and split past one another causes big earthquakes. One example of this connection between plates tectonics and earthquakes is the San Andreas rift that is found in the North American Continent in California. The 1906 San Francisco Earthquake was produced by a great displacement of plates that moved horizontally to one another causing a great destruction in California. The earthquakes caused death, great catastrophes and material losses. Scientists continue to verify and improve this theory to explain the dynamics of the earth system and to understand much better how our planet has been shaped by plate tectonics.

My Response

This research has helped me understand much better how plate tectonics influences the geological process of the earth. The earth continues to evolve and plate tectonic movements are part of this evolution. The constant changes in plate tectonics don't always produce terrible catastrophes. However many times they produce small changes like tremors and high waves in the ocean. But when the movement is too abrupt or drastic it can produce terrible catastrophes like earthquakes and tidal waves. However it is interesting to learn how they work. Through this research I learned that we don't have control over these huge pieces of solid rock that move around the continents and I learned how they affect the land where we live.

Based on my own experience, I can relate to victims of earthquakes. My country, Peru which is located in the south American continent on the Pacific Ocean is in front of the Nazca plate a zone where constant movement of the plate are registered. As a result we get tremors and sometimes earthquakes. On May 31, 1970 at 1.30 p.m. a great earthquake was registered on the coast of Peru. I remembered that I heard a strong sound and after the sound, I started to feel the earth shaking abruptly. Some houses in the street were demolished by the movement and the sound waves. The streets cracked and a few cars fell into the crack. People ran in the street; some were praying and others screaming and crying. It was a very traumatic experience.

These natural phenomena usually bring many consequences and dramatic experiences to those who survive these catastrophes. After doing this research on plate tectonics I understand the movements that are provoked by plate tectonics.


In conclusion the earth's surface is covered by a series of crusted plates. The new theories about plate tectonics explain how this surface is broken and how they move. The earth continues to evolve and plate tectonic movements are part of this evolution. The discoveries of scientists have created new material and new ways of doing things. We and the world where we live are constantly being affected by the changes of science. As a result we need to adapt and learn to live in harmony with the natural processes of the earth.

Works Cited

Major Tectonics Plates of the World.

This Dynamic Earth: The Story of Plate Tectonics

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Irina Pototskaya
ESL 91
Fall, 2000

Earthquakes and Plate Tectonics

Usually the earth under our feet is firm and solid. However, sometimes the ground shakes and moves. Human beings have faced this geological event from ancient times. To be precise, this geological phenomenon has existed 4.5 billion years, a long time before the first human beings appeared. In some places the earthquakes are very common, in others they happen rarely, or not at all. Sometimes it can be a real hazard. For instance it took 45,000 people's life in Greece, Corinth, in 856 year; 100,000 lives, In China, in1290; 300,000 lives, in India, Calcutta, in1737; and in many different places and times, as it was in Japan, Peru, Italy, Turkey, etc. These catastrophes left deep traces in history of human civilization. People made up the theories in compliance with their own beliefs and knowledge trying to explain this phenomenon.

According to Aristotle, strong, wild winds were trapped and hold in caverns under the ground. They struggle to escape, and the earthquake is a result of their struggle. Southern American Indians thought that the ground shaking when gigantic jaguar began hunting. Old Greeks supposed that huge giant Atlas held the Earth on his shoulders and sometimes asked his wife to replace switch him. While they are passing the globe the shaking happens. India: The Earth is held up by four elephants that stand on the back of a turtle. The turtle is balanced on the top of the cobra. When any of these animals move, the earth trembles and shakes. Belgium: When people on Earth are very, very sinful, God sends an angry angel to strike the ear that surrounds our planet. The blows produced a musical tone that is felt on the Earth as a series of shocks. And these two I like the best. Central America: the Square Earth is held up at its four corners by four gods. When they decided the Earth was becoming overpopulated, they tipped it to get rid of surplus people. Mozambique: The Earth is a living creature, and it has the same kind of problems people have. Sometimes it gets sick with fever and chills and we can feel this shaking. No one can really explain the causes and nature of this geographical phenomenon. Even the geologists- scientists who study the physical history and structure of Earth- do not knew much. It is impossible to look inside the Earth and it seems the secret was hidden so well that people may never got the answer.

In the beginning of twenty century, after the famous San Francisco earthquake the American scientist Harry Reid first described the theory of "elastic rebound'' of rock. This elastic rebound is the immediate cause of quake. The crust rocks strained and bounded by geological forces can keep the stress for many years accumulate energy. Suddenly, the fault raptures and million tons of bedrock move released the huge amount of energy as seismic waves. These waves are the earthquake. The fault is a weak place in Earth's crust where the rock's layers have rapture and slipped. The point within the earth from which originates the first motion of a quake is called focus and the point on the Earth's surface directly above the focus called an epicenter. Faults are divided into three main groups, depending on how they move. Normal faults. These occur in response to pulling or tension: the overlying block moves down the dip of the fault plane. Thrust /reverse faults/. These occur in response to squeezing or compression: the overlying moves up the dip of the fault plane. Strike-slip /lateral/ faults. These occur in response to either type of stress; the blocks move horizontally past one another." This theory was good and still alive today, but it explained a mechanic of phenomenon but not the cause. What forces can deform the rocks? On this question scientists will be able to answer only 50 years late.

In the end of nineteen century English engineer John Mile created an invention that detects and records the vibrations in Earth caused by earthquake. This devise is called a seismograph. And in the beginning of twenty century, Russian seismologist invented the modern one using a magnetic pendulum suspended between the poles of an electromagnet. The basic principles of this device with little improvement is applying in now time. Thus," the earthquake is the vibration of the Earth caused by the passage of seismic waves radiating from some source of elastic energy."/p.309 B. Bolt/

There are two basic types of seismic waves. They are surface waves, ones that travel along the Earth surface, and body waves, which are vibrations that travel through the Earth. Of the two, surface waves are the strongest and therefore caused the most damage. These waves literally make the ground roll. Body waves are divided into two different classes; compression ones which are faster, called P-primary waves and can spread through any matters and shear waves or S- secondary waves, which are not so fast and cannot penetrate through liquid substances.

Thousand seismic stations have braided the Earth like huge web collecting information about earthquake. Millions of quakes happened every year - an average every third second- and every one produce seismic vibrations are setting off through the Earth to almost all place on the globe. Because of this diversity of seismic waves, various of their velocities and types of motions, and differences in filtering abilities passing through different substances, seismologist -the scientists who study earthquakes- compeering and analyzing the reflection and refraction of earthquakes waves was recorded by seismographs have got a perspective of Earth's structure.

The Earth has diameter of 7,972 miles or 12,756 km and it made up of four main layers. The outside layer of Earth is a thin layer of solid rocks called crust from 4 miles /6 km/ to 56 miles /90 km/. Beneath the crust lies a layer of extremely hot rocks called mantle. The mantle is 1,800 miles /3,000 km / thick. The most upper part of the mantle is dense solid silicate rocks, but deep part is liquid melted rocks, cause of extremely high temperature created by an incredible pressure inside the earth. Than deeper than more pressure and the temperature is higher. Below the mantle lies Earth's core to be made of nickel-iron alloy. The outer core is liquid because of extremely hot temperature. The inner core although it reached the temperatures of 10,000 degrees Celsius, which is hotter than the sun's surface is solid due to the tremendous amount of pressure that is being on it.

The upper part of the mantle and the entire crust make up the lithosphere, the layer of solid rocks are about 40 to 100miles thick that flows slowly on the asthenosphere. The asthenospere is similar in composition to lithosphere, but unlike the rock in the lithosphere asthenospheric rock is partially meted. This thick liquid layer extends to about 500 km downward from lithosphere and constantly moves. Within an asthenospheric's currents the rock material heats up, expands and rises. As it rises it cools down and contracts. Since it now cooler the density increases and the rock once again sinks. As it sinks the rock material heats up again and rises to continue the cycle. This motion forces the upper layer also to move that crumbs it. Recently scientists discovered that all of the lithosphere is broken up into pieces were called Tectonic plates. Tectonic comes from the Greek word teckton, which means, "to build up and break down."/p.26 Ch.Spenser/

The fact, that the continent's shapes perfectly suit one to another, was noticed by scientists even in previous centuries. This looked like before it used to be one huge continent they didn't know why separated apart. In the beginner of twenty century on the bases of identity between the fossils tiny creatures had been discovered on different continents German meteorologist- the scientist who studies weather and the atmosphere-Alfred Wegener considered that long time before, according to the fossile clue it began 200 million years ago, existed one ancient super continent. He even named it Pangaea. But he couldn't explain what forces move continents.

This explanation came with knowledge about Plate Tectonics and researches of ocean floors. That became possible with inventing of a new tool-sonar a system to measure the distance of objects using an echo-sounding device. U.S. Navy sonar mapped the entire mountain range that snakes along the ocean floor of the planet. The oceanic crust is broken apart in many places, the hot magma called lava rising from the Earth's mantle to fill the gaps. It cools forming new oceanic crust and mid-ocean range, and spreads oceanic floor apart pushing continents to move away at the rate of anywhere from 1 to 12 cm/year. The Pacific seafloor spreading is faster then the Atlantic. /p.22 Ch. Spenser However, in spit of this addition, the Earth is not getting any bigger and earth's crust is not getting any thicker. Why is this? As the new crust ruses up from the mid-ocean ridges, the same amount of old crust moves back into the mantle. This movement of old crack back down into Earth is called subtraction. Far away from the mid-ocean ridges, near the edges of the continent, the old ocean crust dips down. There are two types of crust: continental and oceanic, and because of the rocks of oceanic crust are heavier and thinner than rocks that make up continental crust, the oceanic crust sinks into deeper layer of Earth. We now known that there are seven major crustal plates, subdivided into number of smaller plates named by continents or ocean, which located on them. These plates are constantly moving, whether it be together, apart or side by side each other, where they interact, along their margins, important geological processes take place.

The same picture the scientist noticed about/with quakes. The most word's quakes are not randomly distributed over the Earth surface. They tend to be concentrated in narrow zones, which defined the boundaries of plates. Ninety percent of all earthquakes occurred around the border of the vast Pacific plate. There is so much earthquakes and volcanic activity on the edges of this plate that the outline is called the Pacific Ring of fire. Alaska, California, Mexico, the west coast of Central and South America, and the east coasts of China, Japan, and New Zealand are all located within the Pacific Ring of Fire. Another major earthquake zone stretches through Italy, Greece, and Turkey to the Middle East and into Asia.

Not only the movement of Plate Tectonics cause quakes. Another cause of quakes is volcanic eruptions. It is believed that earthquakes and volcanic eruptions both result from tectonic forces in the rocks and need not occur together.

Other types, called Collapse, earthquakes are small quake occurring in regions of underground caverns and mains. The immediate case of ground shaking is the collapse of the roof of the mine or caverns. "Main bursts" are common in South Africa mines, it has been observed in Canada. Sometimes, quakes may be produced by massive landsliding, like one along Montare River, Peru in 1974. Human case explosion earthquakes produced by the detonation of chemicals or nuclear devices. /p.98 B. Bruce/

To measure and compare the sizes of earthquake we use two scales: the Mercalli scale and Richter scale. " In 1902, the Italian scientist Giuseppe Mercalli created a method of describing earthquake by the effect they have on things." /P.55 Ch. Spenser. / This intensity scales "are used to estimate the amount of shaking at different places. Numbers related how much the earthquake is felt and how much distractions it causes."/pp.40-41 L. Casper/ To avoid confusion with Richter scale numbers, the Mercalli scale uses Roman numerals from I to XII. This descriptive scale is a not precisely scientist, but before inventing seismographs and the Richter scale it was only way to made an estimation of major earthquakes in history. The Richter scale named after the American seismologist Charles Richter created in 1935, is not an actual instrument.

" It is a measure of the amplitude of seismic waves and is related to the amount of energy released. This can be estimated from the recordings of an earthquake on seismograph. The scale is logarithmic, which means that each whole number on the scale increases by 10. For instance: a magnitude 4.0 earthquake is 10 times greater than 3.0, a 5.0 is 100 times greater, and a magnitude 6.0 is 1,000 times greater, etc. The power of quake called magnitude.

I'd like you to give a few statistics. So-called great earthquake a magnitude of 8.0+ approximately occurs once in a year. Major quakes a magnitude of 7.0-7.9 happen 18 times per year. Larges/destructive/ a magnitude of 6.0-6.9 are 120 per year. Moderates /damaging / a magnitude of 5.0-5.9-1,000 per year. Minors /damage slight / a magnitude of 4.0-4.9-- 6,000 times per year. Generally felt quakes a magnitude of 3.03.9 - 49,000 times during year. Potentially perceptible with magnitude of 2.0-2.9 are 300,000 per year. Imperceptible quakes with magnitude less than 2.0 shack earth 600,000+ times per year.

The largest known earthquake, a magnitude of 9.5 was in Chile in 1960 and resulted in over 6,000 deaths by triggered tsunami. The largest one in the United States was the great Alaska earthquake of March 27,1964. " It had a magnitude of 9.2 and 115 people died, with most of deaths due to the tsunami it generated. Shaking was felt for an estimated 7 minutes, and raised or lowered the ground surface as much as 2 meters (6.5 feet) in some areas and 17 meters (approx. 56 feet) in others. The length of the ruptured fault was between 500 and 1000 kilometers (310.5 and 621 miles). The amount of energy released was equal to 12,000 Hiroshima-type blast, or 240 million tons of TNT."

Not the shaking of land is dangerous by itself, yet by its consequents like collapsing people's structures, conflagrations, tsunamis, landslides, water floods, etc. Some of quakes became real disasters caused uncountable distractions and hundred of thousands of deaths, as it happened in Italy, Messina in 1908, when it took 120,000 lives, it magnitude was 7.5; in Japan Kwanto, in 1928, the quake of magnitude 8.2 killed 143,000 people; in China, Shency, 830,000 souls were dead as result of earthquake of 1556 year. How many damage would be done by it and how many lives it would take depends from many causes. The main determinative factor is where it happens. The " major" earthquake would brink less revenges if it happens in desert areas; yet/as/while "largest" or even "moderate " one, if it happens into densely populated areas. Till now, the seismologists almost never can predict quakes. It comes suddenly. Sometimes people noticed not usual behavior of animals and fish, but there is no serious information about it.

I am also can be an eyewitness. I experienced this geological event and even twice. At that time, it was the middle of 80-th; we used to live in a country in Moldova Republic. Once, it was at deep midnight, I was awaked by strange nose. Windows glasses rattled, and I felt little tremor. I was slightly anxious by this phenomenon. However, I found an explanation very quickly: it was just that my crazy neighbors drove this hellish bulldozer. That fall, they had built something in their yard. Of course, they couldn't find better time for doing it than now. After then, I fell asleep again.

Next day, I met my friend in the town. She looked terrible. I asked her "What happened to you? Are you sick?" She answered me " We didn't sleep. We were sitting in the garage all night." It wasn't warm outside, and I known their garage didn't have any hitter. I was really puzzled with this information " What the hell did you do there?" It seemed, she was surprised with my questions also." What did we do? We were waiting till the quake is over. When it began, I awake my children and husband, we grabbed something cloths and run outside. It finished soon, yet we feared to return into house and spent night into iron garage. Well. Where was your family last night?" I said, "We were sleeping in our beds at home. I noticed some little jolting at night, but I didn't pay much attention to it. " We had lived in this area since recently and we wasn't aware about earthquake which have place in this region. Sometimes, it is very useful not to be known about something. Now, applying my new knowledge I am able to estimate that earthquake on the Mercalli scale of intensity as between III and IV and at about 4.0 magnitude on the Richter scale.

Sooner, the second earthquake happened. It was in the daytime and was much stronger. I was in a small town, not far from our village. Because I was outdoors, nothing threatened me. I could survey and analyze it. I was already aware of this geological phenomenon, but anyway at the very first moment I was confused. My life experience came into contradicts with my feeling. The earth under my feet that I accustom to feel firm and solid began shaking and swinging. My eyes said I am on the land, yet my feelings refused to believed in that. It seems to me like I swing into boat. I remembered the people running outside from the supermarket and houses, the dog's howling, and what surprised me the most was the view of swinging huge cast iron light poles. They were moving with such a big amplitude what I didn't have any doubts they were going to fall. Soon, it stopped as suddenly as it began. Nothing terrible happened, nothing was destroyed. A couple of days later, I heard what a few old houses had gotten cracks and no one was injured or hurt.

When I was writing this research project, I found a lot of information about earthquakes: what phenomenon is called the earthquake; what the focus, the epicenter is; how rocks move during the quake; what kind of seismic waves there are; what kind of destructions will happened if ...; how many people were killed by this quake and how much damages brought that one; and so on, so on, so on. It was so much explanations, but I felt something confusing, something uncomfortable. It looked like behind of all these definitions authors, didn't know the real answer. And it is really so. Even in terminology, numbers, explanations there were differences in different sources and various points on the same facts. The knowledge about earthquakes is extremely new. Even today the scientists still don't understand completely the nature of geological processes emanating inside the Earth. They also cannot explain some geological phenomenon and they almost never can predict the earthquakes.

The Theory of Plate Tectonics made the revolution in geology in the end of 60-th and connected many geological phenomenon in one concept including an explanation of the cause of earthquakes. However, some of them don't suit this theory. For instance: a violent quake that shook Missouri in 1811 the area in the heart of the North American continent located too far from any plate's edges. The Lissabon, Portugal was hit by one of the most devastating earthquake in history book in 1755 and it also doesn't lie near the plate boundary. And what about Great Britain, which also lays far away from a plate boundary, but has more than 120 earthquakes every 100 year. "Earthquakes have jolted almost every part of the world. Scientists don't know why they occur in places that should be free of violent plate movement. Fore now, it's a mystery."/ Spenser p.47/

People like to make up explanations for everything in compliance with their own beliefs and knowledge, but I suppose on some questions we will never get answer. Why does the Earth change its skin? The recreation that has been continued for 200 millions of years. Have we ever really answered this question? The geological processes are so global; their durations are so huge. It is unimaginable. Compared to the Earth's age we even not a sparks into wind, but we are only shadows of shadows of these sparks. The Theory of Plate Tectonics is really fine but it only theory.


Bolt Bruce A. (1993). Earthquakes. University of California, Berkeley: W. H. Freeman and Company.

Kasper Loretta . (1998). Interdisciplinary English.. New York: The McGraw-Hill Companies.

Millennium 2000. The American Encyclopedia 2000. Chicago.

Seymour Simon. (1995). Earthquakes. New York: A Mulberry Paperback Book.

Spencer Christian and Anatolia Felix. (1997). Shake, Rattle, and Roll. John Wiley & Sons, Inc.

NASA. [Online].

United States Geological Survey. [Online].

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