Plate Tectonics. A Continental Puzzle. Drifting Continents

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1 Plate Tectonics A Continental Puzzle Drifting Continents In 1910, a meteorologist from Germany named Alfred Wegener (Vay-guh-ner) developed a theory based on many observations. Even though meteorologists study the weather, his theory was about another branch of science called Geology. Geology is the study of the Earth, including the rocks, landforms and the processes that work to change them. Wegener s work as a meteorologist included the study of weather maps. Like many scientists before him, he made the observation that the continents of the earth looked a lot like pieces in a giant puzzle. He noticed that the coastline of Eastern South America matched up very well with the coastline of Western Africa. As he began to further investigate this discovery he found that scientists had found identical fossils of extinct land organisms on both coastlines. He also noticed mountain chains that began on one continent and seemed to continue on other continents. He began to wonder how this could be when they were, in many cases, separated by oceans such as the Atlantic. Wegener developed a theory that he called Continental Drift. Wegener s hypothesis was that the continents were once joined together in a single landmass. According to the age of the fossils, he calculated that the continents were together 300,000,000 years ago. He called this landmass Pangaea (pan-jee-uh) which means all lands. His theory also stated that the continents broke up into many large pieces called plates. He believed that they are still moving to this day and will continue to move until they meet up again to form another giant landmass.. Unfortunately, Wegener had a hard time getting the scientists of the world to accept his theory. Some pointed out the fact that he was trained as a meteorologist, not a geologist. More importantly, and most discouraging for Wegener, he could not explain what was causing the continents to move. While he had theories about that as well, they were not supported with as much evidence. Alfred Wegener spent the rest of his life trying to prove his theory of Continental Drift. He died in 1930 while on a scientific expedition to Greenland. It wasn t until many years after his death that scientists found the evidence to prove his theory to be correct.

2 Sea-Floor Spreading The Mid Atlantic Ridge While serving in the Navy during World War II, a navy officer and scientist named Harry Hess sought to prove Wegener right. After studying Wegener s work he thought he might be able to find the proof for Continental Drift using a new technology used on the Navy ships. While mapping the ocean floor during the 1950s, Hess made an interesting discovery that helped prove Wegener s theory of Continental drift. Using sonar, a device that sends and receives sound waves under water, he discovered the existence of an undersea mountain range that extends down the center of the Atlantic Ocean. This has come to be known as the Mid Atlantic Ridge. As Hess studied the Mid Atlantic Ridge he began developing his own theory about the continents. His hypothesis was that molten lava is constantly oozing up from the interior of the earth along the Mid Atlantic Ridge, creating new sea floor that slowly spreads away. This new seafloor is called oceanic crust. The seafloor then sinks down into deep sea trenches. When the oceanic crust sinks back down to be melted again it is called subduction. The Theory of Plate Tectonics Harry Hess s investigation of the Mid Atlantic Ridge led to a better understanding of how the Earth s surface, both above and below the water is in a constant state of change. The Theory of Plate Tectonics was developed to explain how the plates are formed, move, and are destroyed over time. Boundaries Between the Plates Scientists have identified three different ways that the continental plates interact with each other. They are called divergent boundaries, convergent boundaries, and transform boundaries. Divergent Boundaries are places where two plates move away from each other. This happens at mid ocean ridges where sea floor spreading is happening. At divergent boundaries new crust is being formed. Divergent boundaries also occur on land where they are called rift valleys. An example of this is the Great Rift Valley in Africa.

3 Convergent Boundaries are where two plates are sliding toward each other. Where they collide with each other, several things can happen depending on certain traits of the rock. Because oceanic crust is more dense than continental crust, when the two collide, oceanic crust sinks beneath continental crust in a process called subduction. When two plates with continental crust meet, they both rise up forming huge mountains. Transform Boundaries are places where two plates slide next to each other, each going a different direction. This is where earthquakes often occur. There is a large transform boundary in California called the San Andreas Fault. A fault is a break in the Earth s crust.

4 The Layers of the Earth Why are the Plates Moving? An understanding of how the plates are moving leads to the question of why the plates are moving. To understand that, you need to understand what is inside the earth. Unfortunately, this is a difficult question to answer. To understand the interior of the earth better, many geologists spend time in caves. While some caves seem to be very deep in the earth - the deepest cave in the world is the Krubera Cave in Eastern Europe at a depth of over 2,000 meters - they hardly scratch the surface of our earth. There are two main ways that geologists study the interior of the earth: rock samples from the Earth s surface and indirect evidence from seismic (earthquake) waves. Rock samples can be obtained in several ways. One way is to observe rocks that were brought up from beneath the earth through natural processes such as volcanic eruptions. Another way is through drilling deep holes and bringing up samples. However, the deepest hole ever dug (the Kola Superdeep Borehole, in Russia) only goes down 12 kilometers. This may seem like a great depth but is pretty shallow when compared to how far the center of the earth is. The Layers of the Earth Geologists have discovered that the Earth is divided into four distinct layers. We have learned about the different layers through observations of material ejected from volcanoes and through seismic waves from earthquakes. The layers of the the Earth are the crust, mantle, outer core, and inner core. The Crust is where we live. It is the surface of the Earth. The temperature of the crust is cool compared to the other layers. It is 5-70 kilometers thick, depending on where you are on the Earth. The crust is made up of more than a dozen plates of solid basaltic rock. The plates are constantly moving around and interacting with each other. The crust and the upper part of the mantle is called the lithosphere. The Mantle is below the Crust. It is very hot, with an average temperature of 1,200 degrees Celsius. The mantle is 2,867 km thick and is divided into upper and lower parts. The upper mantle is made up of two parts. The part closest to the crust is called the lithosphere. It is similar to crust except much hotter. Below that is the asthenosphere. It is under a lot of pressure and has a consistency much like plastic. The lower mantle is very hot and mostly solid all the way to the core. The Core is the center of the Earth. It is made up of two parts; the outer and inner core. The core is made up mostly of nickle and iron. It is the hottest layer with

5 an average temperature of 3,200 degrees Celsius. The outer core is made up of molten metal that flows around the inner core. It is 2,266 km thick. The inner core is made up of solid, dense, metal that is under extreme pressure. It is 1,216 km thick. Convection in the Mantle The mantle is not just one temperature. It is cooler in the upper mantel than it is in the lower mantle, which is hotter because of the extreme heat from the core. Cooler materials are more dense than hotter materials, therefore they have a tendency to sink down toward the core, pulled by the force of gravity. The hotter, less dense material rises to the top of the mantle where it eventually becomes cooler and begins sinking down as well. This creates a cycle know as convection currents. Convection currents in the mantle are the cause of the movement of the continents. This has been going on for 4 billion years inside the earth! Convection also occurs in the outer core as well. Convection in the outer core results in the magnetic field that surrounds the earth!

6 Earthquakes, Volcanoes, and Tsunamis Earthquakes The earth seems pretty solid beneath your feet most days. For most of us, that is true almost all of the time. However, if you live in certain regions of the world you may have the misfortune of feeling the ground beneath your feet behaving like a trampoline. This is called an earthquake. An earthquake is caused by a sudden shifting of the crust on a fault line. This results in the release of energy into the crust that travels in the form of seismic waves. The seismic waves travel out in all directions and may cause the ground to noticeably shake. The strength of an earthquake depends upon how deep beneath the surface the shifting occurs and how large the fault is. Earthquake strength is measured using the

7 Richter scale, which was developed in 1935 by Charles Richter. Volcanoes The earth rumbles, a colossal explosion fills the air, and day seems to be turned to night. A volcano just erupted! A volcano is an opening in the earth s crust that allows magma from the mantle to reach the surface. Once magma escapes, it releases gases and becomes lava. Volcanoes are responsible for much of the creation of new land on our planet. The gasses released from an eruption also contribute to our atmosphere. Along with lava and gasses, volcanic ash can be launched many kilometers into the air, sometimes enough to block the sunlight! There are different types of volcanoes that have different kinds of eruptions. For example, Mt. St. Helens in Washington State is a composite volcano that has had very violent, explosive eruptions. Its last major eruption, on May 18, 1980, resulted in the death of 57 people and the destruction of 250 homes as well as 185 miles of highway. A shield volcano, on the other hand, has mostly quiet eruptions. The lava from shield volcano eruptions travel many more miles than more explosive eruptions. The Hawaiian islands are the largest and most well-know shield volcanoes on Earth. Where Are They Found? Most of the world s earthquakes and volcanoes occur or are found on or near continental plate boundaries. This is where the crust is thinnest and weakest. The largest accumulation of earthquakes and volcanoes can be found around the edges of the Pacific Ocean. This is often referred to as the Ring of Fire. States such as California and Alaska as well as countries such as Peru and Japan are accustomed to frequent earthquakes and volcanoes because of their proximity to the Ring of Fire. Tsunamis Water is one of the most destructive forces in the world. When an earthquake happens under water the energy released by the movement of the crust may result in a large wave, called a tsunami, which can travel for thousands of miles and cause catastrophic damage to seaside areas. The word tsunami means harbor wave in Japanese. Japan has had frequent tsunamis throughout history, the most notable of which occurred on March 11, Damages from the earthquake and tsunami are still being calculated. A tsunami can be caused by volcanic eruptions and landslides, but it most often caused by undersea earthquakes. There are several stages to a tsunami:

8 Initiation begins with a disturbance (earthquake) on the ocean floor which results in a large column of water being lifted and dropped down. The waves travel in each direction out from the center of the disturbance. In a process called amplification, as the wave travels over the continental slope toward the shore its height (amplitude) increases and its length (wavelength) decreases. As the tsunami wave travels from the deep-water, continental slope region to the near-shore region, tsunami run up occurs. For more information on earthquakes, volcanoes, and tsunamis, you can visit the website for the United States Geological Survey, a U.S. government organization that collects data about earthquakes from around the world. learn/ A Puzzle Solved? Scientists have been studying the earth and its processes for thousands of years. Each new discovery brings us closer to a deeper understanding of the many changes that occur on a daily basis all around the world. Though he never lived to see it, the contributions of Alfred Wegener led to a revolution in our understanding of the earth and its changing surface. Though we have come a long way there are still mysteries and puzzles to solve for future generations of geologists! Prentice Hall Science Explorer, Inside Earth Copyright 2007, Pearson Education