An in-depth comparison of earthquakes in United States history ultimately leads one to earth science. An exploration of plate tectonics and the thermodynamic cycle of convection in the layers below the earth's crust are fundamental to understanding how most earthquakes occur. How in-depth these phenomena can be studied will depend largely upon the prior knowledge and ability of my students. Since many of my students complete an earth science curriculum in ninth grade, most should have been exposed to some fundamental concepts about the composition and process of the earth's layers. An assessment of prior knowledge and review of earth science principles as they pertain to the composition and process of the earth's interior will be necessary. I plan on using on-line information hosted by the United States Geological Survey (USGS) to teach the scientific components. The USGS site is a comprehensive resource that students and I can repeatedly refer to in order to view data pertaining to geological processes and specific geological events. In some cases, study of earthquakes and history appears to offer an opportunity for interdisciplinary work between a science or math teacher and history teachers. I will explore this as a possibility should any of my students have an earth science or a science/ math elective. I will also share information from this unit with colleagues in my school who teach earth science and math.
Nevertheless, as teaching time permits, I should probe for prior knowledge and expose students to the fundamental composition of the earth's interior and the process of plate tectonics and mantle convection that recycles layers of rock. I will explore information from hardcover texts and on-line resources to reference the composition of the earth, the process of mantle convection and the behavior of plate tectonics. Most of the specific terms associated with earthquakes in regard to characterizing their origin, intensity, and magnitude were unfamiliar to me at the start of this seminar. The plethora of accessible resources made it realistic for me and I predict for students to get a basic grasp of the fundamental scientific principles of earthquakes. The United States Geological Survey (USGS), in particular, hosts online texts, maps, charts, diagrams, and videos that are particularly helpful in presenting key concepts.
The first key point to remind or teach students would be that the earthquakes are a natural by-product of the internal processes of the earth that include planetary convection and cooling. Heat from the interior of the earth, some of which is a by-product from the formation of the earth, fuels a cycle of convection that circulates more buoyant material toward the surface. Material closer to the surface cools and becomes denser and sinks back toward the earth's interior where the cycle of convection and cooling repeats. This process takes place over hundreds of millions of years. However, while the movement of plate material is remarkably slow it is measurable at key geographical locations around the globe. Areas where plate material is sinking such as the northwest Pacific Ocean have been identified (Abbott, 2006, p.62). Spreading centers, such as the Mid-Atlantic Ridge are areas where new and buoyant material is formed. Scientists can measure sea-floor spreading at spreading centers and analyze the movement of the earth's plate material.
The sinking of cooler and denser material pulls the earth's plates which collide and/or rub against each other. The regions between plates, called plate boundaries, become prone to earthquakes.
When two plates collide, the edge of the plate that is colder and denser is driven below the less dense plate with which it collides. The crust of the earth in these subduction zones bends and breaks according to these forces. Violent earthquakes often occur at subduction zones. Alternatively, when plates move sideways with respect to another, scientists refer to this as a transform fault (Abbott, 2006, p.62). The rocks in transform faults bend to the forces of plate tectonics and thus experience stress until snapping violently. The resulting movement of the crust indicates that an earthquake or seismic event has occurred. Areas where long transform faults exist include the northeastern Pacific (Queen Charlotte fault), the San Andreas Fault in California, and the southwestern edge of the Pacific Ocean (Abbott, 2006, p.62).
Students will need to become familiar with the scientific terms that describe geological characteristics of the earth. These terms include definitions that describe the earth's interior: the crust, mantle, and core. The mantle is important to understanding the earth's composition. Scientists point to the importance of the top two sections of the mantle, the lithosphere (solid) and the asthenosphere (plastic) (Abbott, 2006, p.30). In addition students will be able to define plate tectonics and the different types of faults: dip-slip faults (normal and reverse faults), strike slip and /or transform faults (right lateral fault or left lateral fault)(Abbott, 2006, p.86).
Since each of these topics might serve as a topic of extended study, I find that the biggest challenge might be condensing their fundamental meaning and relation to the topic of earthquakes. Using the USGS web site, I have begun summarizing some of the most fundamental topics and concepts. To help contextualize these topics I tried to list an essential question that might be worth exploring with my students.
Layers of the Earth
How does the interior of the earth impact what happens on the surface?
Discussion of the earth's interior involves knowing the layers of the earth's interior and what happens in each region. The USGS on-line publications
The Interior of the Earth
Inside the Earth
describe the layers of the earth's interior. The earth's interior can be divided into three primary layers: crust, mantle, and core. Each layer can be conceptualized as part of a hard boiled egg; the crust is compared to the thin and brittle egg shell, the mantle to the elastic egg white, and the core to the yolk. Each of the earth's layers has unique characteristics.
The characteristics of the core and mantle affect the crust, the thinnest and most brittle of the layers. The mantle is approximately 2,900 km thick and contains more iron, magnesium, and nickel, than the crust. The mantle is semi-solid rock and is subdivided into two layers: the lithosphere and the asthenosphere. The earth's core (approximately 3,400 km thick) consists of a solid inner core (1,250 km thick) and a liquid outer core (2,200 km thick). The liquid outer core creates the Earth's magnetic field due to its fluid motion. The characteristics of the mantle of the Earth have a profound affect on the crust. The upper layer of the mantle, called the lithosphere is where plate tectonics are at work (Watson, J.M. 1999, This Dynamic Earth, USGS).
How does plate tectonics relate to earthquakes?
Alfred Wegener proposed in 1912 that the continents moved or drifted over time. Wegener developed his theory of continental drift to debunk a theory that land bridges were the media by which similar fossilized plants and animals could be found from the same time period on different continents. Central to Wegener's theory is the idea that the continents were once joined in a single landmass that Wegener named Pangaea. Wegener investigated what Abraham Ortelius had declared as far back as the late 16th century; the continents roughly fit together as if they were the pieces of a giant puzzle (Watson, J.M.,2007, Historical Perspective- This Dynamic Earth,USGS).
Unfortunately, Wegener's explanation that the rotation of the earth fueled the continental movement proved inaccurate. He believed that the continents plowed through the lithosphere. His contemporaries led by Harold Jeffereys, dismissed Wegener's idea. Jefferey's argued that the ocean lithosphere was too strong for continents to drift or plow through. Alfred Holmes (1929) proposed thermal convection as the correct causal mechanism that drives the movement of plates. However, the ideas linking the movement of plates with thermal convection were not given attention until the 1960's. Harry Hess (1962), R. Deitz (1961), and S. Keith Runcorn (1962) proposed that mantle convection currents caused sea floor magnetic anomalies, ocean trenches, mid ocean ridges, and island arcs. Sea floor spreading, as this concept came to be known, and plate subduction are two fundamental ideas of plate tectonics (Weil, Anne, 1997, Plate Tectonics: The Rocky History of an Idea).
Geologists have determined that the earth's surface is divided into 12 major plates and several smaller ones. The movement and subsequent collisions of these plates cause earthquakes in areas where plates collide and grind against each other.
(To view a diagram of the plates of the earth's surface visit USGS online http://pubs.usgs.gov/gip/dynamic/slabs.html). The United States (with the exception of Hawaii), Canada, and Greenland sit on the North American Plate. The North American Plate has relative contact with other plates on the West Coast of the United States. Subsequently this is an area of high earthquake activity. Hawaii is subject to earthquakes due to its being a chain of collapsing volcanic islands (Watson, J.M., 2003, dynamic slabs, USGS).
Spreading Centers or Divergent Boundaries
What is the role of spreading centers in causing earthquakes?
Spreading centers, or divergent boundaries, such as the Mid-Atlantic Ridge, in the earth's crust and lithosphere occur due to plate tectonics and the cycle of convection and cooling that drives plate tectonics. They are centers of seismic activity. However, earthquakes there are not as prominent as they are at transform boundaries or convergent boundaries (subduction zones). At transform boundaries, plates slide against each other causing seismic activity. At subduction zones, denser plate material sinks under lighter more bouyant plate material. The friction and grinding of one plate as it subducts under the other not only causes seismic activity but also generates heat and steam which manifests itself in volcanic activity over the subduction zone. Spreading centers occur where two plates move away from each other slowly but at a measurable pace. The East Pacific Rise, in the Southeastern Pacific Ocean and the Mid-Atlantic Ridge, in the Atlantic Ocean demonstrate a predictable and measurable rate of spreading. The range of spreading along the East Pacific Rise ranges from six centimeters per year in the south of the Pacific Ocean to 10 centimeters per year just west of Central America's Pacific West Coast. Spreading along the Mid Atlantic Ridge ranges from approximately two centimeters per year near Iceland to four centimeters per year in the south of the Atlantic Ocean. The Mid-Atlantic Ridge actually bisects Iceland. It is one of the few places in the world where a spreading center exists on land. The spreading center is causing Iceland to gain approximately four centimeters of land per year (Abbott, 2006, p. 52).
Spreading is caused as the older, cooler, and denser plate material sinks. This process pulls plate material behind it. Imagine a towel on a table top. As you pull the towel over the edge, the back end of the towel follows behind. At a certain point the weight of the towel would cause the towel to completely slide off of the table top. If you covered a table with two towels so that each covered one half of the table's surface and you pulled the towels in opposite directions, the space between the towels would spread and subsequently the space between the towels would increase. Scientists believe a similar process occurs in the world's spreading centers. The earth's plates cool and sink at their denser and cooler end. The remaining plate material is dragged behind.
Gary Smith and Aurora Pun in
How Does the Earth Work?
describe the process that occurs at divergent plate boundaries. One characteristic that occurs at a divergent plate boundary is having upwelling of asthenosphere fill the space between separating plates. In addition, submarine volcanic eruptions occur and the crystallization of magma forms oceanic crust. Earthquakes happen because normal faults also occur. These faults break the crust. As a result seismic activity occurs (Smith, G. and Pun, A., 2006, p. 316-317).
How might a new rift or spreading center form?
Imagine a single towel or tablecloth that hangs over opposing edges of the table. If you continued to add weight to opposite and hanging ends of the towel or tablecloth, eventually, the table cloth would tear and spread toward the edges of the table where the weights have been added. Rifts occur where plate material had been ripped apart. As the cooler and denser edges of plates sink, the plates are subject to stresses that eventually tear them. Before the stress in the plate can be relieved through a complete tear or rift, a number of smaller tears and rifts might occur. Tears or rifts that begin but subsequently stop due to stress release in other areas are referred to as failed rifts. These phenomena may remain stable and unknown over time. However, they may become points of relief as stress builds up from plate movement over time. This stress relief can result in small seismic activity or large earthquakes. Seismic activity in the Reelfoot Rift, a failed rift lying below the Mississippi Valley, is the probable cause of the New Madrid earthquakes.
What is the relationship between plate tectonics, subduction zones, and earthquakes?
The process by which a denser, cooler, and older plate collides and slips under a lighter and more buoyant plate and then sinks into the earth's mantle is referred to as subduction. The area where two plates collide and one is driven under the less dense plate is known as a subduction zone, or, convergent boundary. Earthquakes and volcanoes are prone to areas of the earth's crust in subduction zones. Due to the pressures that build up in the earth's crust above subduction zones, cracks or faults in the crust occur. In the case where two land masses collide, mountains are often the result. The Pacific Northwest coast of the United States is above a subduction zone where the Juan de Fuca Plate (a remnant of the Farralon Plate) is subducting under the North American plate. Earthquakes and volcanoes happen in this area because of subduction.
How do faults reflect the type of plate activity that occurs in a region?
Faults are the cracks in the crust that form mostly in plate boundaries under forces of plate tectonics. The land above a fault often deforms due to the relative movement of plates. Geological observations show that changes in elevation relative to the faults may cause pieces of land to rise or fall away from each other or move laterally in relation to each other. These can be characterized or defined as normal faults/reverse faults, strike slip faults, and transform faults.
Plate Tectonics and the United States
How do Plate Tectonics apply to areas of the United States?
The United States is subject to the same geological phenomena as any other continent riding on a plate or plates. Parts of the West Coast have been and are currently experiencing the effects of the subduction of plates below the crust. The Juan de Fuca Plate continues to dive under the Pacific Northwest. In parts of California, the northwestern movement of the Pacific plate continues to cause sliding between plate edges. Faults such as the San Andreas Fault are the result of this plate movement. Some of the most damaging earthquakes can be associated with fault activity.
Why are failed rifts an earthquake risk?
While failed rifts may go unnoticed for hundreds, if not thousands, of years (compared to active faults at subduction zones, where seismic activity is more common) they still pose earthquake risks. Failed rifts are vulnerable to the changing stresses caused by plate movements. While they may not completely tear, they may move. The end result may be the subtle or violent movement of the material above the failed rift. The latter appears to be the case in New Madrid as the ground shifted at times radically in reaction to seismic activity on the underlying failed rift.
The New Madrid earthquakes are an example of seismic activity in an area of failed rifts. Due to the integrity of the earth's crust and lithosphere in these areas any seismic activity potentially poses wider ranging risks than in areas of localized fault lines. Quite simply, the solid material will transfer the disturbance over a much larger area than where a heavily faulted and broken-up region will dampen or scatter the waves. Such was the case in the New Madrid episodes which appear to have been felt from New Madrid to Cincinnati and as far east as Washington and Boston. West Coast quakes are rarely noticed beyond the West Coast. A significant seismic event in the central or eastern United States could resonate throughout much of the central and eastern regions.
A comprehensive study of the New Madrid Earthquake events determined that significant land fissuring and deformation occurred. In addition, sunken lands became new lakes. New lakes were formed in Tennessee (Reelfoot Lake) and Arkansas (St. Francis Lake and Big Lake). Most importantly, paleoliquefaction studies showed that at least two major earthquakes hit the same region in a 1500 year period prior to the 1811. Of primary interest is the assessment that further major seismic events are probable (Jonhston, Arch and Schweig, Eugene, 1996). If the New Madrid Earthquakes are indicative of future probable events, people from the nation's interior particularly from the Mississippi Valley above the Reelfoot Rift will have to be prepared. Municipalities could significantly lessen the destruction of a major event by focusing on the earthquake-compatible types of construction and plans for disaster readiness. The New Madrid events could provide valuable history lessons for our future.
I imagine that the following objectives will drive the activities of this unit. First, I would like students to be able to understand the geological profile of the United States that contributes to disasters such as earthquakes. This will involve learning about plate tectonics, subduction, strike-slip or transform faults, and rift zones. Secondly, I would like students to be able to distinguish between different types of earthquakes. Earthquakes along subduction zones are different from earthquakes on ancient rift zones and on strike-slip zones. Thirdly, I would like my students to be able to name and categorize the potential earthquake zones in the United States. Subsequently this should lead to the identification and analysis of the destructive implications of earthquakes on the landscape and built environments. I imagine this would also extend toward examining city planning, construction, and political response in areas subject to earthquake activity. Specifically, it will lead to case studies in which students will synthesize information about famous earthquakes in American history. My preparing a case study of the New Madrid Earthquakes will provide the anchor set by which students can research, organize, synthesize, and present information about other famous earthquakes. Developing an informed perspective on earthquakes in United States history, the natural processes that bring them about, and the political responses to those earthquakes should serve students well in their thinking about future responses to earthquakes as they occur in our future.
Lastly, working on the above objectives should result in students being able to demonstrate an understanding of key terms and concepts regarding geology and earthquakes: plate tectonics, subduction, rift zones, failed rift zones, fracture zones, fault lines, and earthquake magnitude.
Alignment with the Curriculum
Occurring within ten years of the Louisiana Purchase and Lewis and Clarks Exploration of the Northwest, the New Madrid Earthquakes align with the district unit on Westward Expansion/Manifest Destiny in the New Haven Public School Curriculum. Additionally, the rumblings of the ground metaphorically foreshadow the War of 1812 and the countless troubles in securing the westward lands from Native Americans. Indeed, the curriculum and most textbooks rarely explore the link between natural disasters and United States history. I hope that by introducing a connection with natural disasters, I will be able to help students see relationships between how natural events occur and what citizens can do to minimize the natural disaster risk.