Structure+of+the+Earth

=Understanding the Earth and what makes it dynamic.=

In this section you will **teach yourself about the composition (what makes up) of the Earth**. When you are done with this section you will need to __//know the different layes of the Earth and what elements are most abundant in the earth, its oceans and atmosphere.//__ There is a link below that will help you do this and a fabulous slide show above.

 Let's first take a look at the earth. The earth is made up of 4 main layers-**//the inner core, the outer core, the mantle and the crust//**. The [|crust] is where we live, on the surface of the earth. There are two types of crust, what are they called? Below the crust lies the [|mantle], which is made up of solids, liquids and gases. The mantel is made up of the //Lithosphere// (upper portion of the mantel )and the Asthenosphere (the lower portion of the mantle). Next you have the [|outer core], which is where magma (molten rock) comes from! Finally in the center of the Earth you have the [|inner core.] 




 * What is an Earthquake?**



The lithosphere is made up of the crust and the upper most layer of the mantle, and is divided up into 12 major plates. As the plates of the lithosphere shift, weak spots, or "faults" develop. When this shifting has built up over long periods of time, the crust of the earth weakens and an earthquake occurs. The place where the crust is weakened is called a "fault". Sometimes the movement of plates is slow, with great pressure accumulating over time. Other times, plates become locked together and when pressure has reached a certain point, the plates are released and an earthquake happens. If this earthquake happens in a populated area, the effects on mankind can be devastating.




 * Earthquake Terms**

As described above, a fault is a weak spot, or fracture (break) in the earth's crust. There are three main types of faults. A normal fault occurs as a result of pulling tension that develops as one plate slips away from another plate (Diagram 2, "B"). A thrust (reverse) fault occurs as a result of the compression of plates pushing towards each other (Diagram 2, "C"). A strike-slip (lateral) fault occurs as a result of two adjacent places sliding next to each other in opposite directions. (Diagram 2, "A"). The focal depth of an earthquake is the depth from the Earth's surface to the region where an earthquake's energy originates (the focus). Earthquakes with focal depths from the surface to about 70 kilometers (43.5 miles) are classified as shallow. Earthquakes with focal depths from 70 to 300 kilometers (43.5 to 186 miles) are classified as intermediate. The focus of deep earthquakes may reach depths of more than 700 kilometers (435 miles). The focuses of most earthquakes are concentrated in the crust and upper mantle. The depth to the center of the Earth's core is about 6,370 kilometers (3,960 miles), so event the deepest earthquakes originate in relatively shallow parts of the Earth's interior. The epicenter of an earthquake is the point on the Earth's surface directly above the focus. The location of an earthquake is commonly described by the geographic position of its epicenter and by its focal depth. Earthquakes beneath the ocean floor sometimes generate immense sea waves or tsunamis (Japan's dread "huge wave"). These waves travel across the ocean at speeds as great as 960 kilometers per hour (597 miles per hour) and may be 15 meters (49 feet) high or higher by the time they reach the shore. The vibrations produced by earthquakes are detected, recorded, and measured by instruments call seismographs. The zig-zag line made by a seismograph, called a "seismogram," reflects the changing intensity of the vibrations by responding to the motion of the ground surface beneath the instrument. From the data expressed in seismograms, scientists can determine the time, the epicenter, the focal depth, and the type of faulting of an earthquake and can estimate how much energy was released. The two general types of vibrations produced by earthquakes are surface waves, which travel along the Earth's surface, and body waves, which travel through the Earth. Surface waves usually have the strongest vibrations and probably cause most of the damage done by earthquakes. The severity of an earthquake can be expressed in several ways. The magnitude of an earthquake, usually expressed by the Richter Scale, is a measure of the amplitude of the seismic waves. The moment magnitude of an earthquake is a measure of the amount of energy released - an amount that can be estimated from seismograph readings. The intensity, as expressed by the Modified Mercalli Scale, is a subjective measure that describes how strong a shock was felt at a particular location. The Richter Scale, named after Dr. Charles F. Richter of the California Institute of Technology, is the best known scale for measuring the magnitude of earthquakes. The scale is logarithmic so that a recording of 7, for example, indicates a disturbance with ground motion 10 times as large as a recording of 6. A quake of magnitude 2 is the smallest quake normally felt by people. Earthquakes with a Richter value of 6 or more are commonly considered major; great earthquakes have magnitude of 8 or more on the Richter scale. Seismometers are instruments that measure and record motions of the ground, including those of seismic waves generated by earthquakes, nuclear explosions, and other seismic sources. Records of seismic waves allow seismologists to map the interior of the Earth, and loca te and measure the size of these different sources.
 * Faults**
 * Focus and Focal Depth**
 * Epicenter**
 * Tsunamis**
 * Seismology**
 * Vibrations**
 * Richter Scale**
 * Seismometers**



Convection Currents 

What is a convection current? Based on this animation link, why do you think [|convection currents] change the surface of the Earth?




 * Earthquakes in History**

//Information provided courtesy of the USGS// The earliest earthquake for which we have descriptive information occurred in China in 1177 B.C. The Chinese earthquake catalog describes several dozen large earthquakes in China during the next few thousand years. Earthquakes in Europe are mentioned as early as 580 B.C., but the earliest for which we have some descriptive information occurred in the mid-16th century. The earliest known earthquakes in the Americas were in Mexico in the late 14th century and in Peru in 1471, but descriptions of the effects were not well documented. By the 17th century, descriptions of the effects of earthquakes were being published around the world - although these accounts were often exaggerated or distorted. The most widely felt earthquakes in the recorded history of North America were a series that occurred in 1811-1812 near New Madrid, Missouri. A great earthquake, whose magnitude is estimated to be about 8, occurred on the morning of December 16, 1811. Another great earthquake occurred on January 23, 1812, and a third, the strongest yet, on February 7, 1812. Aftershocks were nearly continuous between these great earthquakes and continued for months afterwards. These earthquakes were felt by people as far away as Boston and Denver. Because the most intense effects were in a sparsely populated region, the destruction of human life and property was slight. The San Francisco earthquakes of 1906 was one of the most destructive in the recorded history of North America - the earthquake and the fire that followed killed nearly 700 people and left the city in ruins.

The Alaska earthquake of March 27, 1964, was of greater magnitude than the San Francisco earthquake; it released perhaps twice as much energy and was felt over an area of almost 500,000 square miles. The ground motion near the epicenter was so violent that the tops of some trees were snapped off. One hundred and fourteen people (some as far away as California) died as a result of this earthquake, but loss of life and property would have been far greater had Alaska been more densely populated. 

=**General Resources about Earthquakes**=

> This USGS publication takes a look at earthquakes in history, where and how they occur, and gives a general overview of how they are measured. > This site offers monitoring of earthquakes, offers teaching resources, and offers information about earthquake preparedness. > Find out what a seismograph is, as well as other earthquake facts, from The Tech Museum of Innovation. > Learn how various cultures have explained earthquakes throughout history! > With a "short answer" and a "long answer" to that question, students of all ages and abilities can learn about the Richter Scale.
 * Online Resources**
 * [|Earthquakes: General Interest Publication]
 * [|USGS Earthquake Hazards Program]
 * [|Earthquake Shake]
 * [|Earthquake Myths and Folklore]
 * [|What is Richter Magnitude?]



=**Homework Assignments: **=