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9) Solid Earth. The student knows Earth’s interior is differentiated chemically, physically, and thermally. The student is expected to: c. Explain how scientists use geophysical methods such as seismic wave analysis, gravity, and magnetism to interpret Earth’s structure; and d. Describe the formation and structure of Earth’s magnetic field, including its interaction with charged solar particles to form the Van Allen belts and auroras. Paleomagnetism Polar wandering Magnetic reversals and mechanism of sea-floor spreading Vocabulary: Mohorovicic discontinuity (Moho); Shadow Zone, PWaves; S-Waves; Magnetosphere; Solar Wind; Aurora Borealis; Aurora Australis We’ve spent a couple of lessons already on the internal structure of the Earth. Just how do we know what lies beneath our feet? Do we have direct evidence from these depths? • 1989 Kola Superdeep Borehole in Russia: 12,262 M (40,230 ft) • 2008 Al Shaheen Oil Well in Qatar: 12,289 M (40,318 ft) (40, 502 ft) • 2011 Sakhalin-I Odoptu Well: Devastation 12,345after M 1906 Earthquake in San Francisco, which killed 700 people Anyway you look at it, we haven’t come close to drilling through even the crust, or outer surface of the Earth yet…so, how do wePersian know?Gulf Scientists have used indirect measurements and seismic waves from 1989 earthquakes to determine the internal structure of the Earth. What exactly is a seismic wave? • Seismic waves are those which are produced by movements and motions of the Earth’s crust along its tectonic boundaries…What types of boundaries are there again? Convergent, divergent, and transform Analysis of the Earth’s internal structure is made possible because earthquakes produce vibrations called seismic waves. These waves travel through the interior of the Earth and can be measured with sensitive detectors called seismographs. Scientists have seismographs set up all over the world to track movement of the Earth’s crust. While we may not have practical experience at these depths, we do have ways of determining Earth’s interior through extrapolation based upon behavior of waves through various media. 1. What are the three types of tectonic boundaries? 2. Describe each of the following: • Convergent • Divergent • Transform 3. What is a seismic wave? Seismic waves are divided into two types: Body waves and surface waves. Body waves include P and S waves, and these are the two types of waves that are used to determine the internal structure of the Earth. There are other types of seismic waves, but they relate to surface damage and only travel through the Earth’s crust, not the entire Earth. Measuring seismic waves traveling through the Earth allowed seismologists to determine that the core begins at a depth of 2900 km, or in other words, the mantle extends to 2900 km; its composition is probably fluid rock. This is based on the velocity of the waves, mass of the Earth and other lines of evidence. As humans have studied, and know how these various body waves travel through different substances (ie solids, fluids…etc), we can extrapolate about the structures found deep within the Earth by determining how these waves act as they move through it. (ie, the speed and direction of travel…etc) The shadow zone is the area of the Earth from angular distances of 104 to 140 degrees from a given earthquake epicenter that does not receive any direct P waves. The shadow zone results from S waves being stopped entirely by the liquid core and P waves being bent (refracted) by the liquid outer core. Through measuring how P and S waves travel through the Earth and out the other side, this seismic wave shadow zone was discovered in 1910. From the lack of S waves and a great slowing of the P wave velocity (by about 40%) it was deduced that the outer core is made of liquid. The shadow zone also defined the diameter of the core. Of Earth’s Interior…there is some! One type of direct evidence is a type of rock called "Xenoliths." Xenoliths are foreign or alien rocks. The word "xeno" means stranger. Xenoliths are basically small pieces of small fragments of rock usually enclosed in volcanic rocks that are thought to be pieces of the Earth's mantle, which are brought up along with the lava during the volcanic eruption. Another type of direct a type of rock structure called Theevidence ophiolite is mantle material looks "Ophiolites". somewhat darker than the gabbro above it The ophiolites are thought to be pieces of the mantle connected to pieces of the crust, which have been uplifted and embedded by tectonic processes in the crust of the Earth. They have a particular structure, which involves a transition from one rock type to another. 4. How do scientists use seismic waves to determine what the Earth’s interior must be like? • Mantle • Core 5. What tool is used to measure the intensity of an Earthquake? 6. Describe each of the following body waves: • P Waves • S Waves 7. What are xenoliths and ophiolites? 8. How are xenoliths and ophiolites direct evidence of Earth’s interior? You can use a spring scale to measure gravity. Notice that when this is hanging without moving, the spring is stretched by a certain amount. If the mass is pulled down, it causes the spring to stretch further. A machine called a gravimeter can be built based on this principle to measure Earth's gravity or differences in Earth's gravity between one place and another. Earth’s rotation causes the polar regions to flatten down slightly, making Earth an ellipsoid instead of a sphere. As a result, the force of gravity is slightly stronger at the poles compared to the equator (generally). Smaller variations in gravity across the Earth’s surface are caused by differences in the thickness and rock density of Earth’s crust, as well as density differences deep in Earth’s interior. • The overall density of the Earth is much higher than the density of the rocks we find in the crust. This tells us that the inside must be made of something much denser than rock. • Meteorites (created at the same time as the Earth, 4.6 billion years ago) have been analyzed. The most common type is called a chondrite and they contain iron, silicon, magnesium and oxygen (Others contain iron and nickel). A meteorite has roughly the same density as the whole Earth. A meteorite minus its iron has a density roughly the same as Mantle rock. • Iron and Nickel are both dense and magnetic. Earth has a Magnetic Field. Convection currents in the liquid outer core force the inner core to spin. The inner core spins faster than the rest of Earth. This creates a magnetic field through something called the dynamo effect. It is Earth’s rotating, convecting, electrically conducting metallic fluid outer core that enables this dynamo, in turn energizing the magnetic field, protecting us from the Sun. 9. Would the force of gravity be stronger at the poles, or equator…and why? 10. Where would gravity be stronger, on the continents or oceanic floor…and why? 11. How can the composition of meteorites suggest the makeup of the Earth’s interior? 12. What is the dynamo effect and how does it contribute to the formation of the magnetic field on Earth? 13. Why is Earth’s magnetic field so important? The Van Allen radiation belt is a ring of energetically charged particles (plasma) around Earth, which is held in place by Earth's magnetic field. It is believed that most of the particles that form the belts originated from solar wind and cosmic rays. The Van Allen Belt is located in the inner region of the Earth's magnetosphere. It is split into two distinct belts, with energetic electrons forming the outer belt and a combination of protons and electrons forming the inner belts The auroras, both surrounding the north magnetic pole • Aurora borealis and south magnetic pole • Aurora australis occur when highly charged electrons from the solar wind interact with elements in the Earth's atmosphere. Different gases give off different colors when they are excited. Oxygen at about 60 miles up gives off the familiar yellow-green color, Oxygen at higher altitudes (about 200 miles above us) gives the all red auroras. Ionic Nitrogen produces the blue light and neutral Nitrogen gives off the red-purple and the rippled edges. 14. What is the Van Allen Belt, and where do the particles that compose it come from? 15. What composes the outer Van Allen Belt? 16. What composes the inner Van Allen Belt? 17. Why does Earth have a Van Allen Belt? 18. Which auroras surround the north magnetic pole? South magnetic pole? 19. Why are auroras composed of different colors? Paleomagnetism is the study of the magnetic properties of rocks over time. Paleomagnetism aids in our understanding of plate tectonics, minerology, petrogenesis, geochronology, and the history of the Earth's magnetic field. Paleomagnetism is possible because some magnetic minerals common in igneous rocks, such as magnetite, and hematite, will "acquire" the magnetic field of their surroundings as the rock cools. As it cools, the surrounding magnetic field becomes "frozen," in the mineral, so at the time of formation, these rocks preserve the direction of the magnetic field. Pink= Normal magnetism White= Magnetic reversal During World War II, geologists employed by the military carried out studies of the sea floor, in order to understand the topography of the sea floor to find hiding places for both allied and enemy submarines. The topographic studies involved measuring the depth to the sea floor. These studies revealed the presence of two important topographic features of the ocean floor: •Oceanic Ridges - long ridges that occupy the middle of the Atlantic Ocean and the eastern part of the Pacific Ocean. • Oceanic Trenches - deep trenches along the margins of continents, particularly surrounding the Pacific Ocean. Polar Wandering is basically the apparent migration of the magnetic poles of the Earth through geologic time. Scientific evidence indicates that the magnetic poles have slowly and erratically wandered across the surface of the Earth. Today, magnetic north is here, and True geographic north is here. Calculations of pole locations over the past 20 million years show that the magnetic pole hasn’t moved greatly. Go back 30 million years, however, and substantial deviations occur. Calculations of polar wandering formed one of the first important pieces of evidence for continental drift. Scientists have determined that the magnetic poles do not actually wander that much; they generally stay close to the geographic poles. Therefore the concept of apparent polar wander is very useful in plate tectonics, since it can retrace the relative motion of continents, as well as the formation and break-up of supercontinents. As the image on the right shows, the paths of polar wandering would be different if the pole were fixed, compared to the path if the continent was fixed (on the left, image a ). 20. What is paleomagnetism, and how do we use it? 21. What two important oceanic features were discovered during WWII? 22. What is polar wandering, and what did its discovery help to prove?