Praxis Review for Earth Science

1. Praxis Review for Earth Science By Frank H. Osborne, Ph. D. 2: Tectonics and Internal Earth Processes

2. Plate Tectonics Plate tectonics is a unifying theory of Earth Science. It can explain events of the past, the present situation, and it can predict what will happen in the future. Formation of mountain ranges Continental drift and sea-floor spreading Ocean bottom is younger than continents Magnetic field reversals

3. Plate Tectonics Plates move as a result of convection currents in the mantle. They carry the continents or ocean bottoms on them. Plates generally move about an inch or two per year. This is about as fast as your fingernails grow. Plates can spread apart, collide or slide next to each other. Plate boundaries are known for presence of volcanoes and earthquakes.

4. Plate Tectonics Divergent plates spread apart at a spreading center. An example is the mid-Atlantic ridge. Convergent plates are colliding. Mountain ranges will form as a result of the collision. An example is the Himalayas formed by the collision of India with Asia. Transform boundaries are found where plates slide together. An example is the San Andreas Fault.

5. Plate Tectonics Continental crust is less dense than ocean bottom. At a convergent boundary between the two, the ocean bottom is drawn under the continent. This is called subduction. About 50-75 miles into the continent a line of volcanoes will form. If the subducted ocean bottom is bringing lots of water with it, the resulting steam will make the volcanoes active.

6. Plate Tectonics When a subduction zone is found under the ocean, a trench forms. An example is the Puerto Rico Trench. A spreading center located in a continent is called a rift valley. An example is the great Rift Valley in East Africa. Sea-floor spreading moves the plates. At the mid-Atlantic ridge, Europe and Africa are moving away from the Americas.

7. Plate Tectonics Evidence for sea-floor spreading comes from paleomagnetism. As new plate materials form, they become magnetized by the Earth�s magnetic field. When the magnetic field reverses, the new material is magnetized by the new magnetic orientation but the original magnetizations remain. This evidence supports plate tectonic theory.

8. Plate Tectonics A hot spot is a point where a hole has been melted through the crust from the mantle. The hot spot stays in one place while the crust moves along over it. Examples are the Hawaiian Island chain and Yellowstone National Park. Plate activity results in earthquakes and volcanic activity. Because of sea-floor spreading the continents move around. They have not always been located in the same places.

9. Plate Tectonics The Ring of Fire is a series of volcanoes that surrounds the Pacific Ocean.

10. Crustal Processes The Crust of the Earth can be deformed. Extension is a stretching process. Such a process is apparently occurring in the Basin and Range area of the western United States. Compression is a squeezing process. This is generally associated with mountain building. Shear results from two portions of crust passing by each other. Example is the San Andreas fault.

11. Crustal Processes Mountain building, folding and faulting of rock layers result from forces in the crust. Mountain ranges result from collisions between plates. The Pacific coast mountain ranges are examples. The mountains resulted from the collision of the North American Plate with the Pacific Ocean Plate. Volcanoes in Washington and Oregon indicate that there is a subduction zone under the Pacific northwest.

12. Crustal Processes Eastern mountains in the United States were produced by a collision between North America and Africa. The collision between plates caused folds and faults in the crust. The resulting mountains were once as high as the Himalayas. Erosion and other processes, in some cases glaciation, caused the mountains to appear the way they do today.

13. Crustal Processes Western mountains in the United States display block faulting. It is suspected that the continent is stretching apart in the west. The process of extension causes separation of the crust and blocks will drop down as a result. Similar block faulting occurred when the Atlantic Ocean began to open. This resulted in such features as Narragansett Bay and the Newark Basin.

14. Isostasy Isostasy refers to the fact that thicker continents (such as Africa) ride higher on the mantle than do thinner continents.

15. Structure of the Earth Earthquakes An earthquake is a sudden motion of rocks in the crust of the Earth after a long buildup of potential energy. Most earthquakes occur along plate boundaries or where there are faults in the rocks. Intensity is recorded on the Richter scale which is logarithmic. This means that a 6 is ten times stronger than a 5.

16. Seismic Waves There are two types of seismic waves Primary (P) waves are of the compression type. Secondary Shear (S) waves are of the longitudinal type. Earthquake wave travel velocities give evidence of the interior structure of the Earth.

17. Locating the Epicenter To locate the epicenter of an earthquake, geologists use three seismographs in widely separated locations. The time interval between the P and S waves gives the distance of the epicenter from the observatory. A circle with this radius is drawn on a map or globe. The same is done with all three stations. There is one point that lies on all three circles and it is the epicenter.

18. Structure of the Earth Earthquakes provide evidence about the internal structure of the Earth. The outer core of the Earth is believed to be liquid because S waves do not pass through it and P wave velocity is sharply decreased. Using similar techniques, oil companies use seismic waves to locate bodies of petroleum under the surface of the Earth.

19. Structure of the Earth The Earth is a series of concentric layers. The inner core is a solid ball of iron and nickel. The outer core is made of molten rocks as well as iron and nickel. The mantle contains liquid rocks that have very slow convection currents. These currents cause continental drift. The crust is the outer layer of the Earth. Continental crust is light and granitic. Ocean bottom crust is dense and basaltic.

20. Structure of the Earth

21. Electricity and Magnetism Magnets A magnet attracts the metals iron, cobalt and nickel. A magnet has two poles called NORTH and SOUTH. Opposite poles attract. Like poles repel.

22. Electricity and Magnetism Magnetic Fields A magnetic field surrounds a magnet. The lines of force emanate from the north pole of the magnet and enter the south pole of the magnet. A compass needle is an example of a magnet. The north pole of the compass needle points in the general direction of the geographic north pole of the Earth.

23. Electricity and Magnetism Diagram of Magnetic Field

24. Electricity and Magnetism Photograph of Magnetic Field

25. Electricity and Magnetism Magnetic Field of the Earth The Earth has a magnetic field. Because the north pole of the compass needle points north, the north magnetic pole is of the south type. The North Magnetic Pole is 11 1/2� away from geographic north so there is generally a compass deviation from true north.

26. Electricity and Magnetism Diagram of Magnetic field of the Earth

27. Electricity and Magnetism Map of North Magnetic Pole

28. Paleomagnetism As new crust is formed due to spreading, the rocks become magnetized as they cool. Magnetized rocks maintain their magnetic orientation. The magnetic field of the Earth reverses occasionally. This causes bands in the magnetic orientation of the sea floor that can be measured with a magnetometer.

29. The End 2: Tectonics and Internal Earth Processes